Unable to concatenate two strings

Question

I was practicing on the topics of string. I am trying to concatenate two strings without using the strcat() function available in C.

This is the code that I came up with:

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

int string_length();
void concatenate();

int main(void) {
    char str[] = "Hello there";
    char str2[] = "How are you";
    char result[100];
    int length;

    length = string_length(str);

    printf("%d\n", length);
    concatenate(result, str, str2);
}

//Function which counts the number of characters of the string
int string_length(char str[]) {
    int i = 0;
    while (str[i] != '\0') {
        i++;
    }
    return i;
}

//Function to concatenate the two strings
void concatenate(char result[], char str[], char str2[]) {
    int i = 0;
    while (str[i] != '\0') {
        result[i] = str[i];
        ++i;
    }
    int j = i + 2, k = 0;
    while (str2[k] != '\0') {
        result[j] = str2[k];
        k++;
        j++;
    }
    printf("%s\n", result);
}

When I ran this code, this is what I get as the output -

    11
    Hello there

The concatenation function is not working properly as output should be-

    11
    Hello there How are you

I used the debugger and found that loops are working fine and as per the counter variable, The two strings should get concatenated.

Answer 1

There are multiple problems in your code:

• in your concatenation function, you initialize j to i + 2, whereas you should just use j = i as the first character of the second string should come just after the last character of the first and i was incremented after each character copied from str. Note that you can just use i, there is no need for j.
• you must set the null terminator at the end of result. You can set at with result[j] = '\0';
• you should output the concatenated string in main instead of concatenate().
• the prototypes of string_length() and concatenate() should include the argument types.

Also note that you should use size_t for the index types.

Here is a modified version:

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

size_t string_length(const char *s);
void concatenate(char *result, const char *str, const char *str2);

int main(void) {
    char str[] = "Hello there";
    char str2[] = "How are you";
    char result[100];
    size_t length;

    length = string_length(str);

    printf("%zu\n", length);
    concatenate(result, str, str2);
    printf("%s\n", result);  // shows Hello thereHow are you
    concatenate(result, "abc", "def");
    printf("%s\n", result);  // shows abcdef
    return 0;
}

//Function which counts the number of characters of the string
size_t string_length(const char *s) {
    size_t i = 0;
    while (str[i] != '\0') {
        i++;
    }
    return i;
}

//Function to concatenate the two strings
void concatenate(char *result, const char *str, const char *str2) {
    size_t i = 0;
    while (str[i] != '\0') {
        result[i] = str[i];
        ++i;
    }
    size_t k = 0;
    while (str2[k] != '\0') {
        result[i] = str2[k];
        k++;
        i++;
    }
    result[i] = '\0';
}

Here is an alternative approach, using pointer arithmetics:

//Function to concatenate the two strings
void concatenate(char *result, const char *str, const char *str2) {
    // copy the bytes from str, including the null terminator
    while ((*result = *str++) != '\0')
        result++;

    // copy the bytes from str2, including the null terminator
    while ((*result = *str2++) != '\0')
        result++;
}