While loop behave different while conditions are the same?

Question

I've done lots of programming and arduino and would like to learn in C, since this is also an requirement for my current study. At this moment, I am trying to make a program which checks if the "age" variable contains a character. This will give the user an error message which loops until the input is a number. This is what I have at the moment:

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

#define MAXIMUM 10
#define BASE 10


char name[MAXIMUM]; // Input[Range to prevent overflow]
int age = 0; // Input
int nage; // nage = age + 1;
int error1 = 0;

void errorProgram() 
{
    if (isdigit(age))
    {
        error1 = 0;
        printf("Error = 0\n");
    }
    else {
        error1 = 1;
        printf("Error = 1\n");
    }
}

void main(void)
{
    system("cls");
    printf("What is your name: ");
    scanf("%15[^\n]", name);
    system("cls");
    printf("What is your age: ");
    scanf("%d", &age);
    errorProgram();
    while (error1 != 0) {
    printf("Age contains character, please enter again: ");
    scanf("%d", &age);
    }
    nage = age + 1;
    system("cls");
    printf("Hello %s, you'll be %d next year.", name, nage);
    return(0);
}

This is what happens when I run the program when we just ignore the what's your name part

Scenario 1(number)

Output

What is your age:

Input

5

Output

Error = 1
Age contains character, please enter again:

After I hit enter, it uses the while loop to ask it again until I have it right which is a wished behavior. However, it seems to detect age as a character?

Scenario 2(Character)

Output

What is your age:

Input

d

Output

    Age contains character, please enter again: d
Age contains character, please enter again: Age contains character, please enter again: Age contains character, please enter again: Age contains character, please enter again: Age contains character, please enter again: Age contains character, please enter again: Age contains character, please enter again: Age contains character, please enter again: Age contains character,

I'd like to know why if I enter a character, it spams the console with the lines. Both 5 and d throws an error1 = 1, but the while function doesn't seem to behave the same way it does things different when it comes to the character.

Can someone explain this behavior?

Btw, I am using a template from my study which uses the "Void main". Hence it's a bit different.

Answer 1

the following proposed code:

1. cleanly compiles
2. properly checks for errors
3. caveat: does not empty stdin and retry getting age
4. eliminates unneeded #defines and variables and useless code
5. properly uses scanf() to input the name string
6. immediately outputs the final printf() rather than waiting for the program to end (wherein the OS will output the final data from the printf()
7. does not use non-portable statements like: system( "cls" );

and now, the proposed code:

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

#define MAXIMUM 100



char name[MAXIMUM]; // Input[Range to prevent overflow]
int age = 0; // Input


int main(void)
{
    //system("cls");
    printf("\nWhat is your name: ");
    if( scanf("%99[^\n]", name) != 1 )
    {
        fprintf( stderr, "scanf for name failed\n" );
        exit( EXIT_FAILURE );
    }

    //system("cls");
    printf("\nWhat is your age: ");
    if( scanf("%d", &age) != 1 )
    {
        fprintf( stderr, "scanf for age failed\n" );
        exit( EXIT_FAILURE );
    }

    //system("cls");
    printf("\nHello %s, you'll be %d next year.\n", name, age+1);
    return(0);
}