C: Use a different condition in for statement depending on value of variable

Question

So I want to implement a for statement whose 'length' and condition depend on the value of the number-of-entries variable I give it. The loop is reading a file primarily.

For example, if the command line input for number-of-entries is a positive integer, I want the loop to run for number-of-entries iterations or until the end of the file is reached; if the input for number-of-entries is -1, I want the loop to run until the end of the file.

Is there a way to do this without writing the for loop twice, each one nested in an if statement; is there a more concise way? Asking because the statements inside the for loop are the same; the only difference is that condition in the for argument.

Here's what I know I can do:

if ( number_of_entries > 0 ) {
    for ( i = 0; i < number_of_entries; i++ ){
        // set of for statements
        // check to see if end of file is reached
        // this case stops when i reaches number_of_entries or EOF
    }
}

else if ( number_of_entries < 0 ) {
    for ( i = 0; i > number_of_entries; i++ ){
        // identical set of for statements
        // check to see if end of file is reached
        // this case stops at EOF because i will never reach number_of_entries
    }
}

Just wondering if I can do this while keeping only one set of for statements; because they'd be the same in either case.

Edit: Let me clarify the i++ in the second case: it should still be i++; the second loop only ends when the end of file is reached. i will keep increasing until then. The only acceptable inputs for number_of_entries are -1 or any positive integer (there is a check for this in the program).

Answer 1

how about this using short-circuit so i is only tested when number_of_entries is positive:

for ( i = 0; number_of_entries < 0 || i < number_of_entries; i++ ){
        // set of for statements
        // check to see if end of file is reached
    }

if number_of_entries is negative, the for loop is an infinite loop (and you have to use a break internally when detecting the end of the file)

So that's an extra test each time if number_of_entries is positive, but given the contents of the loop (file read) the performance shouldn't suffer much. Concision vs raw speed.

Answer 2

If you find that number_of_entries is negative, you can reset it to a very large value like INT_MAX so that it keeps going until you hit EOF.

if (number_of_entries < 0) {
    number_of_entries = INT_MAX;
}
for ( i = 0; i < number_of_entries; i++ ){
    // set of for statements
    // check to see if end of file is reached
}

Answer 3

Accepting you have a typo in your second snippet: you mean i--, this would do it:

for (
    i = 0;
    i < abs(number_of_entries);
    i += (number_of_entries > 0) - (number_of_entries < 0)
)

(number_of_entries > 0) - (number_of_entries < 0) is the idiomatic way of testing the sign of a number in C.

Reference: Fast sign of integer in C

Answer 4

According to your edited question, you could state that in case of negative number_of_entries it should run to at least a large number. This expresses your intent probably more clearly, and it avoids UB due to integer overflows:

for (int i = 0; i < (number_of_entries < 0 ? INT_MAX : number_of_entries); i++ ) {
        // set of for statements
        // check to see if end of file is reached
        // this case stops when i reaches number_of_entries or EOF
}

Answer 5

You can always use a ternary operator for the test condition:

for (i = 0; ne > 0 ? i < ne : i > ne; i++)

which encapsulates the test in a single for condition.

Here is a short example:

#include <stdio.h>
#include <stdlib.h>

#define MAXC 256

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

    char buf[MAXC] = "";
    int ne = argc > 1 ? atoi(argv[1]) : 10;
    FILE *fp = argc > 2 ? fopen (argv[2], "r") : stdin;

    if (!fp) {  /* validate file open for reading */
        fprintf (stderr, "error: file open failed '%s'.\n", argv[1]);
        return 1;
    }

    /* if ne > 0, read ne lines, otherwise read entire file */
    for (int i = 0; ne > 0 ? i < ne : i > ne; i++) {
        if (!fgets (buf, sizeof buf, fp))
            break;
        printf ("%2d : %s", i + 1, buf);
    }

    if (fp != stdin) fclose (fp);     /* close file if not stdin */

    return 0;
}

Example Use/Output

$ ./bin/for_dual_cond 8 <~/tmp/foo.txt
 1 : after block/auto unblock
 2 : remember >> next time!
 3 : after manual unblock w/menu - restored.
 4 : signal blocked saving
 5 : comparing file names.
 6 :
 7 : $ ./bin/gtkwrite
 8 : adding app->mfp_handler: 667

with a negative value for ne:

$ ./bin/for_dual_cond -8 <~/tmp/foo.txt
 1 : after block/auto unblock
 2 : remember >> next time!
 3 : after manual unblock w/menu - restored.
 4 : signal blocked saving
 5 : comparing file names.
 6 :
 7 : $ ./bin/gtkwrite
 8 : adding app->mfp_handler: 667
 9 : blocking changed (667)
...
95 : Testing foreign modification: Yes
96 :
97 :
98 : new test after mod