More Basic Karel

Karel can navigate a world by moving and turning left within it. Karel can also take balls and put balls down. Recall that these are the only four commands Karel knows:

move();
putBall();
takeBall();
turnLeft();

Karel resides in a boxed-in world. Let's review the basic components of Karel's world.

Karel's World

Karel's world is a grid composed of walls, streets, and avenues.

Walls:

Walls block Karel's movement. Walls include the borders of a world as well as any lines running through the world.

Karel cannot move through walls! If Karel attempts a move(); command while directly in front of a wall, Karel will crash into it.

Streets:

A street is a row in the grid.

The 5th row, or 5th street, is highlighted in the example world above.

Avenues:

An avenue is a column in the grid.

The 8th column, or 8th avenue, is highlighted in the example world above.

Karel's Direction

Karel can face one of four directions at any given time - north, east, south, or west. The direction Karel is facing determines which direction Karel will move. If Karel is facing north, a move(); command will make Karel move up. If Karel is facing east, a move(); command will make Karel move to the right. The same is true for the south and west directions.

Karel can always go right, left, or forward from any position.

For example, if Karel is initially facing east, a turnLeft(); command will result in Karel facing north.

Similarly, if Karel is initially facing north, a turnLeft(); command will result in Karel facing west.

Our Second Karel Program

Using our knowledge of direction, let's have Karel make a square out of balls.

Here is the solution. The comments explain which direction Karel ends up facing after each turnLeft(); command.

/* Tennis Ball Square
 * This program has karel place a square of tennis balls
 * and return to his starting point.
 */

//Karel begins facing east
putBall();
move();
turnLeft(); //Karel is now facing north

putBall();
move();
turnLeft(); //Karel is now facing west

putBall();
move();
turnLeft(); //Karel is now facing south

putBall();
move();
turnLeft(); //Karel finishes facing east.