Thread: Obscure loop using scanf and getchar functions - Program for reading int and double

Hi, guys.

I have problems understanding this program.

Code:

/*


    By now you have probably experienced problems with scanf insofar as when an invalid character is typed things go drastically wrong. In “GETVAL.C” write and test two functions:

                double get_double(void);
                int get_int(void);
        
    which loop, prompting the user, until a valid double/valid integer are entered.


*/

#include <stdio.h>


//FUNCTION PROTOTYPES
int get_int(void);
double get_double(void);


//MAIN FUNCTION
int main(void)
{
    int i;
    double d;


    printf("type an integer ");
    i = get_int();
    printf("the integer was %i\n", i);


    printf("type an double ");
    d = get_double();
    printf("the double was %lg\n", d);


    return 0;
}


//FIRST FUNCTION
int get_int(void)
{
    int result;


    printf("> ");


    while(scanf("%i", &result) != 1)
    {
        while(getchar() != '\n');
        printf("> ");
    }


    return result;
}


//SECOND FUNCTION
double get_double(void)
{
    double result;


    printf("> ");


    while(scanf("%lf", &result) != 1)
    {
        while(getchar() != '\n');
        printf("> ");
    }


    return result;
}

I have problems understanding this loop in both subordinate made functions.

Code:

while(scanf("%lf", &result) != 1)
    {
        while(getchar() != '\n');
        printf("> ");
    }

Could you help me to understand? I've looked for additional information without success. I really don't get what's going there.

Thanks in advanced.

Ok, I've been studying a little further the secondary functions after reading your comments.

Code:

double get_double(void)
{
    double result;


    printf("\n> ");


    while(scanf("%lf", &result) != 1)
    {
        while(getchar() != '\n');
        printf("> ");
    }


    return result;
}

What I was not understanding were the conditions inside the while. Let me see if I got this straight.

Outer while. The scanf function is waiting for a double, in case a non-double is entered returns 0, which is compared with the given value 1. Zero means false, and 1 means true. If a double is entered, it returns the boolean value 1. Since 1 != 1 is false, then the while is terminated.

Inner while. The getchar function can only accept chars. While only chars are entered, it will return 1, because getchar is working, is true. This is different than \n, in which case getchar() != \n is true, so the loop will continue. So, it will be possible to enter as many characters as wanted until a new line is entered, in which case the condition of \n != \n is false and the loop is terminated. After this, the character > is printed out and the condition in the outer loop is executed.

I'm having trouble seeing how the condition of the outer loop works with the inner loop. I thought that C was a sequential (line 1 -> line 2 -> line 3 -> etc) programming language. Is the condition in the inner loop executed at the very same time touterter condition is executed?

Thanks for your answers.

Originally Posted by laserlight

That is incorrect. If there is a character to read, getchar reads it as an unsigned char converted to int,

So, what is the integer value that is reading? The ASCII character codes? (ie: 101 for A, etc)

Originally Posted by laserlight

then returns the result. Otherwise, getchar returns EOF. Therefore, it will not return 1 unless the character read as a value of 1 when read as an unsigned char.

I'm not sure what you're trying to explain here. I speak Spanish, and I struggle sometimes with Englsih. For what you've written here, I understand that getchar returns 1 everytime that it reads a character, doesn't it? What happens if I enter instead of a character, the correct value (ie, an integer value)?

Originally Posted by laserlight

The outer loop reads the first part of the input entered by the user, then if that input is not in double format, the inner loop then reads and discards input until the newline that terminates the line of input is read.

Ok. But what's the returning value of scanf every time the correct input is entered? And why? I've tried to found the original function of scanf without success.

Thanks for your answers.

Originally Posted by laserlight

The outer loop reads the first part of the input entered by the user, then if that input is not in double format, the inner loop then reads and discards input until the newline that terminates the line of input is read.

Got it. Thank you.