How to check if user input is not an int value in C

Question

I need to check if a user input value is not an int value. I've tried different combinations of what I know but I either get nothing or random errors

If the user inputs any char it'll raise a warning message

this is what ive written

#include <stdio.h> 

//C program to perform addition, subtraction, multiplication, division + - * / 

int main()   
{    
    int num1,num2;
    char alpha[30]

    printf("enter numbers:\n\n");

    printf("number 1: ");
    scanf("%d",&num1);

    printf("number 2: ");
    scanf("%d",&num2);

    // write a funcntion that when a char is entered to display an error 

    if (num1//and num2 == alpha)
        printf("error");


        else {

    printf("Rezultat: \n");

    printf("sborut im e: %d\n",num1+num2);
    printf("ralikata im e: %d\n",num1-num2);
    printf("proizvedenieto im e: %d\n",num1*num2);
    printf("ralikata im e: %d\n",num1/num2);

            }
    return 0;
}

Select the code. Click the code button {} to format the code correctly. — user3386109
– user3386109, Commented Jan 7, 2017 at 21:46
scanf returns the number of successful conversions. You request one conversion, so the return value should be 1. — user3386109
– user3386109, Commented Jan 7, 2017 at 21:47

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

Scanf has a return value for a reason.

1-3) Number of receiving arguments successfully assigned (which may be zero in case a matching failure occurred before the first receiving argument was assigned), or EOF if input failure occurs before the first receiving argument was assigned.

4-6) Same as (1-3), except that EOF is also returned if there is a runtime constraint violation.

Here is an example program using that information:

#include <stdio.h> 
int main(int argc, char **argv) {
    int inputInteger;
    printf("Please provide some input.\n");
    if(scanf("%d", &inputInteger) == 1) {
        printf("You inputted an integer\n");
    } else {
        printf("You did not enter an integer\n");
    }
    return 0;
}

Output:

./a.out
1[Enter]
You inputted an integer

./a.out
hello[Enter]
You did not enter an integer.

Note: I feel obliged to inform you that scanf() is not the best way to get input. See this answer for more details.

EDIT: I changed if(scanf("%d", &inputInteger)) to if(scanf("%d", &inputInteger) == 1) so that EOF will not output that an integer was found (pointed out by chux in the comments).

edited Jun 20, 2020 at 9:12

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answered Jan 7, 2017 at 22:00

iRove

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5 Comments

Toni03m Over a year ago

okay but how to check for two scanf like my case if (scanf("%d",&num1)( what stays in between them) scanf("%d",&num2){ printf("stuff...") }

iRove Over a year ago

@Toni03m Simply use the && operator inside your if statement. If you don't know how to do that, I would suggest reading a basic book on C.

chux Over a year ago

Note the when stdin is closed (end-of file), scanf("%d", &inputInteger) returns EOF and will then printf("You inputted an integer\n");. Better to use if(scanf("%d", &inputInteger) == 1).

iRove Over a year ago

@chux Edited. Thank you, I didn't catch that.

Flaming_Dorito Over a year ago

I inputted 4.5 and it still worked. It just set the value to 4.

user1213320 · Accepted Answer · 2017-01-07 22:12:24Z

0

a try/catch approach works, with casting to int the test is caught by the compiler

std::string input;
std::getline(std::cin,input);
int input_value;
try {
  input_value=boost::lexical_cast<int>(input));
} catch(boost::bad_lexical_cast &) {
  // process bad input here
}

answered Jan 7, 2017 at 22:12

user1213320

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1 Comment

Toni03m Over a year ago

we are talking for C language

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