Loops (While)
COP-3223H

Recall: exponent on desktop calculator
Exponent in C (with goto)
goto can be confusing
While Loops
Exponent with a while loop
Repeated guessing game
Factorial
do-while
Repeated Guessing Game
Boundary conditions
Considering all inputs
Example: newton's method
Console animation

Recall: exponent on desktop calculator

\(x^e\) for e >= 1

Input x and e
Start with 1 in result
Multiply result by x
Subtract 1 from e
Repeat from 3 only if e > 0

Desktop calculator program for exponent

1: 3
2: store x
3: 5
4: store e
5: 1
6: store result
----------------
7: recall result
8: *
9: recall x
10: =
11: store result
12: recall e
13: -
14: 1
15: =
16: store e
17: if e>0 goto 7
-----------------------
18: recall result

# inputs
1: 3
2: store x
3: 5
4: store e
5: 1
6: store result
----------------
# one multiplication
7: recall result
8: *
9: recall x
10: =
11: store result
# decrement e
12: recall e
13: -
14: 1
15: =
16: store e
# repeat if not done multiplying
17: if e>0 goto 7
-----------------------
18: recall result

Exponent in C (with goto)

Use goto as in desktop calculator program
- To a label instead of a number

repeat:
// C code
goto repeat;

#include <stdio.h>
#include <assert.h>

int main(int argc, char **argv) {
  int x;
  int e;

  scanf("%d", &x);
  scanf("%d", &e);
  assert(e >= 1);

  int result = 1;

 repeat:

  result = result * x;

  e = e - 1;

  if (e > 0) {
    goto repeat;
  }

  printf("%d\n", result);
}

goto can be confusing

Recall goto fail bug
"go to statement considered harmful"
Hard to see what code is repeated

Letters to the editor: go to statement considered harmful Edsger W. Dijkstra

While Loops

while loops

while (condition) {
  instructions;
}

Check the condition
If the condition is true, run the instructions and repeat from #1
If the condition is false, don't run the instructions nor repeat

Flow chart

Diagram

Condition check
instructions, then go back to condition
next code

while true

What will be the output of this program?

while (1) {
}
print("done!\n");

Decrement e with while loop

while (e > 0) {
  e = e - 1;
}

Does the program ever end?
What will be the value of e after the while loop (if it ends)?

#include <stdio.h>

int main(int argc, char **argv) {
  int e;

  scanf("%d", &e);

  int result = 1;

  while (e > 0) {
    e = e - 1;
  }

  printf("%d\n", e);
}

Exponent with a while loop

How do we use while here?

#include <stdio.h>
#include <assert.h>

int main(int argc, char **argv) {
  int x;
  int e;

  scanf("%d", &x);
  scanf("%d", &e);
  assert(e >= 1);

  int result = 1;

 repeat:

  result = result * x;

  e = e - 1;

  if (e > 0) {
    goto repeat;
  }

  printf("%d\n", result);
}

#include <stdio.h>
#include <assert.h>

int main(int argc, char **argv) {
  int x;
  int e;

  scanf("%d", &x);
  scanf("%d", &e);
  assert(e >= 1);

  int result = 1;

  while (e > 0) {
    result = result * x;
    e = e - 1;
  }

  printf("%d\n", result);
}

Differences?

Repeated guessing game

Guess until right
Fixed guesses

#include <stdbool.h>
int answer;
int guess;

printf("enter the answer: ");
scan("%d", &answer);
printf("enter the guess: ");
scan("%d", &guess);

while (guess != answer) {
  printf("enter the guess: ");
  scan("%d", &guess);
}

Factorial

do-while

do {
  instructions;
} while (condition);

Run the instructions
Check the condition
If the condition is true, repeat from #1
If the condition is false, don't repeat repeat

Differences from while

int e;
scan("%d", e);
while (e > 0) {
  printf("%d\n", e);
  e = e - 1;
}

int e;
scan("%d", e);
do {
  printf("%d\n", e);
  e = e - 1;
} while (e > 0);

Summary of differences

while loops: run 0 or more times
do-while loopsl run 1 or more times

Compare exponent implementations

#include <stdio.h>
#include <assert.h>

int main(int argc, char **argv) {
  int x;
  int e;

  scanf("%d", &x);
  scanf("%d", &e);

  int result = 1;

  do {
    result = result * x;
    e = e - 1;
  } while (e > 0);

  printf("%d\n", result);
}

Compare guessing game implementations

Repeated Guessing Game

From homework

Boundary conditions

0, 1, and many

Fencepost problem

0-based or 1-based

Considering all inputs

Tips

[x, y), inclusive start, exclusive end (easy composition)
0 <= x means inclusive, x < 10 means exclusive
Know both 0-base (we'll see arrays later) and 1-base (for math-related stuff)

Clean up: exponent

Clean up: factorial

Clean up: finite guessing game

Example: newton's method

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

int main() {
  float a;  // the number to find square root of
  float x;  // the iterative guess

  printf("enter a number: ");
  /* a = (float) (getchar() - '0'); */
  scanf("%f", &a);
  printf("a: %f\n", a);

  // initially guess half of a
  x = a / 2;
  printf("initial x: %f\n", x);

  while (1) {
    x = x - (x * x - a)/(2 * x);
    /* x = x - (x * x - a)/(2); */
    printf("x: %f\n", x);
    sleep(1);
  }
}

