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cscanfgetchar

Using Scanf after getchar?

I am taking C programming course. I did not understand these codes.

#include <stdio.h>
int main()
{
    int c;
    int ival;
    printf("type : ");
    c = getchar();
    scanf("%d", &ival);
    printf("c     = %d\n", c);  //65
    printf("ival  = %d\n", ival);  //127
    return 0;
}

For example whenever I type Abc, I am getting c = 65; ival = 1. why ival is 1?

Solution

  • ival is never initialized, so it can have any value. The reason is that, c is receiving 'A' (through getchar()) and then scanf fails to read a number (since the next character in the input, 'b', is not a decimal number), so it never touches ival.

    You can check the return value of scanf to see if it fails or succeeds:

    if (scanf("%d", &ival) != 1)
    printf("you need to enter a number\n");
else
    printf("entered: %d\n", ival);

    Note that scanf returns the number of items it successfully read and assigned. For example scanf("%d %f %c", ...) would return 3 if all three items were correctly read.1

    1Note that assigned means that ignored input (such as those with the assignment-suppresion modifier (*)) doesn't count towards the return value of scanf (C11, 7.21.6.2.10,16). Furthermore, %n doesn't affect the return value of scanf (C11, 7.21.6.2.12).