validate numerical entry

**taurus** · 10-05-2007

Hi

is there any function/command to validate numberical entries, like it check to make sure the entered value is a number?
thanks

**Dave_Sinkula** · 10-05-2007

Yup. Several are presented in the FAQ.

**taurus** · 10-05-2007

what am i looking for?

**Dave_Sinkula** · 10-05-2007

The FAQ is searchable; this is a search of it for "validate".
http://faq.cprogramming.com/cgi-bin/...arch&match=all

**taurus** · 10-06-2007

isint there any simple command like strcmp or something like that? do i could say
while(input!<"some command")
bla bla

**dwks** · 10-06-2007

Well, you could, but it likely wouldn't compile . . .

If you're still talking about numerical entry, then perhaps you mean something like this:

Code:

while(scanf("&#37;d", &x) == 1 && x != some_command)

**taurus** · 10-07-2007

but how do i say if its a character, then reinput?

**dwks** · 10-07-2007

You'll need to clear the offending character(s) from the input buffer and try to read another number. For example:

Code:

int get_number() {
    int r, x, c;

    for(;;) {
        printf("Enter a number: ");
        if((r = scanf("&#37;d", %x)) != 1) return x;
        else if(r == 0) {
            while((c = getchar()) != '\n' && c != EOF);
        }
        else {
            return INT_MIN;
        }
    }
}

Note that my code just eats all characters until a newline. You could, for example, eat all non-digit characters if you wanted to:

Code:

while((c = getchar()) != EOF && c != '\n') {
    if(!isdigit(c)) {
        ungetc(stdin);
        break;
    }
}

Something like that. There are better ways to do it, of course. For example, you could read a whole line of input with fgets() and then use sscanf() to parse that line.

**taurus** · 10-07-2007

iv done this:

Code:

do
{
fprintf(output,"enter number");
fscanf(input,"&#37;f",array[i]);
}
while(fscanf(input,"%f",&array[i]) != 1);

But as you can probably see it will keep repeating like the matix or something lol, how could i fix that?

**dwks** · 10-07-2007

You're calling fscanf() twice. (Once with array[i], once with &array[i].

) You want to call it only once. And when it returns something other than 1 (if you have one format specifier), then you have to return whatever characters caused it to fail from the input stream. (Checking for EOF is a good idea, too.)

A simple example?

Code:

for(;;) {
    fprintf(output,"enter number: ");
    if(fscanf(input,"&#37;f", &i) == 1) break;
    while(getchar() != '\n');
}

It has very little error checking, though. I'd definitely check for EOF if I were you, or you'd get another infinite "matrix" loop for another reason.

**taurus** · 10-07-2007

but i dont understand the for(;

?

**dwks** · 10-07-2007

Oh, that's just a forever loop. It's the same as while(1). I just prefer it, because, in the eternal words of a CBoard member, "(;;) is a secret code word for 'ever'."

**taurus** · 10-07-2007

hmmm so how do i still implement that in my code, i typed that in but get an error?

**dwks** · 10-07-2007

Well . . . you shouldn't. This works just fine for me:

Code:

#include <stdio.h>

int main() {
    double n;

    for(;;) {
        printf("Enter number: ");
        if(scanf("&#37;lf", &n) == 1) break;
        while(getchar() != '\n');
    }

    printf("Number: %f\n", n);

    return 0;
}

Can you post the code that fails to compile, or some of it at least?

**taurus** · 10-07-2007

Code:


#include <stdio.h>
#define SIZE 10

FILE * output =NULL;
FILE * input =NULL;

// Procedure
void display(float array[SIZE])
{
int i;
	for(i=0; i<SIZE; i++)
	{
		fprintf(output,"&#37;.4f ",array[i]);// Display array
	}
	fprintf(output,"\n");
}

// Function
float max(float array[SIZE])
{
	int i;
	float max=0.0f;
	for(i=0; i<SIZE; i++)
	{
		// For each number, the number is compared with max and if > it stores as max
		if(array[i]>max)
		{
			max=array[i];
		}
	}
	// Return the maximum integer
	return max;
}

// MAIN program
int main(int argc, char * argv[])
{
output = stdout;
input = stdin;

int i;
float array[SIZE];

	for(i=0; i<SIZE; i++)
	{
		do
		{
    		fprintf(output,"Enter value %i: ",i);
    		fscanf(input,"%f",&array[i]);
		}
		while(fscanf(input,"%f",&array[i]) != 1);

	}

	fprintf(output,"\n");
	fprintf(output,"Your values you entered are as follows:\n");
	display(array);
	fprintf(output,"The maximum value entered is:\n");	
	fprintf(output,"%.4e\n",max(array));
}

here it is

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