How to prevent non-numeric input in C? - c

Question

I wrote a small C program which will get an input from the user and check if the input is even or odd.
#include<stdio.h>
int main()
{
int n;
printf("Enter an integer number: ");
scanf("%d",&n);
if(n%2 == 0)
{
printf("\n%d is an EVEN number.\n",n);
}
else
printf("\n%d is an ODD number.\n",n);
return 0;
}
but when I enter an alphabet or a symbol, it shows the output as 0 and says input is EVEN. How can I prevent user from entering alphabets and symbols? What's the easiest way to do that?

Answer 1

You have to check the return value of scanf. From the documentation:
Return Value
Number of receiving arguments successfully assigned, or EOF if read failure occurs before the first receiving argument was assigned.
Applied to your code:
#include <stdio.h>
#include <stdlib.h>
int
main()
{
int n;
printf("Enter an integer number: ");
if (scanf("%d", &n) != 1)
{
printf("This is not a number.\n");
return EXIT_FAILURE;
}
else if (n % 2 == 0)
{
printf("\n%d is an EVEN number.\n", n);
return EXIT_SUCCESS;
}
else
{
printf("\n%d is an ODD number.\n", n);
return EXIT_SUCCESS;
}
}

Answer 2

Simply check the return value of scanf - it'll tell you how many format objects were successfully parsed. In this case, it'll return 1 if it could parse an int, and 0 if it couldn't.

Answer 3

If the input is an integer, then scanf() will return 1 so you can check
if (scanf("%d", &integer) != 1)
invalidInput();
to ask the user again you should know that there could be characters left in the stdin that need to be read so you can flush them with getchar() so a complete function would be
int scanint(const char *const message)
{
int value;
printf("%s > ", message);
while (scanf("%d", &value) != 1)
{
int chr;
printf("\tinvalid input...\n");
do {
chr = getchar();
} while ((chr != EOF) && (chr != '\n'));
printf("%s > ", message);
}
return value;
}
and you can use it like this
int main()
{
int value = scanint("please input an integer");
printf("your input was: %d\n", value);
return 0;
}

Answer 4

I can give you two approaches:
very easy — check for return value of scanf(). 1 indicates success (integer read) and 0 if any non-integer is put in there (or EOF on EOF).
by writing code for it:
#include <ctype.h>
#include <stdlib.h>
int isNumeric (const char * s)
{
if (s == NULL || *s == '\0' || isspace(*s))
return 0;
char * p;
strtod (s, &p);
return *p == '\0';
}
Now in this case your scanf has to take a string from user and then pass that string to the function isNumeric().