loop + if + scanf

This is a discussion on loop + if + scanf within the C Programming forums, part of the General Programming Boards category; Hi folks, So I made this program to convert bits to bytes, because I'm so sick of seeing ISP's advertise ...

  1. #1
    Registered User SCRIPT_KITTEH's Avatar
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    Question loop + if + scanf

    Hi folks,

    So I made this program to convert bits to bytes, because I'm so sick of seeing ISP's advertise speeds in megabits, which I consider an intentional attempt to decieve :P And I think I've finally understood how the return value of scanf works since the last time I posted here, so my program can check to see if an integer was entered before processing the input, but I'm stuck on how to make the whole program start over if an integer is not entered. I have a hunch it would involve a loop, but I can't figure out how to make the program start over at "How many mb do you need converted?" if an integer is not entered into scanf... Any ideas?

    Here is the code I have so far:

    Code:
    #include <stdio.h>
    
    int main () {
    
        int b, mb, kb, Byte, kB, mB, gB;
        char term;
    
    
            printf ( "\nHow many mb do you need converted? " );
        
            
            if ( scanf ( "%d%c", &mb, &term ) != 2 || term != '\n' ) {
    
                printf ( "\nThis program requires you enter an integer." );
                printf ( "\n" );
            }
    
    
            else {        
                
                printf ( "\n" );
        
                kb = mb * 1024;    
                b = kb * 1024;
                Byte = b / 8;
                kB = Byte / 1024;
                mB = kB / 1024;
                gB = mb / 1024;
    
                if ( kB < 1024 ) {
    
                printf ( "%i mbps is %i kBps\n", mb, kB );
        
                }
    
                else {
        
                printf ( "%i mbps  is %i mBps\n", mb, mB );
        
                }
        
            }
    
        
        printf ( "\n" );
    
        return 0; 
    
    }
    * oh, and my program makes the assumption for now at least, that mb will be inputted because that's the unit of measurement that i usually see advertised, and i didn't bother making an if statement to print a conversion in terms of gigabytes because i've never heard of a connection that fast :P
    Last edited by SCRIPT_KITTEH; 05-13-2013 at 08:05 PM.

  2. #2
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    Look up the do-while loop.

    scanf() if it encounters an error reading an int, will leave data that causes the error to be read next time. That will cause the next call of scanf(), if it is reading an int, to return immediately. Doing that in a loop results in an infinite loop.

    It is often considered better to read data from the user as a string (aka array of char) using fgets(), and then interpret that string to see if required data (two ints in your case) is there. sscanf() offers one way to interpret the content of the string.
    Right 98% of the time, and don't care about the other 3%.

  3. #3
    Registered User SCRIPT_KITTEH's Avatar
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    Quote Originally Posted by grumpy View Post
    Look up the do-while loop.

    scanf() if it encounters an error reading an int, will leave data that causes the error to be read next time. That will cause the next call of scanf(), if it is reading an int, to return immediately. Doing that in a loop results in an infinite loop.

    It is often considered better to read data from the user as a string (aka array of char) using fgets(), and then interpret that string to see if required data (two ints in your case) is there. sscanf() offers one way to interpret the content of the string.
    Ah. I had tried a do loop before I posted it, and indeed it did make an infinite loop. I just assumed I wasn't putting part of the loop in the right part of the program or something XD It's good to know that about scanf, thanks. I've never heard of sscanf or fgets, I'll have to do some reasearch on those. Thanks, grumpy.

  4. #4
    Registered User ApolCor's Avatar
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    Simply move some of the condition statements around.

    Right after the line "char term;" insert "while (1) {" and then put in "else break; }" right after the "if ( scanf..." block. Then remove the "else {" and its corresponding closing bracket "}" above.

  5. #5
    Registered User SCRIPT_KITTEH's Avatar
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    Quote Originally Posted by ApolCor View Post
    Simply move some of the condition statements around.

    Right after the line "char term;" insert "while (1) {" and then put in "else break; }" right after the "if ( scanf..." block. Then remove the "else {" and its corresponding closing bracket "}" above.
    Ermmm.... I tried to do what I thought you were saying several times, but the program wouldn't even compile. The farthest I got was it compiling but resulting in an infinite loop.

    Is this what you meant?

    Code:
    int main () {
    
        int b, mb, kb, Byte, kB, mB, gB;
        char term;
        
        while(1){
    
            printf ( "\nHow many mb do you need converted? " );
    
            if ( scanf ( "%d%c", &mb, &term ) != 2 || term != '\n' ) {
    
                printf ( "\nThis program requires you enter an integer." );
                printf ( "\n" );
                
            }
            
            else break;
    
        }
    followed by removing the else statement that surrounds the rest of the code?

  6. #6
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    Your biggest problem is that scanf() isn't really made for handling user input (the "f" stands for "formatted" which usually doesn't correspond to user input).

    I suggest you read
    FAQ > How do I get a number from the user (C) - Cprogramming.com
    and
    FAQ > Validate user input - Cprogramming.com

    Bye, Andreas

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