C language trouble

My code’s purpose is to take an input of temperature in celsius and convert it to fahrenheit. It is working, but I need to figure out how to reject any non-numeric input. I think I can create an if-loop, but I’m not exactly sure what to put in it in order to differentiate between numeric, and non numeric inputs. I took a screenshot of my code.


Please don’t post screenshots. Copy/paste the code instead. Just make sure to precede it with a separate line of three backticks and follow it with a separate line of three backticks to make easier to read.

See this post to find the backtick on your keyboard. The “preformatted text” tool in the editor (</>) will also add backticks around text.

Note: Backticks are not single quotes.


//initializing the libraries
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <cs50.h>

int main(void)


    //asking the user for the temperature in celsius
    float name;
    printf("C: "); //asking user for temperature in celsius
    scanf("%f", &name);

    //converting the temperature to fahrenheit from celsius
    float temperature = (name * 9)/5 + 32;
    printf("F: %.1f\n", temperature); //displaying temperature in fahrenheit to tenths place

    return 0;

I think I can create an if-loop

There is no such thing as an “if-loop”. But you could use a while loop. What if you had a boolean variable isValidInput that is initially set to false. The while loop continues while that is still false and inside that loop you get the input and if it is valid, you toggle your boolean variable?

I’m skimming your code here but it looks like you just need to say:

if(  isalpha(name) ){
    printf("F: %.1f\n", temperature);
} else {
   printf("Please only use numbers");