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. <lang 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;</lang> A generic function Max to deal with any floating-point type. <lang ada> generic
type Item is digits <>; type Items_Array is array (Positive range <>) of Item;
function Generic_Max (List : Items_Array) return Item; </lang> Implementation of: <lang 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; </lang> 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.
ALGOL 68
# substitute any array type with a scalar element #
MODE FLT = REAL;
# create an exception for the case of an empty array #
PROC raise empty array = VOID:(
GO TO except empty array
);
PROC max = ([]FLT item)FLT:
BEGIN
IF LWB item > UPB item THEN
raise empty array; SKIP
ELSE
FLT max element := item[LWB item];
FOR i FROM LWB item + 1 TO UPB item DO
IF item[i] > max element THEN
max element := item[i]
FI
OD;
max element
FI
END # max #;
[]FLT buf = (-275.0, -111.19, 0.0, -1234568.0, pi, -pi);
print((max(buf),new line)) EXIT
except empty array:
SKIP
Output:
+3.14159265358979e +0
C
This works well with floats. Replace with double, int or what-have-you before passing a different data type. <lang 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;
}</lang>
The following macro can be used with any number and type of arguments, provided that the arguments are simple, i.e. must not contain subexpressions where commas appear (this is because of the way the arguments are counted; the macro can be modified so that it is up to the caller to count the number of arguments passed).
<lang c>#include <stdarg.h>
- define MAX(A,...) ({ inline __typeof__ (A) _max_(__typeof__ (A) a, ...) {\
va_list l; int i,c; char *s = #__VA_ARGS__; __typeof__ (A) max = a;\
__typeof__ (A) t;\
for(c=1;*s!=0;s++) if (*s==',') c++;\
va_start(l, a);\
for(i=0;i<=c;i++) {\
if ((t=va_arg(l,__typeof__ (A))) > max) max = t;\
}\
va_end(l); return max;\
}\ _max_((A),__VA_ARGS__);\ })</lang>
C++
This will work for any type with a < operator defined. Uses the standard library function max_element(). <lang 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);
}</lang>
Common Lisp
The built-in Common Lisp function max takes the max of all its arguments. <lang lisp>(max 1 2 3 4) (reduce #'max values) ; find max of a list (loop for x in values
maximize x) ; alternative way to find max of a list</lang>
D
The standard library (Phobos) has a max function in D2.0:
<lang D>import std.algorithm; max(values);</lang>
E
This function works for any value which responds to max/1:
<lang e>pragma.enable("accumulator") # non-finalized syntax feature
def max([first] + rest) {
return accum first for x in rest { _.max(x) }
}</lang>
<lang e>? max([1, 2, 3])
- value: 3</lang>
To require only the comparison protocol, one needs to write out the algorithm a little more explicitly:
<lang e>def max([var bestSoFar] + rest) {
for x ? (x > bestSoFar) in rest {
bestSoFar := x
}
return bestSoFar
}</lang>
<lang e>? max([1, 3, 2])
- value: 3
? max([[1].asSet(), [2].asSet(), [1, 2].asSet()])
- value: [1, 2].asSet()</lang>
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
Icon
procedure main()
local l
l := [7,8,6,9,4,5,2,3,1]
write(max(l))
end
procedure max(l)
local i, max
max := l[1]
every i := 1 to !l & i > max do
max := i
return max
end
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. <lang 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;
}</lang>
Optionally, if it is OK to rearrange the contents of the original array: <lang java>public static float max(float[] values) throws NoSuchElementException {
if (values.length == 0)
throw new NoSuchElementException();
Arrays.sort(values);//sorts the values in ascending order
return values[values.length-1];
}</lang>
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. <lang 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);
}</lang>
JavaScript
<lang javascript> Array.prototype.max = function() { var max = this[0]; var len = this.length; for (var i = 1; i < len; i++) if (this[i] > max) max = this[i]; return max; }
// Test it a = [0,1,2,5,4]; alert(a.max()); </lang>
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
Modula-3
Modula-3 provides a builtin MAX function, but it only works on two elements (or enumerations) but not arrays or sets.
We provide a generic Maximum implementation: <lang modula3>GENERIC INTERFACE Maximum(Elem);
EXCEPTION Empty;
PROCEDURE Max(READONLY a: ARRAY OF Elem.T): Elem.T RAISES {Empty};
END Maximum.</lang>
<lang modula3>GENERIC MODULE Maximum(Elem);
PROCEDURE Max(READONLY arr: ARRAY OF Elem.T): Elem.T RAISES {Empty} =
VAR max := FIRST(Elem.T);
BEGIN
IF NUMBER(arr) = 0 THEN
RAISE Empty;
END;
FOR i := FIRST(arr) TO LAST(arr) DO
IF arr[i] > max THEN
max := arr[i];
END;
END;
RETURN max;
END Max;
BEGIN END Maximum.</lang>
Elem can be instantiated to any type (any type that can be compared with the '>' function). For convenience Modula-3 provides interfaces/modules for the built in types, like Integer, Real, LongReal, etc, which contain type definitions as well as properties specific to the type.
