Greatest element of a list
You are encouraged to solve this task according to the task description, using any language you may know.
Create a function that returns the maximum value in a provided set of values, where the number of values isn't known until runtime.
Ada
The keys for this task are initializing the compared value to the 'First value of the element type, and use of an unconstrained array type. <ada> with Ada.Text_Io;
procedure Max_Test is
-- substitute any array type with a scalar element type Flt_Array is array (Natural range <>) of Float; -- Create an exception for the case of an empty array Empty_Array : Exception; function Max(Item : Flt_Array) return Float is Max_Element : Float := Float'First; begin if Item'Length = 0 then raise Empty_Array; end if; for I in Item'range loop if Item(I) > Max_Element then Max_Element := Item(I); end if; end loop; return Max_Element; end Max; Buf : Flt_Array := (-275.0, -111.19, 0.0, -1234568.0, 3.14159, -3.14159);
begin
Ada.Text_IO.Put_Line(Float'Image(Max(Buf)));
end Max_Test;</ada> A generic function Max to deal with any floating-point type. <Ada> generic
type Item is digits <>; type Items_Array is array (Positive range <>) of Item;
function Generic_Max (List : Items_Array) return Item; </Ada> Implementation of: <Ada> function Generic_Max (List : Items_Array) return Item is
Result : Item := List (List'First);
begin
for Index in List'First + 1..List'Last loop Result := Item'Max (Result, List (Index)); end loop; return Result;
end Generic_Max; </Ada> When the argument array is empty, Constraint_Error exception is propagated, because array indexing is checked in Ada. Note also use of the floating-type attribute Max.
C
This works well with floats. Replace with double, int or what-have-you before passing a different data type. <C>#include <assert.h>
float max(unsigned int count, float values[]) {
float themax; unsigned int idx; assert(count > 0); themax = values[0]; for(unsigned int idx = 1; idx < count; ++idx) { themax = values[idx] > themax ? values[idx] : themax; } return themax;
}</C>
C++
This will work for any type with a < operator defined. Uses the standard library function max_element()
.
<cpp>#include <algorithm>
- include <cassert>
template<typename Ty> Ty max(unsigned int count, Ty values[]) {
assert(count > 0); return *std::max_element(values, values + count);
}</cpp>
Common Lisp
The built-in Common Lisp function max
takes the max of all its arguments.
<lisp>(max 1 2 3 4)
(reduce #'max values) ; find max of a list</lisp>
Erlang
From here
list_max([Head|Rest]) -> list_max(Rest, Head). list_max([], Res) -> Res; list_max([Head|Rest], Max) when Head > Max -> list_max(Rest, Head); list_max([_Head|Rest], Max) -> list_max(Rest, Max).
Using it.
>list_max([9,4,3,8,5]). 9
Forth
: array-max ( addr len -- max ) dup 0= if nip exit then over @ rot cell+ rot 1- cells bounds ?do i @ max cell +loop ;
: stack-max ( n ... m count -- max ) 1 ?do max loop ;
Haskell
The built-in Haskell function maximum
already does this.
my_max = maximum
It can alternately be defined as a "fold" on the built-in two-argument max
function.
my_max = foldl1 max
J
Verb maxatom returns the maximum value among atoms (smallest indivisible components) of the input, without regard to the shape of the input. The output of verb maxitem has the shape of an item of the input, and the value of each atom is the maximum along the largest axis. For a list of scalars these are equivalent.
maxatom=: >./ @ , maxitem=: >./
Java
The first function works with arrays of floats. Replace with arrays of double, int, or other primitive data type. <java>public static float max(float[] values) throws NoSuchElementException {
if (values.length == 0) throw new NoSuchElementException(); float themax = values[0]; for (int idx = 1; idx < values.length; ++idx) { if (values[idx] < themax) themax = values[idx]; } return themax;
}</java>
The following functions work with arrays or Lists of reference types, respectively. Note that the type is required to implement Comparable, to ensure we can compare them. For Lists, there is a utility method Collections.max()
that already does this.
<java>public static <T extends Comparable<? super T>> T max(T[] values) throws NoSuchElementException {
if (values.length == 0) throw new NoSuchElementException(); T themax = values[0]; for (int idx = 1; idx < values.length; ++idx) { if (values[idx].compareTo(themax) > 0) themax = values[idx]; } return themax;
}
import java.util.List; import java.util.Collections;
public static <T extends Comparable<? super T>> T max(List<T> values) {
return Collections.max(values);
}</java>
Logo
If the template is, like SUM, the name of a procedure that is capable of accepting arbitrarily many inputs, it is more efficient to use APPLY instead of REDUCE. The latter is good for associative procedures that have been written to accept exactly two inputs:
to max :a :b output ifelse :a > :b [:a] [:b] end
print reduce "max [...]
Alternatively, REDUCE can be used to write MAX as a procedure that accepts any number of inputs, as SUM does:
to max [:inputs] 2 if emptyp :inputs ~ [(throw "error [not enough inputs to max])] output reduce [ifelse ?1 > ?2 [?1] [?2]] :inputs end
OCaml
<ocaml>exception Empty_list let rec my_max = function
[] -> raise Empty_list | [x] -> x | x :: xs -> max x (my_max xs)</ocaml>
Perl
<perl>sub max {
my @list = @_; my $themax = $list[0]; foreach ( @list ) { $themax = $_ > $themax ? $_ : $themax; } return $themax;
}</perl>
It is already implemented in the module List::Util
's max()
function:
<perl>use List::Util qw(max);
max(@values);</perl>
PHP
The built-in PHP function max()
already does this.
<php>max($values)</php>
Python
The built-in Python function max()
already does this.
<python>max(values)</python>
Of course this assumes we have a list or tuple (or other sequence like object). (One can even find the max() or min() character of a string since that's treated as a sequence of characters and there are "less than" and "greater than" operations (object methods) associate with those characters).
If we truly were receiving a stream of data ... searching for the minimum would look something like:
<python> largest = 0 # or the minimal or epsilon value of the appropriate type while True:
item = stream.readline() # assuming line delimited inputs; use read(n) for fixed length inputs, etc. if not len(item): break # assuming stream is any file-like object, providing a read() of len() == 0 at EOF try: item = int(item) # using other functions to parse string inputs into any other data types # import pack.unpack() from standard libraries to handle binary packed data except ValueError, e: # or, in Python3: except ValueError as e: ... pass # log or otherwise deal with errors here if item > largest: largest = item
</python>
As explained in the comments this example can be adapted for any sort of data being read over any sort of socket or file-like interface. The initial value of "largest" must be both comparable to the data that will be parsed from the stream ... and guarantee to be lower than the max item therefrom. Similarly a min() function would need some value that's guaranteed to be larger (sys.maxint, for example). (If the values might be large integers or some other intractable type then read one item from the stream before the loop to initialize the initial value ... then let the loop iterate over the rest).
Ruby
max
is a method of all Enumerables
values.max
Scheme
The built-in Scheme function max
takes the max of all its arguments.
<scheme>(max 1 2 3 4)
(apply max values) ; find max of a list</scheme>
V
Assuming it is a list of positive numbers
[4 3 2 7 8 9] 0 [max] fold =9
If it is not
[4 3 2 7 8 9] dup first [max] fold =9