Change multiple permanent for() statements into only required for() statements required from user input

I am trying to make a c program that cycles through base64 symbols. And I did technically manage to achieve that. However, for each symbol, there is a specific for() statement. And I want it so that I can give it (not yet, only until I can get the code to work) user input (through arguments, but that's for later).

Here's my code:

#include <stdio.h>
#include <string.h>

#define BASE64_CHARS "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"

int main() {
  int i, j, k, l, m, a;
    char base64[100];
    int base64_len = strlen(BASE64_CHARS);
    for (i = 0; i < base64_len; i++) {
        base64[0] = BASE64_CHARS[i];
        for (j = 0; j < base64_len; j++) {
            base64[1] = BASE64_CHARS[j];
            for (k = 0; k < base64_len; k++) {
                base64[2] = BASE64_CHARS[k];
                for (l = 0; l < base64_len; l++) {
                    base64[3] = BASE64_CHARS[l];
                    for (m = 0; m < base64_len; m++) {
                        base64[4] = BASE64_CHARS[m];
			            for (a = 0; a < base64_len; a++) {
			                base64[5] = BASE64_CHARS[a];
			                printf("\r%s", base64);
			            }
                    }
                }
            }
        }
    }
    return 0;
}

Any ideas on how to do this would be appreciated.

I suggest you use an array to emulate an arbitrary number of base 64 digits counter. Then you just need a function incr() to add 1 to the counter with carry, print() function to convert your counter to a string, and for generality a done() function to detect when at the maximum value.

Below I used a 2 digit counter { [1] = 0 } which is just fancy way of saying index 1 should be 0 and the ones not specified are zero-initialized. To use a 6 digit counter you would use { [5] = 0 }.

strlen(BASE64_CHARS) == sizeof BASE64_CHARS - 1 but the latter is computed at compile-time.

#include <assert.h>
#include <stdio.h>
#include <string.h>

#define BASE64_CHARS "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
#define BASE64_LEN sizeof BASE64_CHARS - 1

// The most significant digit is at index 0, and the the least
// significant digit at index n-1
void incr(size_t n, unsigned char counter[n]) {
    assert(n);
	for(size_t i = n - 1;; i--) {
		if(counter[i] < BASE64_LEN - 1) {
			counter[i]++;
			break;
		}
		counter[i] = 0;
        // carry over ends at the most significant digit
		if(!i) 
			break;
	}
}

int done(size_t n, unsigned char counter[n]) {
	for(int i = 0; i < n; i++)
		if(counter[i] < BASE64_LEN - 1)
			return 0;
	return 1;
}

void print(size_t n, unsigned char counter[n]) {
	for(size_t i = 0; i < n; i++)
		printf("%c", BASE64_CHARS
0
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]);
	printf("\n");
}

int main(void) {
	unsigned char counter[] = { [1] = 0 };
	size_t counter_len = sizeof counter / sizeof *counter;
	for(;;) {
		print(counter_len, counter);
		if(done(counter_len, counter))
			break;
		incr(counter_len, counter);
	}
}

and here the first and last few lines:

AA
AB
...
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//