Converting a char array for use with isdigit() - c

Question

I've been trying to write out code that takes input from the user, stores it into a char array, and then identifies the input as a positive or negative integer, a positive or negative decimal, or a character string. So far this is the code I have.
void input() {
char userInput[500];
printf("Enter something: ");
fgets(userInput, 500, stdin);
for (int i = 0; i < strlen(userInput); i++) {
if (isdigit(userInput) == 0) {
printf("This is not a digit.\n");
}
}
}
I'm aware that trying to use a char array with isdigit() will not work. I'm having issues figuring out how to get around this. I'm assuming it would be possible to run through the array and identify what the input is. And I assume there is a way to cast the char array into an int so isdigit would work, but I'm a bit stumped. Any suggestions?

Answer 1

There seems to be some confusion between digit, which is a character between '0' and '9' and integer which is a number composed of such digits with an optional leading sign.
You can use isdigit() defined in <ctype.h> to check if a character is a digit, but testing if the string encodes a number is a different question.
Passing a char to isdigit() has undefined behavior is the char value is negative because isdigit() is only defined for a values of the type unsigned char and the special negative value EOF. To test if a char is a digit, you can write isdigit((unsigned char) userInput[i]).
To test if the string contains a number, you can use strtol(), and to test for a floating point value (which I assume you mean by decimal), you can use strtod().
You can also use sscanf(), which is probably easier for a simple program.
Here is a modified version:
#include <stdio.h>
void input() {
char userInput[500];
printf("Enter something: ");
if (fgets(userInput, sizeof(userInput), stdin)) {
int n;
double d;
char c;
/* try converting as an int,
test if the conversion succeeds
check that no trailing character is present.
sscanf returns the number of successful conversions
*/
if (sscanf(userInput, "%d %c", &n, &c) == 1) {
printf("string contains an integer %d\n", n);
} else
if (sscanf(userInput, "%lf %c", &d, &c) == 1) {
printf("string contains a double %g\n", d);
} else {
printf("string is an ordinary string: %s\n", userInput);
}
}
}
Note however that the above tests will accept leading and trailing whitespace and may invoke undefined behavior if the number cannot be represented in the target types.

Answer 2

You can use simple heuristics based on the character types to distinquish between integer, floating point, or string.
Introduce some new variables:
int slash = -1;
int digits = 0;
int chars = 0;
int decimals = 0;
int isNumber = 0;
int isNegative = 0;
int isFloat = 0;
int len;
Then your scanning code is this:
len = strlen(userInput);
for (int i = 0; i < len i++) {
if (userInput[i] == '-') {
slash = i;
}
else if (userInput[i] == '.') {
decimals++;
}
else if ((userInput[i] >= '0') && (userInput[i] <= '9')) {
digits++;
}
else {
chars++;
}
}
And then you can print the result as:
// NOT A NUMBER if:
// a dash anywhere but index 0
// more than one decimal
// no digits
// anything else that isn't a digit
isNumber = ( ((slash == -1) || (slash == 0)) &&
(decimals <= 1) &&
(digits > 0) &&
(chars == 0)
);
if (isNumber) {
isFloat = (decimals == 1);
isNegative = (slashPos == 0);
printf("string contains a %s %s: %s\n", (isNegative ? "negative" : "positive"), (isFloat ? "double" : "integer"), userInput);
}
else {
printf("string contains an ordinary string: %s\n", userInput);
}
}