abfibo

*
  abfact.c
  S. Edward Dolan
  Wednesday, November 20 2019
*/

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

char* fibo(int n) {
    char* out;
    if (n == 1) {
        /*
          `out' has to be malloc'd because the caller of fibo() must call
          free(), marked with ### below. The caller may be main() or the final
          `else' below.
        */
        out = malloc(sizeof(char) * 2); /* two bytes that `out' points to */
        strcpy(out, "b");               /* out => ['a', 0] */
    }
    else if (n == 2) {
        out = malloc(sizeof(char) * 2);
        strcpy(out, "a");
    }
    else {
        /*
          This loosely does what Python does for you:
          return fibo(n-1) + fibo(n-2)
        */
        char* s1 = fibo(n - 1), *s2 = fibo(n - 2); /* caller */
        out = malloc(sizeof(char) * (strlen(s1) + strlen(s2) + 1));
        out = strcat(strcpy(out, s1), s2);
        /*
          You no longer need s1 and s2 because their characters were copied to
          out, which the caller does need.
        */
        free(s1);               /* ### */
        free(s2);               /* ### */
    }
    return out;
}

int main(int argc, char** argv) {
    int i = 1, n = 5;
    if (argc > 1)
        n = atoi(argv[1]);
    while (i < n) {
        char* v = fibo(i++);    /* caller */
        printf("%ld\n", strlen(v));
        /* puts(v); */
        free(v);                /* ### */
    }
    return 0;
}