Soloway's recurring rainfall

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Task
Soloway's recurring rainfall
You are encouraged to solve this task according to the task description, using any language you may know.

Soloway's Recurring Rainfall is commonly used to assess general programming knowledge by requiring basic program structure, input/output, and program exit procedure.

The problem:

Write a program that will read in integers and output their average. Stop reading when the value 99999 is input.

For languages that aren't traditionally interactive, the program can read in values as makes sense and stopping once 99999 is encountered. The classic rainfall problem comes from identifying success of Computer Science programs with their students, so the original problem statement is written above -- though it may not strictly apply to a given language in the modern era.

Implementation Details:

  • Only Integers are to be accepted as input
  • Output should be floating point
  • Rainfall can be negative (https://www.geographyrealm.com/what-is-negative-rainfall/)
  • For languages where the user is inputting data, the number of data inputs can be "infinite"
  • A complete implementation should handle error cases reasonably (asking the user for more input, skipping the bad value when encountered, etc)

The purpose of this problem, as originally proposed in the 1980's through its continued use today, is to just show fundamentals of CS: iteration, branching, program structure, termination, management of data types, input/output (where applicable), etc with things like input validation or management of numerical limits being more "advanced". It isn't meant to literally be a rainfall calculator so implementations should strive to implement the solution clearly and simply.

References:


Ada

with Ada.Text_IO; 
with Ada.Text_IO.Unbounded_IO;
with Ada.Strings.Unbounded;
with Ada.Integer_Text_IO;
with Ada.IO_Exceptions;

procedure RecurringRainfall is
   Current_Average : Float := 0.0;
   Current_Count : Integer := 0;
   Input_Integer : Integer;
   
   -- Recursively attempt to get a new integer
   function Get_Next_Input return Integer is
      Input_Integer : Integer;
      Clear_String : Ada.Strings.Unbounded.Unbounded_String;
   begin
      Ada.Text_IO.Put("Enter rainfall int, 99999 to quit: ");
      Ada.Integer_Text_IO.Get(Input_Integer);
      return Input_Integer;
   exception
      when Ada.IO_Exceptions.Data_Error => 
         Ada.Text_IO.Put_Line("Invalid input");
         -- We need to call Get_Line to make sure we flush the kb buffer
         -- The pragma is to ignore the fact that we are not using the result
         pragma Warnings (Off, Clear_String);
         Clear_String := Ada.Text_IO.Unbounded_IO.Get_Line;
         -- Recursively call self -- it'll break when valid input is hit
         -- We disable the infinite recursion because we're intentionally
         -- doing this.  It will "break" when the user inputs valid input
         -- or kills the program
         pragma Warnings (Off, "infinite recursion");
         return Get_Next_Input;
         pragma Warnings (On, "infinite recursion");
   end Get_Next_Input;

begin
   loop
      Input_Integer := Get_Next_Input;
      exit when Input_Integer = 99999;
      
	  Current_Count := Current_Count + 1;
      Current_Average := Current_Average + (Float(1) / Float(Current_Count))*Float(Input_Integer) - (Float(1) / Float(Current_Count))*Current_Average;
      
      Ada.Text_IO.Put("New Average: ");
      Ada.Text_IO.Put_Line(Float'image(Current_Average));
      
   end loop;
end RecurringRainfall;

ALGOL 68

BEGIN # read a sequence of integers, terminated by 99999 and outpout their average #
    INT end value       = 99999;
    INT sum            := 0;
    INT count          := 0;
    BOOL invalid value := FALSE;
    on value error( stand in, ( REF FILE f )BOOL: invalid value := TRUE );
    WHILE
        INT n := 0;
        WHILE
            print( ( "Enter rainfall (integer) or ", whole( end value, 0 ), " to quit: " ) );
            read( ( n, newline ) );
            invalid value
        DO
            print( ( "Invalid input, please enter an integer", newline ) );
            invalid value := FALSE
        OD;
        n /= end value
    DO
        sum   +:= n;
        count +:= 1;
        print( ( "New average: ", fixed( sum / count, -12, 4 ), newline ) )
    OD
END

