How can I verify that the input value is only a positive number in C?

Question

I need to check that if the user input a value, the program runs only if the value is a positive integer between [1-8] (Inclusive), but if the input value is a letter or word or not input at all (Enter), the program ask again for an input

#include <stdio.h>


int main (){

int height;

do {
    printf  ("Height: ");
    scanf ("%i", &height);
}

while (height < 1 || height > 8);   

{ .
  .
  .
  .

I was thinking to add another condition "|| (or)" in the while validation, something like while "height be different(!=) from an integer" then do...

But I can't figure out how to check that statement in other StackOverflow questions. I hope you can enlighten me

`scanf()` returns the number of successful conversions, you can exploit that for your need — Ctx, Dec 17 '19 at 15:34
`scanf` is not really very useful for interactive user input — Jabberwocky, Dec 17 '19 at 15:34
If `scanf` failed (there was no number to read), you must `fflush(stdin)` — Paul Ogilvie, Dec 17 '19 at 15:41
@PaulOgilvie Problem is, that this will be an endless loop. flushing stdin is undefined and does nothing on most platforms. — Ctx, Dec 17 '19 at 15:41
@PaulOgilvie I'd expect a logic to rescan while it fails, or the value is too low or the value is too high. Yours has an "and". We seem to be thinking differently. — Yunnosch, Dec 17 '19 at 15:43
My favorite consultant for getting input right: http://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html — Yunnosch, Dec 17 '19 at 15:44
Thanks @Yunnosch i'm starting in code and it will help me a lot — Juan Pablo Yepes, Dec 17 '19 at 16:15

pmg · Answer 1 · 2019-12-17T15:53:13.557

1

use fgets()

char buf[1000];
if (!fgets(buf, sizeof buf, stdin)) {
    exit(EXIT_FAILURE);
}
if (buf[1] != '\n') exit(EXIT_FAILURE);
if (*buf < '1') exit(EXIT_FAILURE);
if (*buf > '8') exit(EXIT_FAILURE);
// use *buf, maybe (*buf - '0')

an input of "<ENTER>", "0<ENTER>", "12<ENTER>", "a<ENTER>", "<SPACE>3<ENTER>", ... will exit with failure

edited Dec 17 '19 at 15:53

answered Dec 17 '19 at 15:42

pmg

106,608
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thanks @pmg, but I don't want to exit the program, instead I want to ask the users again and again until they type a correct value, something like that: "Inster a value betwen 1 - 8" Height: -1 Height: 0 Height: 42 Height: 50 Height: 4 Height: Correct!! – Juan Pablo Yepes Dec 17 '19 at 16:06
@JuanPabloYepes, well, put the code inside an infinite loop and replace the `exit` with `continue`. Add whatever prompt you like and a `break` when the input is correct. – pmg Dec 18 '19 at 07:43

jgalaso · Answer 2 · 2019-12-17T15:54:12.807

0

I've found this answered question: Check if input is integer type in C

This question only will solve the part about being able to know is the input of the scanf is a number or not, which seems on of the main points. You will still have to check if the input is positive or if it's in the range you want.

edited Dec 17 '19 at 15:54

answered Dec 17 '19 at 15:44

jgalaso

22
4

Please flag as duplicate, instead of linking to another answer; or elaborate what the relevant difference is. – Yunnosch Dec 17 '19 at 15:45

How can I verify that the input value is only a positive number in C?

2 Answers2