Making a small calculator program, need to check if user input is a letter

Question

I've been trying to make a small calculator program to test my own skills, due to me being pretty green when talking about coding. Anyways I'm trying to make a function with a loop that will ask for a number until a number is inputted and reject letters.

This is what my limited understanding/googling produced:

int is_number(int num){
    do {
        printf("\n:");
        scanf("%d",&num);
        if( isalpha(num)){
            printf("\nYou entered a letter, please input a number\n");
            continue;
        } else {
            printf("Number accepted...\n");
            break;
        }
    } while(isalpha(num));
    return num;
}

The only problem is that it crashes if given a letter, but there is progress... no infinitive spam of same message.

The next code is the whole code not just a snippet:

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

//Records the answer for Y/N question and checks if answer is Y or No

char check_yesno_question(char a){

    do{
        printf("\n:");
        scanf(" %c",&a);
        if((a=='Y')||(a=='N')||(a=='y')||(a=='n')){
            break;
        }
        else{
            printf("\nIncorrect answer, please input Y or N (Y = Yes, N = No)\n");
            continue;
        }
    }while((a!='Y')&&(a!='N')&&(a!='y')&&(a!='n'));

    return a;

}
//My sad attempt to test if number given is a letter

int is_number(int num){

    do{
        printf("\n:");
        scanf("%d",&num);
        if( isalpha(num)){
            printf("\nYou entered a letter, please input a number\n");
            continue;
        }
        else{
            printf("Number accepted...\n");
            break;
        }
    }while(isalpha(num));
    return num;
}

int main(){

    char YesNo1;
    int start;
    int StartWait=500;
    //char Choice;
    int number1;
    int number2;
    int number3;
    int number4;

    printf("Welcome to simple calculator 0.01\n");
    printf("At this stage calculator supports only 4 numbers\n");
    printf("Would you like to start simple calculator?\n");
    printf("Y/N?\n");                                            

    //Calls for check_yesno_question

    YesNo1=check_yesno_question(YesNo1);

    //Checks if answer is yes proceed, if answer is no, return 0;

    if((YesNo1=='y')||(YesNo1=='Y')){
        printf("Awnser = Yes\n");
        printf("Starting up the calculator");
        for(start=0;start<3;start++){
            printf(".");
            Sleep(StartWait);
        }
        printf("\n");
    }
    else{
        printf("Awnser = No\n");
        printf("Turning of the calculator");
        for(start=0;start<3;start++){
            printf(".");
            Sleep(StartWait);
        }
        printf("\n");
        return 0;
    }
    //Me wanting to make things look bit more clean

    system("cls");

    printf("---------------CALCULATOR---------------\n");
    //Calls for is_number
    number1=is_number(number1);

    return 0;
}

As you seen it is unfinished. I'm basically done looking for answers myself. When this was posted my time was 4:04am

Answer 1

From the context it seems pretty obvious that you're not interested in whether it is a letter or not. You're only interested in knowing if it is a number or not. If it is not a number, you don't care if it is a letter, special character or something else.

Assuming above, you are taking the wrong approach and you are also using scanf incorrectly. scanf("%d", &num) will always read a number into num, provided that the reading succeeded. What you need to do is check if the reading succeeded.

do{
    printf("\n:");
    if(scanf("%d",&num) != 1) { // If we did not perform a successful read
        printf("\nRead failed. Please input a number\n");
        continue;
    } else {
...