Arturo

i: 0
sum: 0

while ø [
    n: input "Enter rainfall as integer (99999 to quit): "
    try? -> n: to :integer n
    else [
        print "Input must be an integer."
        continue
    ]
    if 99999 = n -> break
    'i + 1
    'sum + n
    print ~« The current average rainfall is |sum // i|.
]
Output:
Enter rainfall as integer (99999 to quit): 145
The current average rainfall is 145.0.
Enter rainfall as integer (99999 to quit): 43678 
The current average rainfall is 21911.5.
Enter rainfall as integer (99999 to quit): 9
The current average rainfall is 14610.66666666667.
Enter rainfall as integer (99999 to quit): 155
The current average rainfall is 10996.75.
Enter rainfall as integer (99999 to quit): 99999

BASIC

Applesoft BASIC

 10  LET SUM = 0
 20  FOR RAINFALL = 1 TO 1E38
 30      INPUT "ENTER RAINFALL (AS AN INTEGER, OR 99999 TO QUIT): ";I
 40      IF I = 99999 THEN  END 
 50      IF I <  >  INT (I) THEN  PRINT "?REENTER, MUST BE AN INTEGER": GOTO 30
 60      LET SUM = SUM + I
 70      PRINT "  THE NEW AVERAGE RAINFALL IS "SUM / RAINFALL
 80  NEXT RAINFALL

QBasic

Works with: QBasic version 1.1
Works with: QuickBasic version 4.5
Works with: FreeBASIC
n = 0
sum = 0

DO
    INPUT "Enter integral rainfall (99999 to quit): ", i
    IF i = 99999 THEN
        EXIT DO
    ELSEIF (i < 0) OR (i <> INT(i)) THEN
        PRINT "Must be an integer no less than 0, try again."
    ELSE
        n = n + 1
        sum = sum + i
        PRINT "  The current average rainfall is"; sum / n
    END IF
LOOP
Output:
Same as FreeBASIC entry.

BASIC256

n = 0
sum = 0

while True
	input "Enter integral rainfall (99999 to quit): ", i
	if i = 99999 then exit while
	if (i < 0) or (i <> int(i)) then
		print "Must be an integer no less than 0, try again."
	else
		n += 1
		sum += i
		print "  The current average rainfall is "; sum/n
	end if
end while
Output:
Same as FreeBASIC entry.

True BASIC

Works with: QBasic
LET n = 0
LET sum = 0

DO
   PRINT "Enter integral rainfall (99999 to quit): "
   INPUT i
   IF i = 99999 THEN
      EXIT DO
   ELSEIF (i < 0) OR (i <> INT(i)) THEN
      PRINT "Must be an integer no less than 0, try again."
   ELSE
      LET n = n + 1
      LET sum = sum + i
      PRINT "  The current average rainfall is"; sum / n
   END IF
LOOP
END
Output:
Same as QBasic entry.

Yabasic

// Rosetta Code problem: https://rosettacode.org/wiki/Soloway%27s_Recurring_Rainfall
// by Jjuanhdez, 09/2022

n = 0
sum = 0

do
    input "Enter integral rainfall (99999 to quit): " i
    if (i < 0) or (i <> int(i)) then
        print "Must be an integer no less than 0, try again."
    elsif i = 99999 
        break
    else
        n = n + 1
        sum = sum + i
        print "  The current average rainfall is ", sum/n
    end if
loop
Output:
Same as FreeBASIC entry.

C

#include <stdio.h>

int main(int argc, char **argv)
{
	// Unused variables
	(void)argc;
	(void)argv;
	
	float currentAverage = 0;
	unsigned int currentEntryNumber = 0;
	
	for (;;)
	{
		int ret, entry;
		
		printf("Enter rainfall int, 99999 to quit: ");
		ret = scanf("%d", &entry);
		
		if (ret)
		{
			if (entry == 99999)
			{
				printf("User requested quit.\n");
				break;
			}
			else
			{
				currentEntryNumber++;
				currentAverage = currentAverage + (1.0f/currentEntryNumber)*entry - (1.0f/currentEntryNumber)*currentAverage;
				
				printf("New Average: %f\n", currentAverage);
			}
		}
		else
		{
			printf("Invalid input\n");
			while (getchar() != '\n');	// Clear input buffer before asking again
		}
	}
	
	return 0;
}

C++

#include <iostream>
#include <limits>

int main()
{
	float currentAverage = 0;
	unsigned int currentEntryNumber = 0;
	
	for (;;)
	{
		int entry;
		
		std::cout << "Enter rainfall int, 99999 to quit: ";
		std::cin >> entry;
		
