What if ... Karel gets smart

Karel's Sensuary Equipment
IF ... THEN constructions
ITEATRE constructions
WHILE constructions

Karel's Sensuary Equipment

As you know, Karel has three video cameras and a small microphone to help him stay out of trouble. He can request the following information from his sensuary equipment:

front-is-clear front-is-blocked
left-is-clear left-is-blocked
right-is-clear right-is-blocked
facing-north not-facing-north
facing-south not-facing-south
facing-east not-facing-east
facing-west not-facing-west
next-to-a-beeper not-next-to-a-beeper
any-beepers-in-beeper-bag no-beepers-in-beeper-bag

Each request will result in a true or false answer. There are no ambiguitites.

IF .. THEN questions

Karel can check his video camera and microphone using the if ... then ... else construction. That construction could be inside a new definition or inside the execution block of the program. There are two variations of the IF construction, both of which can be nested inside eachother.

Instruction Syntax Explaination 
IF <test> THEN
   <instruction-block>
if test returns true:
Karel with execute the instruction(s) in the instruction block and then the instruction immediately following the instruction block.
if test returns false:
Karel will not execute the instruction(s) in the instruction block and will jump to the instruction immediately following the instruction block. 
IF <test> THEN
   <instruction-block>
ELSE
   <instruction-block>
if test returns true:
Karel with execute the instruction(s) in the first instruction block and then the instruction immediately following the second instruction block; in other words, the first instruction block is executed while the second one is not. Then he continues to execute the instructions after the second instruction block.
if test returns false:
Karel will not execute the instruction(s) in the first instruction block and will jump to the second instruction block, executing that; in other words, the first instructin block is skipped while the second one is not. Then he continues to execute the instructions after the second instruction block.

Here are some examples for IF ... THEN constructions. Note that especially nested IF ... THEN's can be quite confusing, and it is important to make sure that those types of instructions really work as intended.

Sample Program Explaination 
IF next-to-a-beeper THEN
   pickbeeper;
move
Karel checks if a beeper is present on the current corner. If there is one, he will pick it up and move on step forward. If there is no beeper present, he will just move one step forward. 
IF front-is-clear THEN
   move
ELSE
   turnleft;
Karel would move ahead only if his front is not blocked by a wall. If his front is blocked, he would turn left instead. 
DEFINE-NEW-INSTRUCTION face-north-if-south AS
   BEGIN
      IF facing-south THEN
         BEGIN
            turnleft;
            turnleft
         END
   END;
If Karel is facing south, this new command will turn him north. If Karel is not facing south, this new command will do nothing at all. 
DEFINE-NEW-INSTRUCTION escape AS
   BEGIN
      IF font-is-clear THEN
         BEGIN
            move;
            turnleft
         END
      ELSE
         BEGIN
            turnleft;
            move
         END
   END;
Karel would move and turn left if his front is clear. Otherwise, he would first turn left and then move. Note that if his front and left side was blocked, this new command would lead to an error shutoff. 
DEFINE-NEW-INSTRUCTION give-or-take AS
   BEGIN
      IF next-to-a-beeper THEN
         IF any-beepers-in-beeper-bag THEN
            putbeeper
         ELSE
            pickbeeper
   END;
Karel would first check if he is next to a beeper. If so, he would add another beeper to the current corner, but only if there was one in his beeper bag. If he was next to a beeper but without beepers in his beeper bag he would pick up the beeper from the current corner. If he was not near a beeper, he would do nothing.

Repeating instructions: ITERATE

Karel has a simple way to repeat instructions any number of times without us having to retype the commands. That new constructions if defined using the reserved words ITERATE and TIMES. 

   ITERATE <positive-number> TIMES
      <instruction-block>
Sample Program Explaination 
DEFINE-NEW-INSTRUCTION turnright AS
   BEGIN
      ITERATE 3 TIMES
         turnleft;
   END;
The basic instruction turnleft is repeated three times, which is equivalent to a right turn. 
DEFINE-NEW-INSTRUCTION harvest-one-row AS
   BEGIN
      pickbeeper;
      ITERATE 4 TIMES
         BEGIN
            move;
            pickbeeper
         END
   END;
Karel would pick a single beeper, and then repeat the sequence move; pickbeeper four times. The new instruction therefore would execute five pickbeeper instructions and four move's.

Repeating instructions: WHILE

There are many situation where Karel needs to repeat an instruction but it is not yet know how often. For example, if Karel encounters a pile of beepers, he needs to repeatedly execute the pickbeeper command, but since he does not know the number of beepers in the pile he does also not know how often to execute that command.

The WHILE construction is made-to-order for this situation: you can use it to tell Karel to repeat something until a certain test is met; for example to pick up beepers until a pile is empty. 

      WHILE <test> DO
         <instruction-block>
Sample Program Explaination 
DEFINE-NEW-INSTRUCTION get-all-beepers AS
   BEGIN
      WHILE next-to-a-beeper DO
         pickbeeper
   END;
Karel would pick up all beepers on the current corner, regardless how many there are (if it is a finite number, at least). 
DEFINE-NEW-INSTRUCTION harvest-one-row AS
   BEGIN
      WHILE next-to-a-beeper DO
         BEGIN
            pickbeeper;
            move
         END
    END
Karel would pick up all beepers in a row, regardless of how many beepers there are. The beepers must form an uninterrupted line, otherwise Karel would stop. After picking up the last beeper in the row, Karel would move one more step.

(bgw)