Testing the input if it is numeric or not - c

Question

I wrote a C program wherein I am accepting a numeric input from the user. However, if the user inputs a character, then I have to show that it is an invalid input and make the user enter a number again. How do I achieve this? I am writing the program using gcc compiler in Ubuntu. The search results on Google suggest to use isalpha function...however, it is not available in the libraries I guess.
I also tried the below...
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
void main()
{
system("clear");
if (isdigit(1))
printf("this is an alphabet\n");
else
printf("this is not an alphabet\n");
}

Answer 1

You will need to use scanf to get user input with %d as you want to scan an integer. In your case, scanf will return 1 on sucessfull scanning.
int num;
//loop to prompt use to enter valid input
while(scanf("%d",&num)==0) //scanning an integer failed
{
printf("invalid input ! Try again\n");
scanf("%*s"); //remove the invalid data from stdin
}
The functions isalpha() and isdigit() works when you are getting a character input using %c in the scanf. If you want to scan input using %c , then you can simply check like what you have done in your code provided that you get input using %c. Note that character 1 ('1') is note equal to integer 1 . Characters have their integer values as represented by the ASCII table. Your program to prompt the user again when the user enters anything other that a number using %c will look like this:
char ch;
while(1){
printf("Enter a number\n");
scanf(" %c",&ch);
printf(Your input is %c\n",ch);
if(isdigit(ch))
{
printf("This is a number\n");
break;
}
else
printf("This is not a number. Invalid input\n");
}

Answer 2

I tried the below code which worked fine..using isdigit()
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<ctype.h>
void main()
{
system("clear");
char str[1];
printf("Enter a number\n");
scanf("%s",str);
printf("What you entered was %s\n",str);
if(isdigit(str[0]))
printf("this is not an alphabet\n");
else
printf("this is an alphabet\n");
}

Answer 3

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include<ctype.h>
int main()
{
char c;
printf("Enter a character: ");
scanf("%c",&c);
bool check=true;
while(check)
{
if( (c>='a'&& c<='z') || (c>='A' && c<='Z'))
{
printf("%c is an alphabet.",c);
check=true;
break;
}
else
{
printf("%c is not an alphabet.",c);
check=false;
}
}
return 0;
}

Answer 4

You can write your own. It's better to check digits since there're less cases for digits.
bool isDigit(char c) {
bool rtn = false;
switch(c) {
case '0':
case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
rtn = true;
break;
default:
rtn = false;
break;
}
return rtn;
}