Thread: Validating an Integer

  1. #1
    Registered User
    Join Date
    Mar 2022

    Question Validating an Integer

    Dear all,

    I am a total beginner to C - I've got some code that I'm trying to write that ensures you're entering an integer between 1 and 1000 inclusive. I want to verify the integer is the correct length (4), and that it is an integer.

    void enterN(int *inputInteger)
      char str[5] = { };
      int isValid = 0;
      while (isValid == 0) {
        scanf("%5s", str);
        if (strlen(str) > 4) {
          printf("Please enter a number within 1-1000: ");
          char c = 0;
          while ((c = getchar()) != '\n' && c != EOF);
        char *endptr = NULL;
        *inputInteger = strtol(str, &endptr, 10);
        errno = 0;
        if (str == endptr) {
          printf("Not an integer.");
        if (errno != 0) {
          printf("Not an integer, errored.\n");
        if ((strtol(str, &endptr, 10) >= 1) && (strtol(str, &endptr, 10) <= 1000)) {
          isValid = 1;
        else {
          printf("Something wrong with input");
          printf("%s", str);
      printf("%s", str);
    Every time I test my program by typing values such as 11111, it throws up the else condition saying something is wrong, immediately after saying values are too long. I don't want two errors - I have a feeling I'm not flushing a buffer properly but I have no idea how to do this and what to change.

    Values like 111, and 1000 work.

    Could someone please help me edit my code to fix this? Thank you!
    Last edited by Salem; 03-01-2022 at 11:11 PM. Reason: Removed crayola

  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    > char str[5]
    You need to make this larger, if you want to allow invalid input and have a chance to validate it.

    > scanf("%5s", str);
    Or more specifically, the number here needs to be one less than the size of your array.
    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
    Registered User
    Join Date
    Apr 2021
    Quote Originally Posted by isaidilytd View Post
    I don't want two errors
    You have a bunch of tests like:
        if (something)
        if (otherthing)
    When more than one of the something/otherthing conditions are true, you will get more than one message.

    To avoid this, use if/else if/else if/.../else for your tests. That way the branches will be exclusive of each other and only one of them will print a message.
        if (something)
        else if (otherthing)
            print("all ok");

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