Can scanf() store values?

Question

Hi I am now learning the C language and I have a little problem with a exercise of the book I read. My code is this:

#include<stdio.h>
int main()
{
  unsigned char one=0;
  unsigned char two=0;
  printf("Quantity 1 = ");
  scanf("%d",&one);
  printf("Quantity 2 = ");
  scanf("%d",&two);
  printf("The value is %d",one);
  return 0;
}

Why when I am trying to see the value of one the initial value appears and not the value after the scanf?

Answer 1

You need to use int type in conjuction with %d specifier, and char with %c specifier. And %u with unsigned integers.

#include<stdio.h>
int main()
{
    unsigned int one=0; unsigned int two=0;
    printf("Quantity 1 = ");scanf("%u",&one);
    printf("Quantity 2 = ");scanf("%u",&two);
    printf("The value is %u",one);
    return 0;
}

Basicaly, scanf will try to read integer from input and it will try to store it inside memory location that is not large enough, so you will have undefined behavior.

You can find good reference here.

However, if you try to use character for an input type, you may want ask yourself why you won't get a chance to enter a second Quantity (if you type 4 and press enter). This is because second scanf will read enter key as a character. Also, if you try to type 21 (for a twentyone), it will fill the first value with 2 and second with 1 (well, with their ASCII values).

So, be careful - be sure that you always choose the right type for your variables.

Answer 2

Never use scanf. Never use scanf. Seriously, never use scanf.

Use fgets (or getline, if you have it) to read an entire line of input from the user, then convert strings to numbers with strtol or its relatives strtod and strtoul. strsep may also be useful.

Answer 3

Check if scanf() is working properly by reading its return value. For quickstart, read the details about scanf() at this link.

What you are doing is inputting a integer using "%d" into an unsigned char variable, therefore scanf() may not be working as it should.