how field specifier works in C - c

Question

I need to take two int values from user. First value is using field specifier and second is normal integer value.
#include<stdio.h>
int main()
{
int num, num1;
printf("Enter first number: \n");
scanf("%2d", &num);
printf("First number is %2d\n", num);
printf("Enter second number: \n");
scanf("%d", &num1);
printf("Second number is %d\n", num1);
return 0;
}
and the output is
Enter first number:
12345
First number is 12
Enter second number:
Second number is 345
It won't give control to enter second number. I don't know why?

Answer 1

You see this behaviour because you have limited the size of input that the first scanf statement can consume. scanf("%2d", &num) says that scanf should read a field of width at most 2 and convert that into into num.
Change the scanf to scanf("%d", &num) and the entirety of 12345 will be processed.

Answer 2

you don't get to enter second num because your program gets it from first num.
First time, your scanf lets your num get first two digits of the number you entered, and rest remains in the buffer/stream.
next time scanf is executed, it reads the remaining digits from the stream till an enter stroke, hence you don't get to enter the second no.
if you want to read the second no.
try flushing the stream before using second scanf(), you will get what you want.

Answer 3

Why would you expect it to "give control"? The fist scanf() explicitly consumed at most two digits, leaving "345" unconsumed. The next scan begins with the unconsumed input. What else would you expect?
If you want to discard any unconsumed input before the next scan, use fpurge(stdin).

Answer 4

There is always a tradeoff with scanf. If you want to enter a whole number and then consume the trailing newline (left in the input buffer (stdin) as the result of pressing [enter]), you canappend a %*c to read and discard the trailing newline. This itself causes problems if an empty-string is entered.
However, limiting your scanf format string and specifier to %2d and then entering 123456, you intentially leave 3456\n in the input buffer which is taken as your input to the second scanf call. The only way to insure each of your scanf calls will only accept your expected input is to manually empty the input buffer after each read by scanf to insure there are no characters remaining prior to the next call. A simple way to do this is with either a do .. while or simply a while ; using getchar() to read each character in stdin until a '\n' is encountered or EOF:
#include<stdio.h>
int main()
{
int c, num, num1;
printf("Enter first number: \n");
scanf("%2d", &num);
while ((c = getchar()) && c != '\n' && c != EOF) ;
printf("First number is %2d\n", num);
printf("Enter second number: \n");
scanf("%d", &num1);
printf("Second number is %d\n", num1);
return 0;
}
output:
$ ./bin/scanf_tradeoff
Enter first number:
123456
First number is 12
Enter second number:
12345
Second number is 12345

Answer 5

The reason for second value is not getting from you, in first scanf you are mentioning that
get the two values from the input. You are giving more than two that time the remaining value is stored in the buffer. So second scanf will get the value from the buffer.
If you want to avoid this you can use this statement before the second scanf.
while ((c=getchar())!='\n' && c!=EOF);// clearing the buffer.
So now the second input will get from the user.