Switch Statement
COP-3223H

switch statements
Example: even or odd
Like else-if statements
Example: reading user input
Example: moving a character
C gotcha: fall-through

switch statements

switch (expression) {
case value0:
  instructions0;
  break;
case value1:
  instructions1;
  break;
case value2:
  instructions2;
  break;
... // any number of cases
default:
  default_instructions;
  break;
}

Evaluate the expression
Jump to the case corresponding to its value.
The break statement jumps to the end of the switch

The "break" statement is necessary to avoid "fall-through"

The values can be in any order.

Not all values need to be there.

The default is optional; without it, it's possible that none of the cases will run (like an if without an else).

Example: even or odd

switch (x % 2) {
case 0:
  printf("even\n");
  break;
case 1:
  printf("odd\n");
  break;
default:
  printf("this should never happen\n");
  assert(0);
  break;
}

Like else-if statements

if        (0 == (x % 2)) {
  printf("even\n");
} else if (1 == (x % 2)) {
  printf("odd\n");
} else {
  printf("this should never happen\n");
  assert(0);
}

Simpler syntax for a specific case of conditionals

Example: reading user input

switch (c) {
  case 'j':
    printf("pressed j, which is down\n");
    break;
  case 'k':
    printf("pressed k, which is up\n");
    break;
  case 'h':
    printf("pressed h, which is left\n");
    break;
  case 'l':
    printf("pressed l, which is right\n");
    break;
  default:
    printf("unexpected key\n");
    break;
}

if        ('j' == c) {
  printf("pressed j, which is down\n");
} else if ('k' == c) {
  printf("pressed k, which is up\n");
} else if ('h' == c) {
  printf("pressed h, which is left\n");
} else if ('l' == c) {
  printf("pressed l, which is right\n");
} else {
  printf("unexpected key\n");
}

Example: moving a character

// https://sourceware.org/glibc/manual/latest/html_mono/libc.html#Noncanon-Example

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <termios.h>

/* Use this variable to remember original terminal attributes. */

struct termios saved_attributes;

void
reset_input_mode (void)
{
  tcsetattr (STDIN_FILENO, TCSANOW, &saved_attributes);
}

void
set_input_mode (void)
{
  struct termios tattr;

  /* Make sure stdin is a terminal. */
  if (!isatty (STDIN_FILENO))
    {
      fprintf (stderr, "Not a terminal.\n");
      exit (EXIT_FAILURE);
    }

  /* Save the terminal attributes so we can restore them later. */
  tcgetattr (STDIN_FILENO, &saved_attributes);
  atexit (reset_input_mode);

  /* Set the funny terminal modes. */
  tcgetattr (STDIN_FILENO, &tattr);
  tattr.c_lflag &= ~(ICANON|ECHO); /* Clear ICANON and ECHO. */
  tattr.c_cc[VMIN] = 1;
  tattr.c_cc[VTIME] = 0;
  tcsetattr (STDIN_FILENO, TCSAFLUSH, &tattr);
}

int main(int argc, char **argv) {
  char c = '\0';
  int width = 40;
  int height = 20;
  int x = 1;
  int y = 1;
  int dx = 1;
  int dy = 1;

  set_input_mode ();

  while (1) {
    printf("\x1b[2J");
    printf("\x1b[H");
    printf("\033[%d;%dH", y, x);
    putchar('o');
    putchar('\n');
    printf("\033[%d;%dH", height + 1, 0);
    printf("y: %d\n", y);
    printf("x: %d\n", x);
    printf("c: %c\n", c);
    read (STDIN_FILENO, &c, 1);
    dx = 0;
    dy = 0;
    switch (c) {
    case 'j':
      dy = 1;
      break;
    case 'k':
      dy = -1;
      break;
    case 'h':
      dx = -1;
      break;
    case 'l':
      dx = 1;
      break;
    case 's':
      dy = 1;
      break;
    case 'w':
      dy = -1;
      break;
    case 'a':
      dx = -1;
      break;
    case 'd':
      dx = 1;
      break;
    }
    int newx = x + dx;
    int newy = y + dy;
    if ((0 < newx) && (newx < width)) {
      x = newx;
    }
    if ((0 < newy) && (newy < height)) {
      y = newy;
    }
  }
}

C gotcha: fall-through

switch (x % 2) {
case 0:
  printf("even\n");
  break;
case 1:
  printf("odd\n");
default:
  printf("this should never happen\n");
}

Without a "break", the program will continue running the next case.

case labels are just like labels for goto. The "break" here means goto to end of the switch block.

Sometimes fall-through is used intentionally:

Duff's device - Wikipedia

Switch Statement COP-3223H

Table of Contents