To make a generic interface/module for a specific type, you must instantiate it: <lang modula3>INTERFACE RealMax = Maximum(Real) END RealMax.</lang> <lang modula3>MODULE RealMax = Maximum(Real) END RealMax.</lang>
Now we can import RealMax into our source and use the Max function:
<lang modula3>MODULE Main;
IMPORT RealMax, IO, Fmt;
VAR realarr := ARRAY [1..5] OF REAL {1.1, 1.0, 0.0, 2.4, 3.3};
BEGIN
IO.Put(Fmt.Real(RealMax.Max(realarr)) & "\n");
END Main.</lang>
MAXScript
MAXScript has a built-in function called amax(), which will return the maximum of an array or the values supplied to it. The following custom function will return the maximum of the array supplied to it, or 'undefined' if an empty array is supplied. <lang MAXScript> fn MaxValue AnArray = ( if AnArray.count != 0 then ( local maxVal = 0 for i in AnArray do if i > maxVal then maxVal = i maxVal ) else undefined ) </lang>
Nial
The behavior of multi-dimensional arrays is like J
max 1 2 3 4 =4
Objective-C
This code "extends" (through Objective-C categories) the NSArray adding the method
maximumValue; this one iterates over the objects of the collection
calling the method compare, if it exists for the object of the
collection. Since normally comparing makes sense between numbers, the code
also check if the objects being compared are of "kind" NSNumber. If one eliminates
this check (substituting it maybe with one that checks if the two object are of
the same "kind"), the code is able to get a maximum value for any objects for
which make sense a compare method (e.g. strings), that must be implemented.
If there's no a known way of comparing two objects of the collection (or if the objects are not "NSNumber"), the the method return nil (the void object).
<lang objc>#import <Foundation/Foundation.h>
@interface NSArray (WithMaximum) - (id)maximumValue; @end
@implementation NSArray (WithMaximum) - (id)maximumValue {
if ( [self count] == 0 ) return nil;
id maybeMax = [self objectAtIndex: 0];
NSEnumerator *en = [self objectEnumerator];
id el;
while ( (el=[en nextObject]) != nil ) {
if ( [maybeMax respondsToSelector: @selector(compare:)] &&
[el respondsToSelector: @selector(compare:)] && [el isKindOfClass: [NSNumber class]] && [maybeMax isKindOfClass: [NSNumber class]] ) {
if ( [maybeMax compare: el] == NSOrderedAscending )
maybeMax = el;
} else { return nil; }
}
[maybeMax retain]; // let us be sure we now "own" the object
return maybeMax;
} @end</lang>
This example mixes integers with a double value, just to show that everything is fine until they are NSNumber.
<lang objc> int main() {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; NSArray *collection = [NSArray arrayWithObjects:
[NSNumber numberWithInt: 1], [NSNumber numberWithInt: 2], [NSNumber numberWithInt: 10], [NSNumber numberWithInt: 5], [NSNumber numberWithDouble: 10.5], nil];
NSLog(@"%@", [[collection maximumValue] stringValue]); [pool release]; return 0;
}</lang>
OCaml
<lang ocaml>let my_max = function
[] -> invalid_arg "empty list" | x::xs -> List.fold_left max x xs</lang>
# my_max [4;3;5;9;2;3] ;; - : int = 9
Perl
<lang perl>sub max {
my @list = @_;
my $themax = $list[0];
foreach ( @list ) {
$themax = $_ > $themax ? $_ : $themax;
}
return $themax;
}</lang>
It is already implemented in the module List::Util's max() function: <lang perl>use List::Util qw(max);
max(@values);</lang>
PHP
The built-in PHP function max() already does this. <lang php>max($values)</lang>
Python
The built-in Python function max() already does this. <lang python>max(values)</lang>
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 then in Python, such streams are usually iterable, meaning they have a way of generating one item at a time from the stream.
max(), (and min()), can take iterables and a key argument which takes a function that can transform each item into a type that we can compare, for example, if the stream were returning string representations of integers, one to a line, you could do <lang python>>>> floatstrings = ['1\n', ' 2.3\n', '4.5e-1\n', '0.01e4\n', '-1.2'] >>> max(floatstrings, key = float) '0.01e4\n' >>> </lang> Normally we would want the converted form as the maximum and we could just as easily write: <lang python>>>> max(float(x) for x in floatstrings) 100.0 >>> </lang>
Ruby
max is a method of all Enumerables <lang ruby>values.max</lang>
Scheme
The built-in Scheme function max takes the max of all its arguments. <lang scheme>(max 1 2 3 4) (apply max values) ; find max of a list</lang>
Standard ML
Comparisons are specific for each type. Here is a max function for a list of ints: <lang ocaml>fun max_of_ints [] = raise Empty
| max_of_ints (x::xs) = foldl Int.max x xs</lang>
- max_of_ints [4,3,5,9,2,3]; val it = 9 : int
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