C loops-OK to change counter or condition within loop?

Question

Several books show me how to correctly write for, while, and do loops. Lots of online articles compare them to each other.

But I haven't found any place that tells me what not to do. For example, would it screw things up if I change the value of the counter or condition variable within the loop?

I would like an answer that is not machine dependent.

Answer 1

Yes you can change the counter within a loop and it can sometimes be very useful. For example in parsing command line arguments where there is an option flag followed by a value. An example of this is shown below:

Enter the following command:

program -f filename -o option -p another_option

Code:

#include <string.h>

int main(int argc, char** argv)
{

 char *filename, *option, *another_option;

 if(argc > 1){
   for(int i=1; i<argc; i++){
     if(strcmp(argv[i],"-f")==0){
        filename = argv[++i];
     } else if(strcmp(argv[i],"-o")==0){
       option = argv[++i];
     } else if(strcmp(argv[i],"-p")==0){
       another_option = argv[++i];
     } else {
       printf("Option \"%s\" not recognized, skipping\n",argv[i]);
       continue;
     }
   }
 } /* end if argc > 1 */

 return 0;
}

The example program automatically increments the counter to access the correct command line string. There are of course ways to incorporate counters etc., but they would only make the code more cumbersome in this case.

As others have pointed out, this is where many people write bugs and one must be careful when incrementing counters within loops, particularly when the loop is conditional upon the counter value.

Answer 2

It is not invalid to change a loop counter inside a loop in C.

However, it is probably confusing to future readers and that's a good reason not to do it.

Answer 3

It depends on what you mean by "screw things up".

If you know what you are doing, you can change counter. The language has no restrictions on this.