Digit Validation - Endless Loop - c

Question

I have this code to validate user input. Condition: value entered should be a zero or positive number only. Negative values and alphabetic characters are not accepted.
Here is my code for this, that just keeps on looping:
#include <stdio.h>
int main ()
{
int a, b, c, d;
printf ("enter value for a:");
do {
b = 0;
scanf ("%d", &a);
if (!isdigit(a)) {
printf("Number must be numeric!:\n");
b++;
}
else if (a < 0) {
printf ("number must be postive\n");
b++;
} else {
printf("\neverything is goood\n");
}
} while (b != 0);
}

Answer 1

isdigit() is expecting an ASCII encoded character, however scanf with the %d argument is converting an ASCII encoded number (a string) into an actual number.
ie. If you input '1', with the ASCII code of 0x31, scanf("%d",... will convert this to the value 1. The ASCII code of 1 is not numeric.
To fix this, either:
Make a type char and use the scanf format specifier %c, then isdigit() will do what you want it to.
Use strtol to read in more than one character and perform your own error checking with the char **endptr argument.
Also, you should turn up your compiler warnings and include the header file that contains the isdigit() function, ctype.h.

Answer 2

Once a user makes a mistake, b is always > 0. You need to add a line that sets b = 0 after correct information is entered (i.e. after your printf("\neverything is good\n"); statement. Just be certain to add { & } to the else statement preceding it, so that the printf & new b = 0; statement will be included in that branch)

Answer 3

Here are few issues with your code that I can spot:
You use scanf with %d format specifier that already parses an input as integer value. Thus, there is no need to check if it is numeric with isdigit. In fact, isdigit is checking a decimal digit character, so your usage is incorrect.
You never check for scanf return. You should. In case there was an error (i.e. value was non-numeric), the input is not removed from the stream. In other words, you will get stuck trying to parse the same bad value over and over.
You have unused variables and forgot to include certain header files. But these are minor things in your case (but could be major in certain situations!).
That being said, here is some code that might work out for you:
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
static void skip_input(FILE *stream)
{
int c;
do {
c = fgetc (stream);
} while (c != EOF && c != '\n');
}
int main()
{
int a;
int r;
printf("Please enter a value: ");
for (;;) {
r = scanf("%d", &a);
if (r == EOF)
return EXIT_FAILURE;
if (r != 1) {
printf("Number must be numeric!\n");
skip_input(stdin);
} else if (a < 0) {
printf("Number must be postive\n");
skip_input(stdin);
} else {
printf("Everything is goood\n");
break;
}
}
return EXIT_SUCCESS;
}
Hope it helps.

Answer 4

You are using b as your error flag.
In the "everything is good block", try setting b=0;
Additionally, if you do that, you can get rid of the b++; lines, and simply initialize b=1;
You should only be doing b++ if you need to report how many times the user messed up, which I am presuming you don't need?

Answer 5

There are a couple of problems here.
The first is that the return value of scanf() isn't being checked. If input is available then scanf returns the number of variables that were assigned, which can be 0 or 1 in this case (because you're only attempting to assign one variable). If the user enters a non-numeric character then the loop will execute repeatedly without ever waiting for more user input because there's input available but it'll never match your "%d" conversion. If you want to use scanf() then you'll have to check the return value. If the return value is zero then a non-numeric value was entered, which means that you'll have to clear that value out of the input buffer by reading until the end of the line. If the return value is equal to EOF then either an I/O error occurred or you reached the end of the stream. If the return value is 1 then you know that the value was numeric, which brings me to the next problem.
The routine, isdigit(), takes an integer argument, but it expects that integer value to represent a character. Since you're using scanf() to convert the input to an integer, the value stored in a no longer represents a character; it represents an actual number. Because of this, the call to isdigit() will only return a true value if the user enters a number that corresponds to a digit character. In my locale, that means that the validation will only succeed if the user enters a number between 48 and 57, inclusive. If you're using scanf() then the isdigit() check is not necessary because scanf() will return a value of 1 only if the user entered a numeric value.
To be honest, however, I wouldn't use scanf() to read in user input if I could avoid it precisely because of the need to flush the input buffer if the user enters something wrong. I'm not entirely sure what your requirements are, but I'll assume that you're supposed to read a positive integer from the command line and that the number of digits doesn't matter.
In this case, you'll probably want to use fgets() to read the user input then use strtol() to convert the value to a signed long integer and perform the validation at the same time:
#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
#include <errno.h>
#include <ctype.h>
#include <string.h>
int main () {
long result;
long len;
int is_valid = 0;
char buf[128];
char *arg;
char *end;
while (!is_valid) {
/* Prompt the user for the integer. */
printf("Enter a non-negative integer: ");
if (fgets(buf, sizeof(buf), stdin) == NULL) {
printf("Giving up so soon?\n");
break;
}
/* Verify that the input doesn't exceed our buffer length. */
len = strlen(buf);
if (buf[len] != '\n') {
printf("Input buffer length exceeded - aborting.\n");
exit(1);
}
/* Skip any leading whitespace. */
for (arg = buf; isspace(*arg); arg++);
/* Attempt to convert the argument. */
errno = 0;
result = strtol(arg, &end, 10);
if (errno == EINVAL) {
printf("Please enter a numeric value.\n");
continue;
}
if (errno == ERANGE) {
printf("Numeric value out of range.\n");
continue;
}
/* Check for non-whitespace characters after the end of the integer. */
for (; isspace(*end); end++);
if (*end != '\0') {
printf("Please enter a numeric value.\n");
continue;
}
/* Verify that the number is non-negative. */
if (result < 0) {
printf("Please enter a positive value.\n");
continue;
}
/* The number is valid. */
printf("Excellent!\n");
is_valid = 1;
}
return 0;
}
This isn't perfect; aborting if the input buffer length is exceeded isn't exactly user-friendly. It should take care of the validation problem, however.