Thread: How would you check this?

I have a float that must be entered from the keyboard and I want to check to make sure it is a number and not something else. How would you do that? Here is my function so far:

Code:

float getfloat(char str[], float min){
	float num ;
	
	printf("Please enter the %s of raffel tickets.\n", str);
	scanf("%f%*c", &num);
        
       while(num < min){
		printf("Please enter a number greater than %.2f\n", min);
		scanf("%f%*c", &num);
	}
	
	return num;
}

Thanks for any help.

How would you check the ascii value of a number instead of the numerical value?

Thanks. I'm just not quite sure how to get the ascii number of the string of characters the user inputs. Like after I get input into a float variable, how can I check it from there? If a non-numeric character gets scanf()ed into a float, does the float just put it back into the buffer?

If you read input into a float variable, you're toast as far as trying to check later if it was valid (scanf will just stop at illegal input).

If you store it as characters, then by definition you'll have the character values.

You might want to check the returned value of scanf(). It will tell you whether the input was a number. And clean up the format string to be just "%f".

Also, you're checking if the user entered something less than the minimum. Yet the error says it was erroneously entered exceeding it. What if it's equal to the minimum? Change either the condition, or the message.

so if I scanf() some input into a char array with %s, it will be stored sequentially by character with a terminating null. I can then check the ascii values. If it turns out to be an number, how can I the number back out of the array as a float? also if I scanf() a string into a char array that already holds values, what happens? Do the values get over-written starting from the first index of the array?

1. strtod/strtof
2. Yes

Here's what I ended up coming up with if anyone is interested.

Code:

printf("Enter a number: ");
while(scanf("%f", &num) != 1 || isalpha(getchar())) {
   printf("Just a number, no characters please.");
   while(getchar() != '\n');
}

It was for an exam program, but unfortunately I came up with it after it was due(actually we weren't instructed to validate but I wanted to do it anyway). It was more important that I understand how to do it anyway.

Originally Posted by Salem

> Here's what I ended up coming up with if anyone is interested.
It would still go into an infinite loop if you pressed ctrl-z (dos/win) or crtl-d (unix/linux)

Good point. I didn't consider that. How would one check for those?

scanf returns EOF in those cases (since that's what it is).