Thread: Measuring the length of a string with a for loop

  1. #1
    Registered User
    Join Date
    Jun 2020

    Question Measuring the length of a string with a for loop

    So I have a simple program that measures the length of a string using a for loop.

    #include <stdio.h>
    int main(void) {
        unsigned i;
        char str[16] = "ABBA";
        for (i = 0; str[i] != '\0'; ++i);
        printf("Len: %u", i);
        return 0;
    It works fine, but there's one thing I can't wrap my head around. If I set the initial value of i to 1 inside of the loop the program still outputs 4 in fact it always outputs 4. Even if I set the value of i to something like 2 or 3.

    Here's the thing, shouldn't it output 3 if i starts at str[1]? The loop should start at 'B' and finish at 'A' which is three characters. But that's not the case for some reason. Can anybody explain this to me?

    Thanks in advance.

  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    for (i = 1; str[i] != '\0'; ++i);

    Is no different to this, having gone round the loop once.
    for (i = 0; str[i] != '\0'; ++i);

    Compare with
    int main(void) {
        unsigned i, count;
        char str[16] = "ABBA";
        for (count = 0, i = 0; str[i] != '\0'; ++i, ++count);
        printf("Len: %u", count);
        return 0;
    Now you'll see a difference whether you start at i=0 or i=1
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
    null pointer Structure's Avatar
    Join Date
    May 2019

    Thumbs up

    int stringLength(char *string) {
        int i = 0;
        while (string[i] != '\0') i++;
        return i;
    Last edited by Structure; 04-28-2021 at 12:33 PM.
    "without goto we would be wtf'd"

  4. #4
    Registered User
    Join Date
    Apr 2021
    I want to point out that setting i to values above 0 does actually do something bad in your case: if your string's length is less than the initial value of i, you will be accessing memory you shouldn't be accessing, potentially causing a segfault or worse side effect, including returning the length of a different string!
    Last edited by erikkonstas; 04-28-2021 at 03:52 PM.

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