CS156 IO: Input/Output

Reading input

int zipcode;
double weight;
printf("Enter zip code: ");
scanf("%d", &zipcode);
printf("Enter weight: ");
scanf("%lf", &weight);
printf("And now both: ");
scanf("%d %lf", &zipcode, &weight);

Read input from the keyboard using scanf.
The usage is similar to printf. The formatting symbols are the same, except double is %lf (letter l, letter f).

Example reading in from stdin

#include <stdio.h>
int main() { 
  double friction, number;
  int numPeople;

  // read in some values
  printf("Enter the amount of friction: ");  // prompt stays on line
  scanf("%lf", &friction );
  printf("Enter the number of people:\n");  // prompt goes to next line
  scanf("%d", &numPeople);

  number = friction + 3.2 * 2;    // This makes no sense

  printf("The final number was %f\n", number);
  printf("The value of \"friction\" was %f.\n", friction );
  printf("There are %d people here.\n", numPeople);
  return 0; 
}

What is this &?

It’s an “ampersand”, from the latin “Et”, which means “and”.
The value of a variable is stored in a particular place in the computers memory.
The variable name, alone, gets you the variable’s value.
The & in front of the variable’s name tells the program to use the variable’s address in memory rather than its value.

Memory

Address	Value
9278	00111011
9279	00000000
9280	11111110
9281	10101010
9282	01010101
9283	11110000
9284	11110000
9285	11110000
9286	00010110

Memory is divided into many memory locations (or cells).

Each memory cell has a numeric address, which uniquely identifies it.

Every location has a value, whether you put one there or not. The statement “there’s nothing in there” is meaningless.

Memory by type

int i;
float f;
double d;
char c;
short int si;
long int li;
long double ld;

// sizeof operator gives the size of a variable
printf("char: %zu byte\n",         sizeof(c));
printf("short int: %zu bytes\n",   sizeof(si));
printf("int: %zu bytes\n",         sizeof(i));
printf("long int: %zu bytes\n",    sizeof(li));
printf("float: %zu bytes\n",       sizeof(f));
printf("double: %zu bytes\n",      sizeof(d));
printf("long double: %zu bytes\n", sizeof(ld));

char: 1 byte
short int: 2 bytes
int: 4 bytes
long int: 8 bytes
float: 4 bytes
double: 8 bytes
long double: 16 bytes

But they’re not the same everywhere!

Address

Each house has an address (like memory does).
The person who lives at that address is the value held at that address.
People may move, but the house and address remain the same (the value varies, hence we call them variables).
If you want to know who lives at address 654, you want the value at the address, so reference the variable and you’ll get “Mary”.
If you want to know the address of the house, we use the & to designate we want the address, not the value inside

scanf reads values IN …

scanf needs to know where to store them the values
- It reads in what you tell it
- converts it from whatever base you tell it
- converts it to whatever type you tell it
- and puts the result where you tell it
Where = address
Why doesn’t printf need the address too?

Stress the ampersand

int x=3;
printf("x is now %d\n", x);  // No ampersand
printf("Gimme: ");
scanf("%d", &x);             // Ampersand!
printf("x is now %d\n", x);  // No ampersand

If you use c11 -Wall, it tells you when you forget the ampersand, and when you put in an extra one. Use -Wall!

CS156: Intro to C, Part I

Spring 2018

IO