Console animation

ssh terminal.shop

telnet starwarstel.net

ssh starwarstel.net

ttyquake / aaquake2 - Text Mode Quake II

Bouncing ball

44 lines of C

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

int main(int argc, char **argv) {
  int width = 40;
  int height = 20;
  int x = 0;
  int y = 0;
  int dx = 1;
  int dy = 1;
  while (1) {
    printf("\033[2J");
    printf("\033[H");
    for (int j = 0; j < height; j++) {
      for (int i = 0; i < width; i++) {
        if ((i == x) && (j == y)) {
          putchar('o');
        } else {
          putchar(' ');
        }
      }
      putchar('|');
      putchar('\n');
    }
    for (int i = 0; i < width; i++) {
      putchar('^');
    }
    putchar('\n');
    if ((0 <= (x + dx)) && ((x + dx) < width)) {
      dx = dx;
    } else {
      dx = dx * -1;
    }
    if ((0 <= (y + dy)) && ((y + dy) < height)) {
      dy = dy;
    } else {
      dy = dy * -1;
    }
    x = x + dx;
    y = y + dy;
    usleep(50 * 1000);
  }
  return 0;
}

The `char` type

A single character

char c;

c = 'a';

putchar(a);  // prints a character
fflush(stdout);  // ensure output

Roughly corresponding to keyboard keys. Run man ascii in bash for full set (there are other encodings as well, e.g., EBCDIC, ANSI, and Unicode).

Can also use %c in printf, e.g.,

printf("%c\n", c);

fflush is necessary in the default terminal mode, where output is only sent line-by-line

The newline character `\n`

\n means "go to the next line"

This is a non-printable character. Instead it controls the cursor of the terminal.

hello, world!\n vs. hello, world!

Terminal controls

Special characters to manipulate the terminal screen

\033[ followed by a code, e.g.,

clear screen 2J
move cursor back to top-left H

printf("\033[2J");  // clear screen
printf("\033[H");   // move cursor to top-left of terminal

ANSI Escape Sequences cheatsheet · GitHub

ANSI/VT100 Terminal Control

Console Virtual Terminal Sequences - Windows Console | Microsoft Learn

ANSI stands for the American National Standards Institute

Timing: sleep

Sleep: wait for a given amount of time

usleep(x) sleep for x microseconds

#include <unistd.h>
usleep(50 * 1000);  // sleep for 50 milliseconds (50,000 microseconds)

Example: draw with while

while_putchar.c

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

int main(int argc, char **argv) {
  printf("\x1b[2J");  // clear screen
  printf("\x1b[H");

  while (1) {

    putchar('o');
    fflush(stdout);

    usleep(50 * 1000);
  }
}

Example: movement

move.c

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

int main(int argc, char **argv) {
  int max = 40;
  int x;

  x = 0;
  while (1) {
    printf("\033[2J");
    printf("\033[H");

    int i = 0;
    while (i < max) {
      if (x == i) {
        putchar('o');
      } else {
        putchar(' ');
      }
      i = i + 1;
    }

    fflush(stdout);

    x = (x + 1) % max;

    usleep(50 * 1000);
  }
}

Start with just printing a characater

Example: 1-dimensional bounce

Add code to change direction

1d_bounce.c

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

int main(int argc, char **argv) {
  int max = 40;
  int x;
  int dx;

  x = 0;
  dx = 1;
  while (1) {
    printf("\x1b[2J");
    printf("\x1b[H");

    int i = 0;
    while (i < max) {
      if (x == i) {
        putchar('o');
      } else {
        putchar(' ');
      }
      i = i + 1;
    }

    fflush(stdout);

    x = x + dx;
    if (0 < x && x < (max - 1)) {
      dx = dx;
    } else {
      dx = dx * -1;
    }

    usleep(50 * 1000);
  }
}

Example: 2-dimensional bounce

Add a y-axis

2d_bounce.c

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

int main(int argc, char **argv) {
  int width = 40;
  int height = 20;
  int x = 0;
  int y = 0;
  int dx = 1;
  int dy = 1;
  while (1) {
    printf("\033[2J");
    printf("\033[H");
    int j = 0;
    while(j < height) {
      int i = 0;
      while (i < width) {
        if ((i == x) && (j == y)) {
          putchar('o');
        } else {
          putchar(' ');
        }
        i++;
      }
      putchar('|');
      putchar('\n');
      j++;
    }
    for (int i = 0; i < width; i++) {
      putchar('^');
    }
    putchar('\n');
    if ((0 <= (x + dx)) && ((x + dx) < width)) {
      dx = dx;
    } else {
      dx = dx * -1;
    }
    if ((0 <= (y + dy)) && ((y + dy) < height)) {
      dy = dy;
    } else {
      dy = dy * -1;
    }
    x = x + dx;
    y = y + dy;
    usleep(50 * 1000);
  }
  return 0;
}

Build from 1d bounce.

ANSI cursor control

Simplify positioning, horizontal/vertical positioning:

e.g., 3;2H

printf("\033[%d;%dH", 3, 2);

Terminal is a grid of characters (grid in xournal++), starting from 1, 1

bounce with ANSI

bounce_ansi.c

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

int main(int argc, char **argv) {
  int width = 40;
  int height = 20;
  int x = 1;
  int y = 1;
  int dx = 1;
  int dy = 1;
  while (1) {
    printf("\033[2J");
    printf("\033[H");
    printf("\033[%d;%dH", (int)y, (int)x);
    putchar('o');
    putchar('\n');
    if ((0 < (x + dx)) && ((x + dx) < width)) {
      dx = dx;
    } else {
      dx = dx * -1;
    }
    if ((0 < (y + dy)) && ((y + dy) < height)) {
      dy = dy;
    } else {
      dy = dy * -1;
    }
    printf("\033[%d;%dH", height + 1, 0);
    printf("y: %d\n", y);
    printf("x: %d\n", x);
    x = x + dx;
    y = y + dy;
    usleep(50 * 1000);
  }
  return 0;
}

Loops (While) COP-3223H

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