		if (!std::cin.fail())
		{
			if (entry == 99999)
			{
				std::cout << "User requested quit." << std::endl;
				break;
			}
			else
			{
				currentEntryNumber++;
				currentAverage = currentAverage + (1.0f/currentEntryNumber)*entry - (1.0f/currentEntryNumber)*currentAverage;
				
				std::cout << "New Average: " << currentAverage << std::endl;
			}
		}
		else
		{
			std::cout << "Invalid input" << std::endl;
			std::cin.clear();
			std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
		}
	}
	
	return 0;
}

C#

namespace RosettaCode
{
	class CSharpRecurringRainfall
    {		
		static int ReadNextInput()
		{
			System.Console.Write("Enter rainfall int, 99999 to quit: ");
			string input = System.Console.ReadLine();
			
			if (System.Int32.TryParse(input, out int num))
			{
				return num;
			}
			else
			{
				System.Console.WriteLine("Invalid input");
				return ReadNextInput();
			}
		}
		
        static void Main()
        {
			double currentAverage = 0;
			int currentEntryNumber = 0;
			
            for (int lastInput = ReadNextInput(); lastInput != 99999; lastInput = ReadNextInput())
            {
				currentEntryNumber++;
				currentAverage = currentAverage + (1.0/(float)currentEntryNumber)*lastInput - (1.0/(float)currentEntryNumber)*currentAverage;
				System.Console.WriteLine("New Average: " + currentAverage);
			}
        }
    }
}

D

import std.stdio;

void main()
{
	float currentAverage = 0;
	uint currentEntryNumber = 0;
	
	for (;;)
	{
		int entry;
				
		write("Enter rainfall int, 99999 to quit: ");
		
		try {
			readf("%d", entry);
			readln();
		}
		catch (Exception e) {
			writeln("Invalid input");
			readln();
			continue;
		}
		
		if (entry == 99999) {
			writeln("User requested quit.");
			break;
		} else {
			currentEntryNumber++;
			currentAverage = currentAverage + (1.0/currentEntryNumber)*entry - (1.0/currentEntryNumber)*currentAverage;
			
			writeln("New Average: ", currentAverage);
		}
	}
}

Delphi

Works with: Delphi version 6.0

Although Delphi supports console applications that use simple keyboard input and teletype output, the primary focus of the language is GUI development. Consequently, I've produced a GUI version of the problem that uses the "TMemo" component to simulate a console interface.

To get the TMemo to behave like a console application, I've created an object that captures keyboard into to the TMemo and can return the keystrokes a chars, integers or reals. The object, which is called "TKeyWaiter," is simply created and attached to a TMemo component. While the object is alive, it will capture key stroke and return chars, integers or reals. It can be destroyed as soon as it is no longer needed. In fact, in this code, I create and destroy it for every input. The creation overhead is so low that there is no reason to create a global copy and keep it alive while the program is operating.

{This code would normally be in a library somewhere, but it is included here for clarity}

{TKeywaiter interface}

type TKeyWaiter = class(TObject)
 private
  FControl: TWinControl;
  FControlCAbort: boolean;
 protected
  procedure HandleKeyPress(Sender: TObject; var Key: Char);
 public
  KeyChar: Char;
  ValidKey: boolean;
  AbortWait: boolean;
  constructor Create(Control: TWinControl);
  function WaitForKey: char;
  function WaitForInteger: integer;
  function WaitForReal: double;
  property ControlCAbort: boolean read FControlCAbort write FControlCAbort;
 end;


{ TMemoWaiter implementation }

type TControlHack = class(TWinControl) end;

constructor TKeyWaiter.Create(Control: TWinControl);
{Save the control we want to wait on}
begin
FControl:=Control;
FControlCAbort:=False;
end;

procedure TKeyWaiter.HandleKeyPress(Sender: TObject; var Key: Char);
{Handle captured key press}
begin
KeyChar:=Key;
ValidKey:=True;
if ControlCAbort then AbortWait:=KeyChar = #$03;
end;


function TKeyWaiter.WaitForKey: char;
{Capture keypress event and wait for key press control}
{Spends most of its time sleep and aborts if the user}
{sets the abort flag or the program terminates}
begin
ValidKey:=False;
AbortWait:=False;
TControlHack(FControl).OnKeyPress:=HandleKeyPress;
repeat
	begin
	Application.ProcessMessages;
	Sleep(100);
	end
until ValidKey or Application.Terminated or AbortWait;
Result:=KeyChar;
end;


