Thread: How do I check input for nonnumeric value?

See my code below:

Code:

/*
 * ======================================================================
 *
 *       Filename:  ex08.c
 *
 *    Description:  This program asks the user to enter a Fahrenheit
 *                temperature and then converts it into Celsius and
 *                Kelvin temperature.
 *
 *        Version:  1.0
 *        Created:  03/20/2013 04:55:38 PM
 *       Revision:  none
 *       Compiler:  gcc
 *
 *         Author:  Vincent 
 *   Organization:  Vincent`s Workshop
 *
 * ======================================================================
 */

#include <stdio.h>

void Temperatures(double temp_F);

int main(void) {
    double temp;
    
    printf("Enter a temperature in Fahrenheit:\n");
    scanf("%lf", &temp);

    while ( temp /* some code here? */ ) {
        Temperatures(temp);
        printf("Enter a temperature in Fahrenheit "
                "(enter a nonnumeric value to exit):\n");
        scanf("%lf", &temp);
    }

    return 0;
}

void Temperatures(double temp_F) {
    double temp_C;
    double temp_K;

    const double C_SCALE = 1.8;
    const double C_ADD = 32.0;
    const double K_ADD = 273.16;

    temp_C = (temp_F - C_ADD) / C_SCALE;
    temp_K = temp_C + K_ADD;

    printf("%.2f F is %.2f C and %.2f K.\n", temp_F, temp_C, temp_K);
}

You can roll your own logic on this, or use isdigit() to test each entered char, to see if it's a digit or not. (include <ctype.h>)

If you want something done at least once, a do while(test condition), is usually easiest and clearest.

Use a while loop when you want the test condition to be true, even before entering the loop one time.

I followed what you suggested, but right now the program loops once no matter what value I input. I need to make it loop again when the input is a numeric value.

Here is the modified program:

Code:

/*
 * ======================================================================
 *
 *       Filename:  ex08.c
 *
 *    Description:  This program asks the user to enter a Fahrenheit
 *                temperature and then converts it into Celsius and
 *                Kelvin temperature.
 *
 *        Version:  1.0
 *        Created:  03/20/2013 04:55:38 PM
 *       Revision:  none
 *       Compiler:  gcc
 *
 *         Author:  Vincent 
 *   Organization:  Vincent`s Workshop
 *
 * ======================================================================
 */

#include <stdio.h>
#include <ctype.h>

void Temperatures(double temp_F);

int main(void) {
    double temp;
    
    do {
        printf("Enter a temperature in Fahrenheit "
                "(enter a nonnumeric value to exit):\n");
        scanf("%lf", &temp);
        Temperatures(temp);
    } while (isdigit(temp));

    return 0;
}

void Temperatures(double temp_F) {
    double temp_C;
    double temp_K;

    const double C_SCALE = 1.8;
    const double C_ADD = 32.0;
    const double K_ADD = 273.16;

    temp_C = (temp_F - C_ADD) / C_SCALE;
    temp_K = temp_C + K_ADD;

    printf("%.2f F is %.2f C and %.2f K.\n", temp_F, temp_C, temp_K);
}

Ok now, I modified again and it worked pretty well. However, if I typed something like "10a" I get something like:

Enter a temperature in Fahrenheit (enter a nonnumeric value to exit):
10a
10.00 F is -12.22 C and 260.94 K.
Enter a temperature in Fahrenheit (enter a nonnumeric value to exit):

The program exits to the command line right after it asks me the 2nd question.

Here is the code:

Code:

#include <stdio.h>
 
void Temperatures(double temp_F);
 
int main(void) {
    double temp;
     
    printf("Enter a temperature in Fahrenheit:\n");
 
    while ( scanf("%lf", &temp) == 1 ) {
        Temperatures(temp);
        printf("Enter a temperature in Fahrenheit "
                "(enter a nonnumeric value to exit):\n");
    }
 
    return 0;
}
 
void Temperatures(double temp_F) {
    double temp_C;
    double temp_K;
 
    const double C_SCALE = 1.8;
    const double C_ADD = 32.0;
    const double K_ADD = 273.16;
 
    temp_C = (temp_F - C_ADD) / C_SCALE;
    temp_K = temp_C + K_ADD;
 
    printf("%.2f F is %.2f C and %.2f K.\n", temp_F, temp_C, temp_K);
}

Is there a way to fix that in a way that the program will exit right away after I enter something like "10a"?

Not really. If you stick to using scanf you need a more specific format string. I've written about it before, and it's up to you to decide if it's worth the trouble.

Ok, I now see why it exits like that. And yeah, it's not worth a trouble fixing this over a homework problem. Thanks much!