How can I access the certain indexes of string using the function?

Question

I was practicing and found some exercise on the internet and it says that I should copy str2 to str1, perhaps the main problem is that I should copy only a certain index of that string (so from str2[a] to str2[b]). Therefore A and B should be entered so they reduce the range of copied indexes to str1. So here I used the function method to tackle this problem but something went wrong. So I don't get why function does not work, there must be a big mistake in my understanding or I did it all wrong.

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

#define N 50

void myfunction(char str1[], char str2[], int a, int b);

int main() {
    char str1[N], str2[N];
    int a, b;
    printf("please input str1:");
    gets(str1);
    printf("please input str2:");
    gets(str2);
    printf("please input a and b:");
    scanf("%d%d", &a, &b);
    printf("str2=%s, str1=%s", str2, str1);
    myfunction(str1, str2, a, b);
    return 0;
}

void myfunction(char str1[], char str2[], int a, int b) {
    int i, j;
    for (i = a; i <= b; i++) {
        str1 += str2[i];
    }
}

Answer 1

There are multiple problems in your program:

• you should not use gets()
• myfunction just increments str1 instead of copying characters.
• you should output the resulting string.

Here is modified version:

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

#define N  50

void myfunction(char str1[], char str2[], int a, int b);

// safe replacement for gets()
char *mygets(char *buf, size_t n) {
    int c;
    size_t i;
    while ((c = getchar()) != '\n') {
        if (c == EOF) {
            if (i == 0)
                return NULL;
            break;
        }
        if (i + 1 < n)
            buf[i++] = c;
    }
    if (i < n) {
        buf[i] = '\0';
    }
    return buf;
}

int main() {
    char str1[N], str2[N];
    int a, b;

    printf("please input str1: ");
    if (!mygets(str1, sizeof str1))
        return 1;
    printf("please input str2: ");
    if (!mygets(str2, sizeof str2))
        return 1;
    printf("please input a and b: ");
    if (scanf("%d%d", &a, &b) != 2)
        return 1;
    printf("str2=%s, str1=%s\n", str2, str1);
    myfunction(str1, str2, a, b);
    printf("str1=%s\n", str1);
    return 0;
}

void myfunction(char str1[], char str2[], int a, int b) {
    int len1 = strlen(str1);
    int len2 = strlen(str2);
    int i, j;
    if (a < 0)
        a = 0;
    if (b > len2)
        b = len2;
    for (i = 0, j = a; j <= b; i++, j++) {
        str1[i] = str2[j];
    }
    if (i > len1) {
        /* do not truncate str1 but set the null terminator if needed */
        str1[i] = '\0';
    }
}