function TKeyWaiter.WaitForInteger: integer;
var C: char;
var S: string;
begin
Result:=0;
S:='';
{Wait for first numeric characters}
repeat
	begin
	C:=WaitForKey;
	if AbortWait or Application.Terminated then exit;
	end
until C in ['+','-','0'..'9'];
{Read characters and convert to}
{integer until non-integer arrives}
repeat
	begin
	S:=S+C;
	C:=WaitForKey;
	if AbortWait or Application.Terminated then exit;
	end
until not (C in  ['+','-','0'..'9']);
Result:=StrToInt(S);
end;


type TCharSet = set of char;

function TKeyWaiter.WaitForReal: double;
var C: char;
var S: string;
const RealSet: TCharSet = ['-','+','.','0'..'9'];
begin
Result:=0;
S:='';
{Wait for first numeric characters}
repeat
	begin
	C:=WaitForKey;
	if AbortWait or Application.Terminated then exit;
	end
until C in RealSet;
{Read characters and convert to}
{integer until non-integer arrives}
repeat
	begin
	S:=S+C;
	C:=WaitForKey;
	if AbortWait or Application.Terminated then exit;
	end
until not (C in RealSet);
Result:=StrToFloat(S);
end;



{===========================================================}


function WaitForReal(Memo: TMemo; Prompt: string): double;
{Wait for double entered into TMemo component}
var KW: TKeyWaiter;
begin
{Use custom object to wait for and capture reals}
KW:=TKeyWaiter.Create(Memo);
try
Memo.Lines.Add(Prompt);
Memo.SelStart:=Memo.SelStart-1;
Memo.SetFocus;
Result:=KW.WaitForReal;
finally KW.Free; end;
end;

var Count: integer;
var Average: double;

procedure RecurringRainfall(Memo: TMemo);
var D: double;
begin
Count:=0;
Average:=0;
while true do
	begin
	D:=WaitForReal(Memo,'Enter integer rainfall (99999 to quit): ');
	if Application.Terminated then exit;
	if (Trunc(D)<>D) or (D<0) then
		begin
		Memo.Lines.Add('Must be integer >=0');
		continue;
		end;
	if D=99999 then break;
	Inc(Count);
	Average := Average + (1 / Count) * D - (1 / Count)*Average;
	Memo.Lines.Add('New Average: '+FloatToStrF(Average,ffFixed,18,2));
	end;
end;
Output:
Enter integer rainfall (99999 to quit): 
5.4

Must be integer >=0
Enter integer rainfall (99999 to quit): 
-2

Must be integer >=0
Enter integer rainfall (99999 to quit): 
5

New Average: 5.00
Enter integer rainfall (99999 to quit): 
2

New Average: 3.50
Enter integer rainfall (99999 to quit): 
10

New Average: 5.67
Enter integer rainfall (99999 to quit): 
99999


Elapsed Time: 01:56.914 min

Fortran

Fortran 90

function getNextInput() result(Input)
    implicit none
    integer :: Input
    integer :: Reason
    Reason = 1
    
    do while (Reason > 0)
        print *, "Enter rainfall int, 99999 to quit: "
        read (*,*,IOSTAT=Reason) Input
        
        if (Reason > 0) then
            print *, "Invalid input"
        end if
    enddo
    
end function getNextInput

program recurringrainfall
    implicit none
    real        :: currentAverage
    integer     :: currentCount
    integer     :: lastInput
    integer     :: getNextInput
    
    currentAverage = 0
    currentCount = 0

    do
        lastInput = getNextInput()
        
        if (lastInput == 99999) exit
        
        currentCount = currentCount + 1
        currentAverage = currentAverage + (1/real(currentCount))*lastInput - (1/real(currentCount))*currentAverage
        
        print *, 'New Average: ', currentAverage
    enddo 

    
end program recurringrainfall

FreeBASIC

Dim As Integer n = 0, sum = 0
Dim As Single i

Do
    Input "Enter integral rainfall (99999 to quit): ", i
    If i = 99999 Then 
        Exit Do
    Elseif (i < 0) Or (i <> Int(i)) Then
        Print "Must be an integer no less than 0, try again."
    Else
        n += 1
        sum += i
        Print "  The current average rainfall is"; sum/n
    End If
Loop

Sleep
Output:
Enter integral rainfall (99999 to quit): 5.4
Must be an integer no less than 0, try again.
Enter integral rainfall (99999 to quit): -2 
Must be an integer no less than 0, try again.
Enter integral rainfall (99999 to quit): 5
  The current average rainfall is 5
Enter integral rainfall (99999 to quit): 2
  The current average rainfall is 3.5
Enter integral rainfall (99999 to quit): 10
  The current average rainfall is 5.6666666666667
Enter integral rainfall (99999 to quit): 99999

J

The specification for this task seems inadequate. So, we'll copy some features from other implementations of this task.

