Taking formatted input : sscanf not ignoring white spaces

Question

I have to find out the input hours and minutes after taking inputs from the user of the form :

( Number1 : Number2 ) 

eg: ( 12 : 21 )

I should report 12 hours and 21 minutes and then again wait for input. If there is a mismatch in the given format, I should report it as invalid input. I wrote this code :

#include<stdio.h>
int main()
{
    int hourInput=0,minutesInput=0; 
    char *buffer = NULL;
    size_t size;

    do
    {
        puts("\nEnter current time : ");
        getline ( &buffer, &size, stdin );

        if ( 2 == sscanf( buffer, "%d:%d", &hourInput, &minutesInput ) && hourInput >= 0 && hourInput <= 24 && minutesInput >=0 && minutesInput <= 60  )
        {

            printf("Time is : %d Hours %d Minutes", hourInput, minutesInput );
        }

        else
        {
            puts("\nInvalid Input");
        }   
    }

    while ( buffer!=NULL && buffer[0] != '\n' );

    return 0;
}

Q. if someone gives spaces between the number and :, my program considers it as invalid input, while I should treat it as valid.

Can someone explain why it is happening and any idea to get rid of this issue ? As far as I understand, sscanf should ignore all the white spaces ?

Answer 1

To allow optional spaces before ':', replace

"%d:%d"

with

"%d :%d"

sscanf() ignores white space where its format directives tell it to ignore, not everywhere. A whitespace character in the directive such as ' ' will ignore all white spaces. %d as well as other integer and floating point directives will ignore leading white space. Thus a space before %d is redundant.

C11 7,21,6,2,8 Input white-space characters (as specified by the isspace function) are skipped, unless the specification includes a [, c, or n specifier.)

---

Additional considerations include using %u and unsigned as an alternate way to not accept negative numbers. strptime() is a common function used for scanning strings for time info.

Answer 2

I think if you put a space before and after the colon, it will ignore any whitespace and still work when they don't put spaces before and after the colon. Like this:

sscanf( buffer, "%d : %d", &hourInput, &minutesInput )

Answer 3

use "%d : %d" as format string. it will work with and without spaces.

Answer 4

1st thing allocate memory before using buffer

2nd is this a C++ program or C as getline is not a C standard function.

Check this

int main()
{
        int x=0,y=0;
        char bff[]="7        8";
        sscanf(bff,"%d%d",&x,&y);
        printf("%d %d",x,y);
}

o/p-7 8

Answer 5

On sscanf man pagez

RETURN VALUE OF SSCANF -

These functions return the number of input items assigned. This can be fewer than provided for, or even zero, in the event of a matching fail- ure. Zero indicates that, although there was input available, no conver- sions were assigned; typically this is due to an invalid input character, such as an alphabetic character for a `%d' conversion. The value EOF is returned if an input failure occurs before any conversion such as an end- of-file occurs. If an error or end-of-file occurs after conversion has begun, the number of conversions which were successfully completed is returned.

Now,

if someone gives spaces between the number and :, my program considers it as invalid input

Yes , it should consider it wrong because the sscanf reads the from the buffer in exactly the same way as %d:%d,but if a character in the input stream conflicts with format-string, the function ends, ending with a matching failure.

Characters outside of conversion specifications are expected to match the sequence of characters in the input stream; the matched characters in the input stream are scanned but not stored. (Please see the emphasis on the sentence in bold)

i.e, sscanf while writing to the memory ignores whitespaces.

Answer 6

Avoid comparing sscanf() return value. In your case, it is always depends on the user input. If user gives spaces between input this value changes.