Thread: checking scanf return value

While tidying up the code on my latest project i decided its about time i learnt to check that the user is inputting the correct data. my understanding of the return value of scanf is an int to the value of he number of items read in. ie
scanf("%d", &a); will return 1. if the user just hit enter scanf would sit there waiting for a number to be entered.

Code:

int main()
{
    int a, b, x;

    printf("please enter 2 numbers between 1 and 10 ");
    x = scanf(" %d %d", &a, &b);
    while (x != 2)
    {
        printf("incorrect input please enter 2 numbers between 1 and 10 ");
        x = scanf(" %d %d", &a, &b);
    }


    return 0;
}

if the above is correct why is this an infinite loop
coop

thanks i have it working

Code:

int main()
{
    int a, b, x;
    char array[4];

    printf("please enter 2 numbers between 1 and 10 ");
    fgets(array, sizeof(array), stdin);
    x = sscanf(array, " %1d%1d", &a, &b);
    while (x != 2)
    {
        printf("inccorect entery please enter 2 numbers beteen 1 and 10 ");
        fgets(array, sizeof(array), stdin);
        x = sscanf(array, " %1d%1d", &a, &b);
    }
    printf("x = %d a = %d b = %d\n", x, a ,b);

    return 0;
}

why does the array have to be four to allow entry of "3 4" though
coop

never mind looking at my deceleration its a char array so even though i am dealing with numbers its still a string and there for to hold the string "Coop" in an array i would declare it with a size of 5 for the \0 character
coop

ok now what am i doing wrong

Code:

#include <stdio.h>
#include <stdlib.h>

typedef struct
{
    char name[10];
    int age;
    char age_array[3];
}DETAILS;

int main()
{
    int a, x = 0;
    DETAILS record;

    printf("Please enter your name upto 10 characters: ");
    while (x != 1)
    {
        fgets(DETAILS.name, sizeof(DETAILS.name), stdin);
        x = sscanf(DETAILS.age_array, " %s", DETAILS.name);
        if (x != 1)
        {
            printf("incorrect entry Please enter your name upto 10 characters: ");
        }
    }

    return 0;
}

coop

Originally Posted by laserlight

You need to get rid of the invalid input, otherwise the next scanf call will try to read it, and fail again.

Because this kind of interactive I/O tends to be line-based in that you enter your input on a line, then output is printed, then you enter your input on another line, one approach is to combine fgets with sscanf instead of just using scanf. So, the fgets call reads the line from the input buffer into an array, then sscanf parses the content of the array into your output variables. If sscanf fails, you don't have to worry about discarding invalid input because the line of input has already been successfully read.

Originally Posted by flp1969

If you need to enter 2 values between 1 and 10 you'll need, at least 7 chars: "XX XX\n\0" (fgets() will get the \n from stdin). BUT, what if you enter "10__________20" (10 spaces -- change each '_' for a space)?

Instead of using fgets() I recommend using POSIX getline() to allocate the buffer dinamically...

Code:

char *buffer = NULL;
size_t size = 0;
if ( getline( &buffer, &size, stdin ) == -1 )
{ ... error ... }
... do something ...
free( buffer ); // free the allocated buffer.

so which is the right one? which do i research to answer my question.
coop

That sensation of cursing is back...

cooper1200, laserlight said "one approach is..." meaning there are others.
I said: "I recommend..." means this is ONE of those other approaches...

BOTH are right! research BOTH!

Originally Posted by flp1969

That sensation of cursing is back...

glad were on the same wave length chuck