Thread: &...how important?

Code:

#include <stdio.h>

int userNum = 0; /*Declare all variables*/

int main()
{
	printf("Please enter a number: \n"); /*Ask for user input and record it to: userNum*/
	scanf("%d",userNum);

	if( userNum = 1)					/*Compare userNum to 1 and print corresponding answer*/

	printf(" Your number equals 1" );
   
	else
      printf(" Your number does not equal 1");

   
   return 0;
}

This message was supposed to be me asking for help with this program but I found the problem but came up with a question. In the above code I forgot the & sign right after scanf, no big deal just a typo but my compiler didn't catch it yet every time I ran the program it would crash and ask if I wanted to send an error report, can someone elaborate a bit on why it is so important but doesn't show up?

P.S. Here is the fixed one:

Code:

scanf("%d",userNum);

Code:

scanf("%d", &userNum);