Category talk:ALGOL 68-rows: Difference between revisions
→Source code: Added AVERAGE, MEDIAN and QUICKSELECT
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(→Source code: Added AVERAGE, MEDIAN and QUICKSELECT) |
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===Source code===
<syntaxhighlight lang=algol68>
# rows.incl.a68: array related utilities for Algol 68 RC tasks #
# prints the elements of an array of integers separated by spaces #
Line 22 ⟶ 23:
# if we were to support multiple sort methods, could retuen the array #
# plus a code to specify sorting using quicksort #
OP QUICKSORT = ( REF[]INT a )REF[]INT: a;
OP QUICKSORT = ( REF[]REAL a )REF[]REAL: a;
OP QUICKSORT = ( REF[]STRING a )REF[]STRING: a;
# constructs a SORTBOUNDS from its parameters #
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a
FI # FROMELEMENT # ;
# in-place quick sort an array of strings #
OP FROMELEMENT = ( REF[]STRING a, SORTBOUNDS bounds )REF[]STRING:
IF INT lb = lb OF bounds;
INT ub = ub OF bounds;
ub <= lb
THEN
# empty array or only 1 element #
a
ELSE
# more than one element, so must sort #
INT left := lb;
INT right := ub;
# choosing the middle element of the array as the pivot #
STRING pivot := a[ left + ( ( right + 1 ) - left ) OVER 2 ];
WHILE
WHILE IF left <= ub THEN a[ left ] < pivot ELSE FALSE FI
DO
left +:= 1
OD;
WHILE IF right >= lb THEN a[ right ] > pivot ELSE FALSE FI
DO
right -:= 1
OD;
left <= right
DO
STRING t := a[ left ];
a[ left ] := a[ right ];
a[ right ] := t;
left +:= 1;
right -:= 1
OD;
QUICKSORT a FROMELEMENT lb TOELEMENT right;
QUICKSORT a FROMELEMENT left TOELEMENT ub;
a
FI # FROMELEMENT # ;
# returns the kth lowest element of list usng the quick select algorithm #
PRIO QUICKSELECT = 9;
OP QUICKSELECT = ( REF[]INT list, INT k )INT:
IF LWB list > UPB list THEN
# empty list #
0
ELSE
# non-empty list #
# partitions the subset of list from left to right #
PROC partition = ( REF[]INT list, INT left, right, pivot index )INT:
BEGIN
# swaps elements a and b in list #
PROC swap = ( REF[]INT list, INT a, b )VOID:
BEGIN
INT t = list[ a ];
list[ a ] := list[ b ];
list[ b ] := t
END # swap # ;
INT pivot value = list[ pivot index ];
swap( list, pivot index, right );
INT store index := left;
FOR i FROM left TO right - 1 DO
IF list[ i ] < pivot value THEN
swap( list, store index, i );
store index +:= 1
FI
OD;
swap( list, right, store index );
store index
END # partition # ;
INT left := LWB list, right := UPB list, result := 0;
BOOL found := FALSE;
WHILE NOT found DO
IF left = right THEN
result := list[ left ];
found := TRUE
ELSE
INT pivot index = partition( list
, left
, right
, left + ENTIER ( ( random * ( right - left ) + 1 ) )
);
IF k = pivot index THEN
result := list[ k ];
found := TRUE
ELIF k < pivot index THEN
right := pivot index - 1
ELSE
left := pivot index + 1
FI
FI
OD;
result
FI # QUICKSELECT # ;
# returns the median element from data #
OP MEDIAN = ( REF[]INT data )INT:
IF INT len = ( UPB data - LWB data ) + 1;
INT mid = ( len OVER 2 ) + LWB data;
ODD len
THEN data QUICKSELECT mid
ELSE ROUND ( ( data QUICKSELECT ( mid - 1 )
+ data QUICKSELECT mid
)
/ 2
)
FI # MEDIAN # ;
# returns the average of the elements of a #
OP AVERAGE = ( []INT a )INT:
IF INT len = ( UPB a - LWB a ) + 1;
len < 1
THEN 0
ELSE INT sum := 0;
FOR i FROM LWB a TO UPB a DO
sum +:= a[ i ]
OD;
ROUND ( sum / len )
FI # AVERAGE # ;
# END rows.incl.a68 #
</syntaxhighlight>
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