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