Specifically, we'll issue a prompt for every number requested, and reject lines which contain something that is not a number.

Implementation:
require'general/misc/prompt'

task=: {{
  list=. ''
  while. do.
    y=. (#~ >.=<.)_.".prompt'Enter rainfall int, 99999 to quit: '
    if. 99999 e. y do. (+/%#)list return. end.
    if. y-:y do. echo 'New average: ',":(+/%#)list=. list,y
    else. echo 'invalid input, reenter'
    end.
  end.
}}
Example use:
   task''
Enter rainfall int, 99999 to quit: 2
New average: 2
Enter rainfall int, 99999 to quit: 3
New average: 2.5
Enter rainfall int, 99999 to quit: 5
New average: 3.33333
Enter rainfall int, 99999 to quit: 7
New average: 4.25
Enter rainfall int, 99999 to quit: 11
New average: 5.6
Enter rainfall int, 99999 to quit: 99999
5.6

Java

class recurringrainfall
{ 
	private static int GetNextInt()
	{
		while (true)
		{
			System.out.print("Enter rainfall int, 99999 to quit: ");
			String input = System.console().readLine();
        
			try
			{
				int n = Integer.parseInt(input);
				return n;
			}
			catch (Exception e)
			{
				System.out.println("Invalid input");
			}
		}
	}
	
    private static void recurringRainfall() {
		float currentAverage = 0;
		int currentEntryNumber = 0;
		
		while (true) {
			int entry = GetNextInt();
			
			if (entry == 99999)
				return;
			
			currentEntryNumber++;
			currentAverage = currentAverage + ((float)1/currentEntryNumber)*entry - ((float)1/currentEntryNumber)*currentAverage;
			
			System.out.println("New Average: " + currentAverage);
		}
    }
    
    public static void main(String args[]) { 
        recurringRainfall();
    } 
}

jq

The following program is mainly intended for interactive use. An appropriate invocation would be: jq -nrR -f compute-averages.jq Please note:

  • a null response is regarded as an invalid entry;
  • if there are no valid entries, the average is not printed;
  • leading and trailing spaces in entries are ignored, and a single + or - prefix is allowed;
  • an "end-of-file" condition is equivalent to the occurrence of 99999.
def isinteger: test("^ *[+-]?[0-9]+ *$");

def soloway:
  label $out
  | foreach (inputs, null) as $x (null;
      .invalid = false
      | if $x == null then .break = true
        elif ($x|isinteger) then ($x|tonumber) as $n
        | if $n == 99999
          then .break = true
          else .sum += $n | .n += 1
          end
        else .invalid = $x
        end;
      if .break then ., break $out
      elif .invalid then .
      else empty
      end)
  | (select(.invalid) | "Invalid entry (\(.invalid)). Please try again."),
    (select(.break and .sum) | "Average is: \(.sum / .n)") ;

soloway

Julia

Written for simplicity of reading rather than brevity. Changed from the originals to allow negative input, though I note that allowing negative integer input and disallowing floating point seems strange given most "real-life" negative rainfall daily numbers are less than 1 inch.

"""
Two annotated example outputs
were given: 1) a run with three positive inputs, a zero, and
a negative number before the sentinel; 2) a run in which
the sentinel was the first and only input.
"""
function rainfall_problem(sentinel = 999999, allownegative = true)
    total, entries = 0, 0
    while true
        print("Enter rainfall as $(allownegative ? "" : "nonnegative ")integer ($sentinel to exit): ")
        n = tryparse(Int, readline())
        if n == sentinel
            break
        elseif n == nothing || !allownegative && n < 0
             println("Error: bad input. Try again\n")
        else
            total += n
            entries += 1
            println("Average rainfall is currently ", total / entries)
        end
    end
    if entries == 0
        println("No entries to calculate!")
    end
end

rainfall_problem()

Nim

We catch exceptions related to wrong input, i.e. non integer values. Negative values are allowed. If an end of file is encountered, the program terminates with an error message.

import std/[strformat, strutils]

const EndValue = 99999


var sum, count = 0.0

while true:

  var value: int

  # Read input until a valid integer if found.
  var input: string
  while true:
    stdout.write &"Enter an integer value, {EndValue} to terminate: "
    stdout.flushFile()
    try:
      input = stdin.readLine()
      value = input.parseInt()
      break
    except ValueError:
      echo &"Expected an integer: got “{input}”"
    except EOFError:
      quit "\nEncountered end of file. Exiting.", QuitFailure

  # Process value.
  if value == EndValue:
    echo "End of processing."
    break
  count += 1
  sum += value.toFloat
  echo &"  Current average is {sum / count}."
Output:

Sample session.

