SWAP

The SWAP statement is used to exchange two variable or array element values.

Syntax

SWAP variable1, variable2

Description

• variable1 and variable2 are any type variables whose values will be exchanged.
• If either variable1 or variable2 is an array, then an element in the array must be designated.
• SWAP can be used with string or number variable values. Both must be of the same type.
• SWAP is often used to sort array elements into greater or lesser numerical or cumulative ASCII STRING values.
• SWAP can be used in page flipping to change between source and destination pages.

Example 1: A simple SWAP of string values.

a$ = "one" b$ = "two" SWAP a$, b$ PRINT a$ PRINT b$

two one

Example 2: Creating Cryptograms by scrambling EVERY capital letter in the alphabet.

DIM Letter$(65 TO 90) RANDOMIZE TIMER CLS FOR a = 65 TO 90 'set ASCII codes and letters in order Letter$(a) = CHR$(a) 'create capitalized characters NEXT a COLOR 11: LOCATE 10, 10 FOR i = 65 TO 90 IF Letter$(i) = CHR$(i) THEN 'find characters the same as the ASCII code index DO: j = INT(RND * 26) + 65: LOOP WHILE j = i 'loop until j <> i SWAP Letter$(i), Letter$(j) 'swap corresponding letter characters END IF PRINT CHR$(i); " "; 'print normal alphabetical order NEXT COLOR 14: LOCATE 12, 10 FOR a = 65 TO 90 'display new alphabetical order PRINT Letter$(a); " "; NEXT text$ = "This is how a normal sentence would look before being encrypted." COLOR 11: LOCATE 20, 5: PRINT text$ L = LEN(text$) DIM Code(L) 'place ASCII code solution into an array COLOR 14: LOCATE 22, 5 FOR i = 1 TO L Code(i) = ASC(UCASE$(text$), i) 'in QB64, ASC can read by character position IF Code(i) >= 65 AND Code(i) <= 90 THEN PRINT Letter$(Code(i)); ELSE PRINT CHR$(Code(i)); NEXT END

Code by Ted Weissgerber

Explanation: The Letter$ STRING array is first created with the letters matching the ASCII code index value. Every index is swapped when the letter matches it's index code until every letter is different. The Code array holds the text code solution.

Example 3: A very quick array sorting SUB procedure using recursion sorts 10 thousand numbers in milliseconds.

DEFINT A-Z DIM SHARED swap2 AS LONG 'Demo only DIM array(10000) AS SINGLE 'array can hold any type of value RANDOMIZE TIMER FOR i = 0 TO 10000 array(i) = RND * 1000 'populate array with random values to sort NEXT start = LBOUND(array) 'lowest element finish = UBOUND(array) 'highest element swap2 = 0 'count swaps for demo only start! = TIMER(.001) CALL QuickSort(start, finish, array()) ending! = TIMER(.001) tmp$ = " array(0)= ##.##### array(5000)= ###.#### array(10000)= ###.####" PRINT USING tmp$; array(0); array(5000); array(10000) PRINT USING " Elapsed time: #.###### seconds with #######, swaps"; ending! - start!; swap2& FOR n = 0 TO 10000 'check array sort order IF array(n) >= max! THEN 'max should match the array type max! = array(n) ELSE BEEP PRINT "Bad sort order!" EXIT FOR END IF NEXT END SUB QuickSort (start AS INTEGER, finish AS INTEGER, array() AS SINGLE) DIM Hi AS INTEGER, Lo AS INTEGER, Middle AS SINGLE Hi = finish: Lo = start Middle = array((Lo + Hi) / 2) 'find middle of array DO DO WHILE array(Lo) < Middle: Lo = Lo + 1: LOOP DO WHILE array(Hi) > Middle: Hi = Hi - 1: LOOP IF Lo <= Hi THEN SWAP array(Lo), array(Hi) swap2 = swap2 + 1 'count swaps for demo only Lo = Lo + 1: Hi = Hi - 1 END IF 'If homework, you will fail LOOP UNTIL Lo > Hi IF Hi > start THEN CALL QuickSort(start, Hi, array()) IF Lo < finish THEN CALL QuickSort(Lo, finish, array()) END SUB

array(0)= 0.20200 array(5000)= 525.8505 array(10000)= 999.6196 Elapsed time: 0.023438 seconds with 33,759 swaps

NOTE: The swap2 shared value is used to count the swaps for the demo and can be removed from the SUB procedure for speed.

See also

• RND, RANDOMIZE
• CHR$, ASC (function)
• ASCII, Arrays

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