QB64-PE/internal/help/SWAP.txt

The {{KW|SWAP}} statement is used to exchange two variable or array element values.


{{PageSyntax}}
:{{KW|SWAP}} {{Parameter|variable1}}, {{Parameter|variable2}}


{{PageDescription}}

* {{Parameter|variable1}} and {{Parameter|variable2}} are any type variables whose values will be exchanged.
* If either {{Parameter|variable1}} or {{Parameter|variable2}} is an array, then an element in the array must be designated.
* {{KW|SWAP}} can be used with string or number variable values. Both must be of the same type.
* It is often used to sort array elements into greater or lesser values. 
* SWAP can be used in page flipping to change between 2 page numbers.


''Example 1:'' A simple SWAP of [[STRING|string]] values.
{{CodeStart}}
a$ = "one"
b$ = "two"

{{Cl|SWAP}} a$, b$

{{Cl|PRINT}} a$
{{Cl|PRINT}} b$
{{CodeEnd}}
{{OutputStart}}two
one
{{OutputEnd}}


''Example 2:'' Creating Cryptograms by scrambling EVERY capital letter in the alphabet.
{{CodeStart}} '' ''
{{Cl|DIM}} Letter$(65 {{Cl|TO}} 90)
{{Cl|RANDOMIZE}} {{Cl|TIMER}}
{{Cl|CLS}}
{{Cl|FOR...NEXT|FOR}} a = 65 {{Cl|TO}} 90                    'set ASCII codes and letters in order
  Letter$(a) = {{Cl|CHR$}}(a)              'create capitalized characters
{{Cl|NEXT}} a

{{Cl|COLOR}} 11: {{Cl|LOCATE}} 10, 10
{{Cl|FOR...NEXT|FOR}} i = 65 {{Cl|TO}} 90
  {{Cl|IF...THEN|IF}} Letter$(i) = {{Cl|CHR$}}(i) {{Cl|THEN}}      'find characters the same as the {{Cl|ASCII}} code index
    {{Cl|DO...LOOP|DO}}: j = {{Cl|INT}}({{Cl|RND}} * 26) + 65: {{Cl|LOOP}} {{Cl|WHILE}} j = i    'loop until j <> i
    {{Cl|SWAP}} Letter$(i), Letter$(j)     'swap corresponding letter characters
  {{Cl|END IF}}
  {{Cl|PRINT}} {{Cl|CHR$}}(i); " ";               'print normal alphabetical order
{{Cl|NEXT}}

{{Cl|COLOR}} 14: {{Cl|LOCATE}} 12, 10
{{Cl|FOR...NEXT|FOR}} a = 65 {{Cl|TO}} 90                    'display new alphabetical order
  {{Cl|PRINT}} Letter$(a); " ";
{{Cl|NEXT}}

