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Iteration (Loops) - PowerPoint PPT Presentation

Iteration (Loops). Loops repeat a set of instructions Two types of loops: Definite loops ( for ) perform instructions explicit (known) number of times. Indefinite loops ( while ) perform instructions an indefinite (unknown) number of times (until boolean test fails). for Loops.

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• Loops repeat a set of instructions

• Two types of loops:

• Definite loops ( for ) perform instructions explicit (known) number of times.

• Indefinite loops ( while ) perform instructions an indefinite (unknown) number of times (until boolean test fails).

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for Loops

• General form:for ( <initialize variable>; <Boolean test>; <increment> ){ <instruction set>}

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for Loops (cont.)

• <initialize variable> sets a variable/identifier to a certain value (variable is usually count, i, j)

• <Boolean test> is a test that is evaluated each time the body is about to be executed (when false, loop is exited)

• <increment> changes the loop control variable (usually i++ or i--)

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• What does i++ mean

• i++ is the same as i = i + 1;

• That is not true you say.

• This does not imply i = i + 1, it increases the value of i by 1

• E.g., if i has the value 10, then i++ changes the value of i to 11

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• What does i-- mean

• i-- is the same as i = i - 1;

• That is not true you say.

• This does not imply i equals i - 1, it decreases the value of i by 1

• E.g., if i has the value 10, then i-- changes the value of i to 9

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for Loops (cont.)

• Example:for (int x=1; x <= 5; x++){ karel.move();}

• This causes karel to move 5 times

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for Loops (cont.)

Flow of for loops:

• Initialization statement executes

• If test is true, proceed to step 3; if test is false, go to step 6

• Instructions in body of loop are executed

• Increment statement executes

• Program proceeds to statements after loop

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while Loops

• General form:while ( <boolean test> ){ <instruction list>}

What do you know here?

• Test = false

• Loop continues until test is false

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while Loops (cont.)

• Example:while (karel.frontIsClear()){ karel.move ();}

• What do you know? – karel is facing a wall

• Causes karel to move continuously until there is a wall in front of it

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while Loops (cont.)

Flow of while loops

• If test is true, proceed to step 2; if test is false, go to step 4

• Instructions in body of loop are executed

• Go to step 1

• Statements after loop are executed

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• In a for or while loop, it is possible for the test to never be false

• When this happens, the loop continues infinitely

• Depending on the compiler, application, and other circumstances, an error may occur or the app may crash

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while Loops (cont.)

• Write a method that turns the Robot Left until the Robot is facing North.

public void faceNorth()

{

while (! facingNorth() )

turnLeft();

}

* note: the number of left turns needed is unknown

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• Pick up seven beepers.

public void pick7Beepers()

{

// what type of loop?

}

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• Pick up seven beepers.

public void pick7Beepers()

{

for(int j = 1; j <= 7; j++)

pickBeeper();

}

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• Move forward until next to another Robot.

public void moveToRobot()

{

// what type of loop

}

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• Move forward until next to another Robot.

public void moveToRobot()

{

while (! nextToARobot() )

move();

}

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• Create a method that moves the Robot forward a specific number of times.

public void moveForward(int numMoves)

{

for(int j = 1; j <= numMoves; j++)

move();

}

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public void moveForward(int numMoves)

{

for(int j = 0; j < numMoves; j++)

move();

}

• numMoves is a parameter provided by the client.

Sample calls:

moveForward(11); // moves the robot forward 11 times

moveForward(7); // moves the robot forward 7 times

moveForward(15+7); // moves the robot forward 22 times

moveForward(15/4+7); // moves the robot forward 10 times

moveForward(num); // moves the robot forward num times

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• Create a method that has the Robot put a specific number of Beepers.

public void put_N_Beepers(int numBeeps)

{

for(int j = 1; j <= numBeeps; j++)

putBeepers();

}

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• Like nested if’s, it is possible to have a loop inside a loop

for(int j = 0; j < 3; j++)

{

for(int k = 0; k < 2; k++)

System.out.println(j + “,“ + k);

}

• Generates the following output:

Note: Each pair of numbers are on separate lines

0,0 0,1 1,0 1,1 2,0 2,1

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for(int j = 0; j < 2; j++)

{

int k = 0;

while(k < 4)

{

System.out.println(j + “,“ + k);

k++;

}

}

• Generates the following output:

Note: Each pair of numbers are on separate lines

0,0 0,1 0,2 0,3 1,0 1,1 1,2 1,3

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for(int j = 0; j < 5; j++)

{

int k = 0;

while(k < 3)

{

k++;

System.out.println(j + “,“ + k);

}

}

• Generates the following output:

Note: Each pair of numbers are on separate lines

0,1 0,2 0,3 1,1 1,2 1,3 2,1 2,2 2,3

3,1 3,2 3,3 4,1 4,2 4,3

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Suppose

public int calculate(int a, int b) {

return 2 * a - b + 7;

}

then

for(int j = 0; j < 3; j++)

{

int k = 0;

while(k <= 3)

{

k++;

System.out.println( calculate(j, k) );

}

• Generates the following output:

Note: Each number is on a separate line.

6, 5, 4, 3, 8, 7, 6, 5, 10, 9, 8, 7, 6

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• Implement methods declared in TreasureSeekerBot class.

• Invoke MainDriver1 to test your implementation. If you correctly implement each method, each Robot will perform a specific task.

• You will then implement one additional method required by MainDriver2 allowing a Robot to following a specific series of commands to find the Hidden treasure!

• See handout (Karel_part5_loops.doc) for details.

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• Your assignment is to implement the NewAndImprovedBeeperSweeperRobotRobot that picks ALL beepers on every corner in a room.

• Invoke MainDriver1 to test your implementation. If you correctly implement the class, the Robot will collect ALL beepers from every corner in the Room.

• The are also different in this assignment. The Rooms will no longer be a constant size. All rooms will 6 wide, but the length will be unknown.

• Did you hear me say while loop?

• In MainDriver2, you will construct Robots to collect All beepers from ALL corner in ALL rooms!

• See handout (Karel_part5-1_loops.doc) for details.

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• Your assignment is to do lab 16.

• All three (oops that is four) parts!

• See handout (Karel_part6.doc) for details.

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