Thread: Beginner practice, scanf

Hello,

I cannot figure out why, whenever I use the scanf function more than once, it never "executes" more than once. I will have three questions that require input from the user, but only the first question allows for input from the keyboard. Once I hit enter, the rest of the code just executes with me not able to input anything. Can someone tell me why?

Code:

#include <stdio.h>


int main()
{
    char name[20];
    int DOB;
    int phoneNumber;
    printf("Please tell us your name: ");
    scanf("%s", name);


    printf("When were you born? ");
    scanf("%d", &DOB);


    printf("What is your phone number? ");
    scanf("%d", &phoneNumber);


    printf("Here's your information:\n Name: %s\n Date of Birth: %d\n Phone Number: %d", name, DOB, phoneNumber);


    return 0;
}

Are you considering the newline character?

Code:

scanf("%s", name);

If the user inputs "John Doe", then name would only contain "John". '%s' as is, only inputs the first word, not the entire string. Then the other scanf() calls will fail as the remainder of the input buffer do not match the format strings for numerical strings.

fgets() would be preferred to input text strings. Yes, an alternative version of the format string in scanf() could be used but fgets() is simpler, although it does also bring in the newline that needs to be dealt with.

Phone numbers should be input as a text string, and input using fgets(), not as a number because of the many styles of phone numbers. "1234567890" "(123) 456-7890", ...

Originally Posted by rstanley

Phone numbers should be input as a text string, and input using fgets(), not as a number because of the many styles of phone numbers. "1234567890" "(123) 456-7890", ...

Plus a plain numeric-only phone number might not even fit in an int, such as one in the 408 area code (4081234567). With 32-bit int, that may get stored as a large negative number (4081234567 may be stored/interpreted as -213732729) (though it's probably actually undefined behavior to try storing a larger value into an int than it will hold, so I'd avoid doing that and instead read/store it as a string).

Originally Posted by christop

Plus a plain numeric-only phone number might not even fit in an int, such as one in the 408 area code (4081234567). With 32-bit int, that may get stored as a large negative number (4081234567 may be stored/interpreted as -213732729) (though it's probably actually undefined behavior to try storing a larger value into an int than it will hold, so I'd avoid doing that and instead read/store it as a string).

It's a moot point as no phone number is ever a simple numerical number, in any integer type!

Please see: "National conventions for writing telephone numbers" for more information. It should always be input as a string , then processed as needed.

Hello,

So are you saying that when the printf prompt comes up, asking me my name and I type in John Doe and hit Enter, the computer is essentially doing the following:

%s = John
1st %d = ' ' (a blank space)
2nd %d = Doe

and therefore nothing else in the code executes because it already has the input from my first entry it just doesn't make any sense?

Originally Posted by Arzakon12589

Hello,

So are you saying that when the printf prompt comes up, asking me my name and I type in John Doe and hit Enter, the computer is essentially doing the following:

%s = John
1st %d = ' ' (a blank space)
2nd %d = Doe

and therefore nothing else in the code executes because it already has the input from my first entry it just doesn't make any sense?

First of all, you do not initialize the local variables, "name", "DOB", and "phoneNumber". They contain whatever garbage values are in the variables when the program starts.

Initialize ALL local variables!

Yes, the first scanf() inputs one word, John.

The second scanf() ignores the space and attempts to convert "Doe" to an int, and fails, leaving the text in the input buffer, and assigns nothing to DOB, and leaves whatever garbage value was in DOB when called.

The third scanf() also attempts to convert "Doe" to an int and fails, as with the second scanf().

Within a while loop, if you capture the return value from scanf(), you will see when the scanf() fails. You can clear the input buffer, but NOT with fflush(stdin)!!! fflush() is only for use with output buffers. You then can prompt the user for the value again, until you get the correct value.

You need to study a good up to date book on the C Programming Language! Please see my other post, here.

You need to read the scanf manpage for more information, or run "man scanf" on a Linux system.