Enter an integer value, 99999 to terminate: 10
  Current average is 10.0.
Enter an integer value, 99999 to terminate: -20
  Current average is -5.0.
Enter an integer value, 99999 to terminate: 50
  Current average is 13.33333333333333.
Enter an integer value, 99999 to terminate: 3.5
Expected an integer: got “3.5”
Enter an integer value, 99999 to terminate: aaa
Expected an integer: got “aaa”
Enter an integer value, 99999 to terminate: 10
  Current average is 12.5.
Enter an integer value, 99999 to terminate: 99999
End of processing.

Perl

FWIW

use strict;
use warnings;

use Scalar::Util qw(looks_like_number);

my ($periods, $accumulation, $rainfall);

sub so_far { printf "Average rainfall %.2f units over %d time periods.\n", ($accumulation / $periods) || 0, $periods }

while () {
    while () {
        print 'Integer units of rainfall in this time period? (999999 to finalize and exit)>: ';
        $rainfall = <>;
        last if looks_like_number($rainfall) and $rainfall and $rainfall == int $rainfall;
        print "Invalid input, try again.\n";
    }
    (so_far, exit) if $rainfall == 999999;
    $periods++;
    $accumulation += $rainfall;
    so_far;
}
Output:
Integer units of rainfall in this time period? (999999 to finalize and exit)>: raindrops
Invalid input, try again.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 2.00
Average rainfall 2.00 units over 1 time periods.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: hail
Invalid input, try again.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 10**0
Invalid input, try again.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 1
Average rainfall 1.50 units over 2 time periods.
Integer units of rainfall in this time period? (999999 to finalize and exit)>:
Invalid input, try again.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 999999
Average rainfall 1.50 units over 2 time periods.

Phix

with javascript_semantics
constant inputs = {5.4,"five",5,-2,4,10,99999}
atom n = 0, average = 0
procedure show_average()
    printf(1,"  The current average rainfall is %g\n",average)
end procedure

for r in inputs do
    printf(1,"Enter integral rainfall, 99999 to quit: %v\n",{r})
    if not integer(r) /*or r<0*/ then
--      printf(1,"Must be a non-negative integer, try again.\n")
        printf(1,"Must be an integer, try again.\n")
    else
        if r=99999 then exit end if
        n += 1
        average += (r-average)/n
        show_average()
    end if
end for
show_average()
Output:
Enter integral rainfall, 99999 to quit: 5.4
Must be an integer, try again.
Enter integral rainfall, 99999 to quit: "five"
Must be an integer, try again.
Enter integral rainfall, 99999 to quit: 5
  The current average rainfall is 5
Enter integral rainfall, 99999 to quit: -2
  The current average rainfall is 1.5
Enter integral rainfall, 99999 to quit: 4
  The current average rainfall is 2.33333
Enter integral rainfall, 99999 to quit: 10
  The current average rainfall is 4.25
Enter integral rainfall, 99999 to quit: 99999
  The current average rainfall is 4.25

PureBasic

Define.i n=0, sum=0
Define.f i

If OpenConsole()
  Repeat
    Print("Enter integral rainfall (99999 to quit): ")
    i=ValF(Input())
    If i=99999
      Break
    ElseIf i<0 Or i<>Int(i)
      PrintN("Must be an integer no less than 0, try again.")
    Else
      n+1
      sum+i
      PrintN("  The current average rainfall is "+StrF(sum/n,5))
    EndIf
  ForEver
EndIf
Output:
Same as FreeBASIC entry.

Python

import sys

def get_next_input():
    try:
        num = int(input("Enter rainfall int, 99999 to quit: "))
    except:
        print("Invalid input")
        return get_next_input()
    return num

current_average = 0.0
current_count = 0

while True:
    next = get_next_input()

    if next == 99999:
        sys.exit()
    else:
        current_count += 1
        current_average = current_average + (1.0/current_count)*next - (1.0/current_count)*current_average
        
        print("New average: ", current_average)

Quackery

  [ $ "bigrat.qky" loadfile ] now!
  
