Having Difficulty with isdigit() in C - c

Question

I have been trying to add some experience in C to my experience with Python and started with a basic addition program. One thing that I'm trying to do is check if the input is a number or a character as seen here:
#include <stdio.h>
#include <ctype.h>
int main()
{
int n, sum=0,c,value;
printf("Enter the Number of Integers You Want to Add\n");
scanf("%d", &n);
if(isdigit(n))
{
printf("Enter %d Integers\n", n);
for(c=1; c<=n; c++)
{
scanf("%d", &value);
if(isalpha(value))
{
printf("ENTER INTEGER NOT CHARACTER\n");
break;
}
else
{
sum = sum + value;
}
}
printf("Sum of Entered Integers = %d\n",sum);
}
else
{
printf("ENTER INTEGER NOT CHARACTER\n");
break;
}
return 0;
}
Initially I had tried this using isalpha(), and the program worked fine when adding numbers but interpreted characters as zeros instead of printing the "not an integer" statement. However, now that I reworked it to use isdigit(), it does not recognize ANY input as an integer, whether or not it is. Is there something that I'm just doing wrong?

Answer 1

When you use scanf to read an integer, you get just that, an integer. (To read a single character, you need %c and a pointer-to-char).
When you use isdigit(), you need to supply the representation of that character (e.g. in ASCII, the character '0' has the representation 48, which is indeed its value as an integer). To recap:
isdigit(0) is false
isdigit(48) is true (for ASCII, ISO8859, UTF-8)
isdigit('0') is true (no matter the character set)
isdigit('0' + n) is true for integers n = 0 ... 9
PS: Not testing the return value from scanf is asking for trouble...

Answer 2

Neither isdigit nor isalpha work as you think they do. The intent of those library functions is to check whether a given code point, represented as an int, is within a subset of points defined by the standard to be digit characters or alpha characters.
You should be checking the results of your scanf calls rather than assuming they just work, and acting on those results accordingly. If you request an integer and one is successfully scanned, then it will tell you so. If that fails, your course of action is probably to consume the rest of the line (through newline or EOF) and possibly try again:
#include <stdio.h>
int main()
{
int n,value,sum=0;
printf("Enter the Number of Integers You Want to Add\n");
if (scanf("%d", &n) == 1 && n > 0)
{
printf("Enter %d Integers\n", n);
while (n--)
{
if (scanf("%d", &value) == 1)
{
sum = sum + value;
}
else
{
// consume the rest of the line. if not EOF, we
// loop around and try again, otherwise break.
while ((value = fgetc(stdin)) != EOF && value != '\n');
if (value == EOF)
break;
++n;
}
}
printf("Sum of Entered Integers = %d\n", sum);
}
return 0;
}
Properly done you should be able to enter valid integers beyond single digits (i.e. values > 10 or < 0), which the above allows.

Answer 3

The %d marker to scanf tells it to interpret the input as a number (more accurately, it indicates that the pointer in the arguments points to an integer type). It can't do anything but put an integer into that argument. If it can't interpret the input as a number, scanf stops scanning the input and returns immediately.
isdigit() evaluates its argument as a character code, as Jens points out above. However, scanf already turned the character code into a pure number.

Answer 4

From the scanf man page:
On success, the function returns the number of items of the argument list
successfully filled.
In your program, you are trying to read just one item from stdin, so scanf should return 1. So check for that and you'll know that it all worked out ok:
printf("Enter the Number of Integers You Want to Add\n");
while(scanf("%d", &n) != 1) {
printf("That's not a valid integer. Please try again.\n");
}
You cannot use isdigit() the way you are using it because you're already using scanf to convert the user input to an integer. If the user had not input an integer, scanf would have already failed.
Look at the man pages for all the C functions you are using, they will show you what the function expects and what the return values will be under different circumstances.
In the case of isdigit(), the input is expected to be an unsigned char representing an ASCII character. This can be a bit confusing because ASCII characters are in fact represented as a type of integer, and a string is an array of those. Unlike languages like Python which hide all that from you. But there is a big difference between the STRING of a number (array of characters that contain the characters of the digits of the number) and the INTEGER itself which is in a form the processor actually uses to do math with... (simplified explanation, but you get the idea).