Question

The following code only concatenates the first string and ignores the second.. from what I gather, it has something to do with Null terminated strings. As I am new to C, this is a new concept to me. Could someone help make the code below work? That would really help me a lot in understanding this.

void concatTest();

int main()
{
    concatTest();

    system("PAUSE");
    return 0;
}

void concatTest()
{
    char string1[20], string2[20], string3[40];
    char *ptr1, *ptr2, *ptr3;
    ptr1 = &string1[0];
    ptr2 = &string2[0];
    ptr3 = &string3[0];
    int i;

    printf("You need to enter 2 strings.. each of which is no more than 20 chars in length: \n");

    printf("Enter string #1: \n");
    scanf("%s", string1);

    printf("Enter string #2: \n");
    scanf("%s", string2);

    int len1 = strlen(string1);
    int len2 = strlen(string2);

    for (i = 0; i < len1; i++)
    {
        ptr3[i] = ptr1[i];
    }
    for (i = len1; i < len1 + len2; i++)
    {
        ptr3[i] = ptr2[i];
    }

    printf("%s\n", string3);
}

Answer 1

You are indexing ptr2[i] using i which ranges from len1 to len1 + len2. This value will probably be out of bounds of the string2 array (unless the first string you type happens to be empty).

I might write your second loop as follows:

for (i = 0; i < len2; i++) {
    ptr3[len1 + i] = ptr2[i];
}