  [ 0 0
    [ $ "Enter an integer (99999 to end): "
      [ input $->n not iff
          [ drop
            $ "                      Try again: " ]
            again ]
      dup 99999 != while
      + dip 1+
      again ]
    drop
    cr
    over 0 = iff
      [ 2drop
        say "No data entered." ] done
    say "Average: "
    swap 10 point$ echo$ ]                          is srr ( --> )
Output:

As a dialogue in the Quackery shell.

/O> srr
... 
Enter an integer (99999 to end): 99999

No data entered.
Stack empty.

/O> srr
... 
Enter an integer (99999 to end): not an integer
                      Try again: 3.14159
                      Try again: 
                      Try again: 2
Enter an integer (99999 to end): -3
Enter an integer (99999 to end): 5
Enter an integer (99999 to end): -7
Enter an integer (99999 to end): 11
Enter an integer (99999 to end): -13
Enter an integer (99999 to end): 17
Enter an integer (99999 to end): 99999

Average: 1.7142857143
Stack empty.

Raku

This task seems to be more about following (kind of vague) instructions and anticipating error conditions rather than solving a rather simple problem.

I notice that units are specified for neither measurement; time nor rainfall accumulation. Time is assumed to always increment in units of one, rainfall in some non-negative integer. "Integer" gets a little leeway in this entry. Floating point numbers or Rational numbers that are exactly equal to an Integer are allowed, even if they are technically not integers.

After update to spec:

Not going to bother to allow negative rainfall amounts. The "Negative rainfall" cited in the linked article references "total accumulated groundwater" NOT precipitation. Evaporation isn't rainfall. Much like dew or fog is not rainfall.

Further, the linked reference article for Soloway's Recurring Rainfall SPECIFICALLY discusses marking an implementation as incorrect, or at least lacking, if it doesn't ignore and discard negative entries. You can't have it both ways.

# Write a program that will read in integers and
# output their average. Stop reading when the
# value 99999 is input.


my ($periods, $accumulation, $rainfall) = 0, 0;

loop {
    loop {
        $rainfall = prompt 'Integer units of rainfall in this time period? (999999 to finalize and exit)>: ';
        last if $rainfall.chars and $rainfall.Numeric !~~ Failure and $rainfall.narrow ~~ Int and $rainfall0;
        say 'Invalid input, try again.';
    }
    last if $rainfall == 999999;
    ++$periods;
    $accumulation += $rainfall;
    say-it;
}

say-it;

sub say-it { printf "Average rainfall %.2f units over %d time periods.\n", ($accumulation / $periods) || 0, $periods }
Output:
# Normal operation
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 0
Average rainfall 0.00 units over 1 time periods.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 2
Average rainfall 1.00 units over 2 time periods.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 4
Average rainfall 2.00 units over 3 time periods.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 6
Average rainfall 3.00 units over 4 time periods.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 8
Average rainfall 4.00 units over 5 time periods.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 999999
Average rainfall 4.00 units over 5 time periods.


# Invalid entries and valid but not traditional "Integer" entries demonstrated  
Integer units of rainfall in this time period? (999999 to finalize and exit)>: a
Invalid input, try again.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 1.1
Invalid input, try again.
Integer units of rainfall in this time period? (999999 to finalize and exit)>:
Invalid input, try again.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 2.0
Average rainfall 2.00 units over 1 time periods.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: .3e1
Average rainfall 2.50 units over 2 time periods.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 999999
Average rainfall 2.50 units over 2 time periods.


# Valid summary with no entries demonstrated.
Integer units of rainfall in this time period? (999999 to finalize and exit)>: 999999
Average rainfall 0.00 units over 0 time periods.

