SWAP

Syntax

SWAP var1,var2 or

SWAP var1$,var2$

Location SWAP, Math Package

This command exchanges the values of the two variables. The parameters can be either numeric variables (integer and floating point) or strings. Arrays are not allowed and both variables have to be the same type: SWAP a$,b is illegal, even if a$ contained a valid number. Also, constant expressions such as SWAP a%,3 are not allowed, since this would not make any sense. Unfortunately, it is not possible to SWAP two elements of an array, the example shows why this would be practicable. The Math Package variant also allows you to swap whole arrays.

Example

In most kinds of sorting routines, a lot of swapping is necessary and an assembler routine which takes over this work makes the process quicker. Here is a Quicksort algorithm as a general subroutine. field$ is sorted from the left element to the right.

100 DEFine PROCedure QSort (field$,left,right)
110   LOCal i,j,last$
120   i=left: j=right: last$=field$(j)
130   REPeat SortLoop1
140     REPeat SortLoop2:IF field$(i)<last$:i=i+1:ELSE EXIT SortLoop2
150     REPeat SortLoop2:IF field$(j)>last$:j=j-1:ELSE EXIT SortLoop2
160     IF i<=j THEN
170       f1$=field$(i): f2$=field$(j): SWAP f1$,f2$
180       field$(i)=f1$: field$(j)=f2$
190       i=i+1: j=j-1
200     END IF
210     IF i>j THEN EXIT SortLoop1
220   END REPeat SortLoop1
230   IF left<j THEN QSort field$,left,j
240   IF right>i THEN QSort field$,i,right
250 END DEFine QSort

Compilers have a fixed stack size - you might have to raise this because this procedure iterates (ie. calls itself), which is something which eats up the stack very quickly. The SuperBASIC interpreter uses a flexible stack.

CROSS-REFERENCE

LET