Do Not Nest FOR/NEXT Loops with Same Counter Index Variable (36809)

SUMMARY

Nesting FOR...NEXT loops that use the same counter index variable is not legal in compiled Basic.

BC.EXE properly gives you a "FOR index variable already in use" error message when you nest FOR...NEXT loops with the same counter index variable. However, the QB.EXE environment will mistakenly allow a program to run.

This information applies to Microsoft QuickBasic Versions 4.00, 4.00b and 4.50, to Microsoft Basic Compiler Versions 6.00 and 6.00b for MS-DOS and MS OS/2, and to Microsoft Basic PDS Version 7.00 for MS-DOS and MS OS/2.

MORE INFORMATION

The program below will run without any error message inside the QB.EXE editing environment (and also in the GW-Basic Interpreter Versions 3.20, 3.22, and 3.23). The result is an infinite loop, because T% is always equal to "6" upon exiting the inner loop and never makes it to 20 to terminate the outer loop.

BC.EXE correctly produces the error message "FOR Index variable already in use" when it attempts to compile this program. This is not a warning error, but a severe error. When you choose the Make EXE File command in the QB.EXE environment, the link step is skipped and no .EXE file is produced.

Microsoft QuickBasic Versions 3.00 and earlier correctly do not allow nested loops with the same index variable, either inside the environment or when compiled using the separate compilation method.

The following code example shows the illegal counter-index (T%) usage:

   10 FOR T% = 1 TO 20
   20   PRINT "t% =";T%
   30   FOR T% = 1 TO 5
   40     PRINT "Inside loop, t% =";T%
   50   NEXT T%
   60 NEXT T%