base26.c

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

char* toBase26(int num) {
    char* str;
    int length = 1;

    int tmp = num;

    // Determine length
    while(tmp > 0) {
        tmp--;
        tmp /= 26;
        length++;
    }

    // Allocate string
    str = malloc(length + 1);

    char* temp = str + length;
    *temp = '\0';

    while(num > 0) {
        // 'A' has meaning, even though it represents zero:
        num--;

        // Figure out the character and add it to the string
        temp--;
        *temp = (num % 26) + 'A';

        // Go to next base26 digit
        num /= 26;
    }

    // Return result
    return temp;
}

int toBase10(char* str) {
    int len = strlen(str);
    int num = 0;

    int i;
    int val;

    // Remember to start at end of string
    for(i = 0; i < len; i++) {
        // Shift the digit
        num = num * 26;

        // Add the value
        val = str[i] - 'A';

        // Remember the A has meaning rule!
        val++;

        // Add
        num += val;
    }

    return num;
}

int main() {
    printf("%s\n", toBase26(28));
    printf("%d\n", toBase10("AB"));
}