Infinite loop when assigning a float to an integer

Question

Please i'm having an issue with my program. Whenever i try to input a float it's getting into an infinity loop. I know that the input is stored as an integer. How can prevent the user from entering a float (how to filter the input).

Why is the program getting into an infinite loop when the input is a float.

This is an example:

#include <stdio.h>

main()
{
    int i = 0;
    while(i<10){
        system("cls>null");
        printf("%d^2 = %d\n", i, i*i);

        printf("Index: ");
        scanf("%d", &i);
    }
}

Read the manual for `scanf`. Make sure you handle *every* way in which it can behave. — Kerrek SB, Dec 15 '13 at 11:32
get the input as a string, parse it to int and check if it could be parsed or not. — bolov, Dec 15 '13 at 11:33
Why the program doesn't ask anymore for an entry when a float is assigned? — rullof, Dec 15 '13 at 11:38
When i try to input a number like '4.5' the program never ask for an input and goes into an infinity loop? how scanf works exactly? I'm not familiar with buffer. — rullof, Dec 15 '13 at 11:43
possible duplicate http://stackoverflow.com/questions/4016073/scanf-fails-why — qwr, Dec 15 '13 at 11:46

score 2 · Answer 1 · answered Dec 15 '13 at 11:44

Better use fgets() to read a complete line from stdin, and strtol() to parse it into an number, for example:

char buffer[256];
char *endp;
int i;
while (fgets(buffer, sizeof(buffer), stdin) != NULL) {
    // buffer now contains one line (including the terminating newline)
    i = (int)strtol(buffer, &endp, 10);
    // endp points to the first character after the parsed number:
    if (endp > buffer && (*endp == 0 || isspace(*endp))) {
        printf("%d^2 = %d\n", i, i*i);
    } else {
        printf("invalid input\n");
    }
}

score 2 · Accepted Answer · answered Dec 15 '13 at 11:44

2

When you call scanf to read a number, but the input contains something incompatible with the input format specifier, scanf does not consume such incorrect input, leaving it in the buffer. Your program does not clear the buffer on input mismatch, entering an infinite loop: scanf tries to read an int again, sees that it's not there, and exits without modifying i. Your loop sees that i is less than 10, and calls the scanf again.

To fix this, check that scanf returned one input. Use the input when it is correct, or call scanf again with the %*[^\n]\n specifier, which means "read to the end of the string, and discard the input":

if (scanf("%d", &i) != 1) {
    scanf("%*[^\n]\n");
}

Note the asterisk - it means that the consumed input needs to be discarded, rather than being written into a variable.

answered Dec 15 '13 at 11:44

Sergey Kalinichenko

714,442
84
1,110
1,523

How does it get the input when it read it to the end? is it works like the casting? – rullof Dec 15 '13 at 11:50
The `[^\n]` format means "string consisting of any characters except newline, repeated any number of times". Asterisk tells `scanf` to discard that string. `\n` at the end removes the `\n` from the buffer. So `scanf("%*[^\n]\n");` call removes the current line from the input buffer, without even looking at its content. By the way, inside that `if (scanf("%d", &i) != 1)` is a good place to add a `printf` telling the user that the input is not valid, and that he should please enter another integer value. – Sergey Kalinichenko Dec 15 '13 at 11:55

Kevin Dong · Answer 3 · 2013-12-15T11:47:19.983

0

#include <math.h>
#include <stdio.h>

int main (void)
{
    int i = 0;
    float j = 0;
    while(i<10)
    {
        system("cls");
        printf("%d^2 = %d\n", i, i*i);

        printf("Index: ");
        if (scanf("%f", &j) <= 0 && j-fabs(j) != 0)
        {
             printf ("The input is not an interger");
        }
    }
}

edited Dec 15 '13 at 11:47

answered Dec 15 '13 at 11:34

Kevin Dong

5,001
9
29
62

@dasblinkenligh Well... I think read the input as a string, check whether all are digits, and then use `sprintf()`.will solve the problem. Right? – Kevin Dong Dec 15 '13 at 11:41
@KevinDongNaiJia You should use the return value of `scanf`. i.e. should return 1 – Ed Heal Dec 15 '13 at 11:42
@KevinDongNaiJia Yes, that's one way to fix it. Another one is checking the return value of `scanf`, as pointed by Ed. – Sergey Kalinichenko Dec 15 '13 at 11:47

Infinite loop when assigning a float to an integer

3 Answers3