DecompositionSolving a large problem by breaking it into smaller

subproblems

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Decomposition

• Avoiding repeating the same command sequences for the same task

• Breaking a large problem down into smaller pieces (subproblems) that are easier to solve

• Ex.

• turnRight()

• turnAround() and backup()

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PotholeFillingKarel

Before After

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PotholeFillingKarel

Public void run() {

move();

fillPothole();

move();

}

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A small exercise

• Create two more potholes

• Command Karel to fill that holes

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Control Statementsin Karel

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Every day we repeat

• Our life

• Wake up

• Get dress

• Go school

• Study

• Go back home

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Two types of control statements

• Conditional statements

• Iterative statements

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Conditional Statements

• Is there a beeper in a hole?

• if yes, Karel should not put a new beeper in it

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Conditions in Karel• Karel can test the following conditions:

frontIsClear() frontIsBlocked()

leftIsClear() leftIsBlocked()

rightIsClear() rightIsBlocked()

beepersPresent() noBeepersPresent()

beepersInBag() noBeepersInBag()

facingNorth() notFacingNorth()

facingEast() notFacingEast()

facingSouth() notFacingSouth()

facingWest() notFacingWest()

positive condition negative condition

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IF

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IF Syntax

if (conditional test) {

statements

}

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PotholeFillingWithCondition

public void run() {

move(); fillPothole(); move(); }

private void fillPothole() { turnRight(); move(); if(noBeepersPresent()){ putBeeper(); } turnAround(); move(); turnRight(); }

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Iterative Statements

• Tell a machine to repeat the same procedure for the same purpose

• Powerful tool

• Your mission is to instruct Karel to fill beepers for all potholes along the street

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The for Statement

• In Karel, the syntax of for statement is:

for (int i = 0; i < count; i++) { statements to be repeated}

for (int i = 0; i < count; i++) { statements to be repeated}

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The for Statement

• In Karel, the syntax of for statement is:

for (int i = 0; i < count; i++) { statements to be repeated}

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The for Statement

• In Karel, the syntax of for statement is:

for (int i = 0; i < count; i++) { statements to be repeated}

for (int i = 0; i < count; i++) { statements to be repeated}

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Improve turnRight()

public void turnRight() {

for(int i = 0; i < 3; i++) {

turnLeft();

}

}

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KarelFillManyPotholes

for(int i=0; i< 5; i++) {

move();

fillPothole();

move();

}

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If you don’t know a number of repeated steps

While (conditions) {

statements to be repeated

}

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KarelFillManyPotholes

while ( frontIsClear() ) {

move();

fillPothole();

move();

}

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Solving a general problem(KarelFixPotholes.java)

• Your algorithm should work with any length of the street

• Potholes may occur any position of the street

• Some potholes are already fixed

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Exercise

• You need to create a world for your buddy to test his/her KarelFixPotholes.java

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Fencepost Error

How many fenceposts do you need?

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Assignment 2

• There is an earthquake and Karel is hired to repair broken stones

• Instruction

• Download Assignment2.zip from course website

• Write your codes in StoneMasonKarel.java

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Summary

• Control statements

• IF

• For loop

• While loop

• Decomposition will help you to improve your programming skill.

• Good luck for your assignments

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