Get a valid integer from keyboard.

It will check if there are non digits or too many digits.

https://onlinegdb.com/AvF3_-gE4

/*
Desc: Get a valid integer from keyboard.
It will check if there are non digits or too many digits.
A valid integer may include a sign and at most 9 digits.

Auth: Liutong XU
*/

#include<stdio.h>
#include<stdlib.h>
#define MAXCHARS 81
#define TRUE 1
#define FALSE 0

int getanInt();
int isValidInt(char val[MAXCHARS]);

int main()
{
    int number;

    number = getanInt();
    printf("The number you entered is %d\n", number);

    return 0;
}

int getanInt()
{
    char value[MAXCHARS];
    // typical scenario to use do-while
    do
    {
        printf("Enter an integer, at most 9 digits plus one sign: ");
        fgets(value, MAXCHARS, stdin);
    } while (!isValidInt(value));  
    // while NOT valid, re-input. checking with a function

    return (atoi(value));       
    // convert a string into a number, or you can write a piece of code to convert
}

int isValidInt(char val[MAXCHARS])
{
    int start = 0;
    int pos;
    int valid = TRUE;
    int sign = FALSE;


    // empty string is NOT valid
    if (val[0] == '\0' || val[0] == '\n')
        return FALSE;

    // we have a sign
    if (val[0] == '-' || val[0] == '+')
    {
        sign = TRUE;
        start = 1;
    }

    // a sign without any digit, invalid
    if (sign == TRUE && val[1] == '\0' && val[1] == '\n')
        return FALSE;

    // now start processing digits
    pos = start;
    while (valid == TRUE && val[pos] != '\0' && val[pos] != '\n')
    {
        if (val[pos] < '0' || val[pos] > '9')   // non-digit
           valid = FALSE;
        pos++;
    }

    // too many digits CANNOT fit into an int type
    if (pos - sign > 9)
    {
        valid = FALSE;
        printf("Too many digits...");
    }

    // print a message
    if (!valid)
    {
        printf("Invalid integer\n");
    }
    return valid;
}

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