text$ = "This is how a normal sentence would look before being encrypted."
{{Cl|COLOR}} 11: {{Cl|LOCATE}} 20, 5: {{Cl|PRINT}} text$
L = {{Cl|LEN}}(text$)
{{Cl|DIM}} Code(L)                         'place ASCII code solution into an array
{{Cl|COLOR}} 14: {{Cl|LOCATE}} 22, 5
{{Cl|FOR...NEXT|FOR}} i = 1 {{Cl|TO}} L
  Code(i) = {{Cl|ASC}}({{Cl|UCASE$}}(text$), i)   'in QB64, ASC can read by character position
  {{Cl|IF...THEN|IF}} Code(i) >= 65 {{Cl|AND (boolean)|AND}} Code(i) <= 90 {{Cl|THEN}} {{Cl|PRINT}} Letter$(Code(i)); {{Cl|ELSE}} {{Cl|PRINT}} {{Cl|CHR$}}(Code(i));
{{Cl|NEXT}}
{{Cl|END}} '' ''
{{CodeEnd}}
{{small|Code by Ted Weissgerber}}
:'' Explanation:'' The Letter$ [[STRING]] [[Arrays|array]] is first created with the letters matching the [[ASCII]] code index value. Every index is '''swap'''ped 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.
{{CodeStart}} '' ''
{{Cl|DEFINT}} A-Z
{{Cl|DIM}} {{Cl|SHARED}} swap2 {{Cl|AS}} {{Cl|LONG}}  'Demo only
{{Cl|DIM}} array(10000) {{Cl|AS}} {{Cl|SINGLE}} 'array can hold any type of value
{{Cl|RANDOMIZE}} {{Cl|TIMER}}
{{Cl|FOR...NEXT|FOR}} i = 0 {{Cl|TO}} 10000
  array(i) = {{Cl|RND}} * 1000 'populate array with random values to sort
{{Cl|NEXT}}
start = {{Cl|LBOUND}}(array)  'lowest element
finish = {{Cl|UBOUND}}(array) 'highest element
swap2 = 0                     'count swaps for demo only
start! = {{Cl|TIMER}}(.001)
{{Cl|CALL}} QuickSort(start, finish, array())
ending! = {{Cl|TIMER}}(.001)
tmp$ = " array(0)= ##.#####     array(5000)= ###.####   array(10000)= ###.####"
{{Cl|PRINT USING}} tmp$; array(0); array(5000); array(10000) 
{{Cl|PRINT USING}} " Elapsed time: #.###### seconds with #######, swaps"; ending! - start!; swap2&
{{Cl|FOR...NEXT|FOR}} n = 0 {{Cl|TO}} 10000             'check array sort order
  {{Cl|IF}} array(n) >= max! {{Cl|THEN}}     'max should match the array type
    max! = array(n)
  {{Cl|ELSE}} {{Cl|BEEP}}
    {{Cl|PRINT}} "Bad sort order!"
    {{Cl|EXIT}} {{Cl|FOR...NEXT|FOR}}
  {{Cl|END IF}}
{{Cl|NEXT}}
{{Cl|END}}

{{Cl|SUB}} QuickSort (start {{Cl|AS}} {{Cl|INTEGER}}, finish {{Cl|AS}} {{Cl|INTEGER}}, array() {{Cl|AS}} {{Cl|SINGLE}})    
{{Cl|DIM}} Hi {{Cl|AS}} {{Cl|INTEGER}}, Lo {{Cl|AS}} {{Cl|INTEGER}}, Middle {{Cl|AS}} {{Cl|SINGLE}}
Hi = finish: Lo = start
Middle = array((Lo + Hi) / 2) 'find middle of array
{{Cl|DO}}
  {{Cl|DO}} {{Cl|WHILE}} array(Lo) < Middle: Lo = Lo + 1: {{Cl|LOOP}}
  {{Cl|DO}} {{Cl|WHILE}} array(Hi) > Middle: Hi = Hi - 1: {{Cl|LOOP}}
  {{Cl|IF}} Lo <= Hi {{Cl|THEN}}
    {{Cl|SWAP}} array(Lo), array(Hi)
    swap2 = swap2 + 1                  'count swaps for demo only    
    Lo = Lo + 1: Hi = Hi - 1
  {{Cl|END IF}}                               'If homework, you will fail
{{Cl|LOOP}} {{Cl|UNTIL}} Lo > Hi
{{Cl|IF}} Hi > start {{Cl|THEN}} {{Cl|CALL}} QuickSort(start, Hi, array())
{{Cl|IF}} Lo < finish {{Cl|THEN}} {{Cl|CALL}} QuickSort(Lo, finish, array())
{{Cl|END SUB}} '' ''
{{CodeEnd}}
{{OutputStart}} array(0)= 0.20200    array(5000)= 525.8505   array(10000)= 999.6196
 Elapsed time: 0.023438 seconds with 33,759 swaps
{{OutputEnd}}
:'''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]] 
* [[ASCII]], [[Arrays]]


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