RPL

Works with: HP version 28
RPL code Comment
≪ "Enter rainfall, then ENTER" 1 DISP
   "" "0123456789" 
   DO DO UNTIL KEY END
      IF DUP2 POS THEN
         ROT OVER + DUP 2 DISP ROT ROT 
      END 
   UNTIL "ENTER" == END
   DROP STR→ 
≫ 'READB' STO

≪ "Enter 9999 to end" 4 DISP
   (0,0) 1 CF CLLCD
   DO READB
      IF DUP 9999 == THEN 1 SF 
      ELSE 
         1 R→C +
         "Average = " OVER RE LAST IM / →STR + 
         3 DISP 
      END
   UNTIL 1 FS? END
   SWAP DROP CLMF 
   IFERR RE LAST IM / THEN DROP2 END
≫ 'RFALL' STO
READB ( → n ) 
Initialize variables
Loop
   if keystroke is an accepted char
      add char to output and display it

until ENTER is pressed
clean stack and convert to integer


RFALL ( → ) 
initialize counters and flag, clear screen 
loop
   get rainfall
   if break value then set flag
   otherwise
     accumulate rainfall and increment count
     display average rainfall
     
 until flag set
 clean stack, unfreeze display
 put average rainfall in stack if count ≠ 0


Ruby

QUIT  = 99999
num   = nil
count = 0
avg   = 0.0

loop do
  begin
    print "Enter rainfall int, #{QUIT} to quit: "
    input = gets
    num = Integer(input)
  rescue ArgumentError
    puts "Invalid input #{input}"
    redo
  end
  break if num == QUIT
  count += 1
  inv_count = 1.0/count
  avg = avg + inv_count*num - inv_count*avg
end

puts "#{count} numbers entered, averaging #{average}"

Rust

fn main() {
	
	let mut current_average:f32 = 0.0;
	let mut current_entry_number:u32 = 0;
	
	loop
	{
		let current_entry;

		println!("Enter rainfall int, 99999 to quit: "); 
		let mut input_text = String::new();
		std::io::stdin().read_line(&mut input_text).expect("Failed to read from stdin");
		let trimmed = input_text.trim();
		match trimmed.parse::<u32>() {
			Ok(new_entry) => current_entry = new_entry,
			Err(..) => { println!("Invalid input"); continue; }
		};

		if current_entry == 99999
		{
			println!("User requested quit.");
			break;
		}
		else
		{
			current_entry_number = current_entry_number + 1;
			current_average = current_average + (1.0 / current_entry_number as f32)*(current_entry as f32) - (1.0 / current_entry_number as f32)*current_average;

			println!("New Average: {}", current_average);
		}
	}
}

Wren

Library: Wren-ioutil
import "./ioutil" for Input

var n = 0
var sum = 0
while (true) {
    var i = Input.integer("Enter integral rainfall (99999 to quit): ")
    if (i == 99999) break
    n = n + 1
    sum = sum + i
    System.print("  The current average rainfall is %(sum/n)")
}
Output:

Sample session:

Enter integral rainfall (99999 to quit): 5.4
Must be an integer, try again.
Enter integral rainfall (99999 to quit): five
Must be an integer, try again.
Enter integral rainfall (99999 to quit): 5
  The current average rainfall is 5
Enter integral rainfall (99999 to quit): -2
  The current average rainfall is 1.5
Enter integral rainfall (99999 to quit): 4
  The current average rainfall is 2.3333333333333
Enter integral rainfall (99999 to quit): 10
  The current average rainfall is 4.25
Enter integral rainfall (99999 to quit): 99999

XPL0

The problem is very simple except for the vague requirements for the user interface and how to deal with unaccepted rainfall amounts.

real Rain, Sum, Count;
[Sum:= 0.;  Count:= 0.;
loop    [loop   [Text(0, "Enter rainfall amount, or 99999 to quit: ");
                Rain:= RlIn(0);
                if Rain < 0. then
                        Text(0, "Must not be negative.^m^j")
                else if Floor(Rain) # Rain then
                        Text(0, "Must be an integer.^m^j")
                else quit;
                ];
        if Rain = 99999. then quit;
        Sum:= Sum + Rain;
        Count:= Count + 1.;
        Text(0, "Average rainfall is ");        
        RlOut(0, Sum/Count);
        CrLf(0);
        ];
]
Output:
Enter rainfall amount, or 99999 to quit: 5.4
Must be an integer.
Enter rainfall amount, or 99999 to quit: five
5
Average rainfall is     5.00000
Enter rainfall amount, or 99999 to quit: -2
Must not be negative.
Enter rainfall amount, or 99999 to quit: 4
Average rainfall is     4.50000
Enter rainfall amount, or 99999 to quit: 
2.00000001
Must be an integer.
Enter rainfall amount, or 99999 to quit: .3e1
Average rainfall is     4.00000
Enter rainfall amount, or 99999 to quit: 99999