for_loop CIS 1068 FOR LOOP PPT

8/11/2019 for_loop CIS 1068 FOR LOOP PPT

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The forloop


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Repetition

How can we eliminate this redundancy?

System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");

System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");System.out.println("I will not throw the principals toupee down the toilet");


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Control Structure

The forloop is a control structurea syntactic

structure that controlsthe execution of other

statements.

Example:

Shampoo hair. Rinse. Repeat.


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forloop over range of ints

We'll write forloops over integers in a given range.

The declares a loop counter variable that is used in the test,update, and body of the loop.

for (int = 1;


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forloop flow diagramfor (; ; ) {

;

;

...

;

}


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Loop walkthrough

Code:for (int i = 1; i


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Loop example

Code:System.out.println("+----+");

for (int i = 1; i


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Varying the forloop

The initial and final values for the loop counter variable can be arbitraryexpressions:

Example:for (int i = -3; i


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Varying the forloop

The update can be a --(or any other operator). Caution: This requires changing the test from =.

System.out.println("T-minus");

for (int i = 3; i >=1; i--) {

System.out.println(i);

}

System.out.println("Blastoff!");

Output:T-minus

3

21

Blastoff!


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Aside: System.out.print

What if we wanted the output to be the following?

T-minus 3 2 1 Blastoff!

System.out.printprints the given output without

moving to the next line.

System.out.print("T-minus ");

for (int i = 3; i >= 1; i--) {

System.out.print(i + "");

}

System.out.println("Blastoff!");


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Errors in coding

When controlling a single statement, the {}braces are optional.

for (int i = 1; i


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Errors in coding

ERROR: Loops that never execute.

for (int i = 10; i < 5; i++) {

System.out.println("How many times do I print?");

}

ERROR: Loop tests that never fail.

A loop that never terminates is called an infinite loop.

for (int i = 10; i >= 1; i++) {

System.out.println("Runaway Java program!!!");

}


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forloop exercises

Write a loop that produces the following output.

On day #1 of Christmas, my true love sent to me



On day #4 of Christmas, my true love sent to meOn day #5 of Christmas, my true love sent to me

...


Write a loop that produces the following output.2 4 6 8

Who do we appreciate


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Special case: If a variable is declared in thepart of a forloop, its scope is the

forloop.

public static void main(String [] args) {

int x = 3;

for (int i = 1; i


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Errors in coding

ERROR: Using a variable outside of its scope.

public static void main(String[] args) {

for (int i = 1; i


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Loops with if/else

Loops can be used with if/elsestatements.

int nonnegatives = 0, negatives = 0;

for (int i = 1; i = 0) {

nonnegatives++;

} else {negatives++;

}

}

Exercise: Describe in English what this code does.


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Exercises

Write a program named CountFactors that reads in an

integer and displays its number of factors.

For example, if the user enters 60, CountFactors displays 12

because 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and 60 are all factors

of 60.


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Nested forloops

nested loop: Loops placed inside one another.

Caution:Make sure the inner loop's counter variable has a different name!

for (int i = 1; i


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Nested loops example



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Exercise: Nested loops

What nested forloops produce the following output?

....1...2

..3

.4

5

Key idea: outer "vertical" loop for each of the lines

inner "horizontal" loop(s) for the patterns within each line

inner loop (repeated characters on each line)

outer loop (loops 5 times because there are 5 lines)


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Nested loops

First, write the outer loop from 1to the number of lines desired.

for (int line = 1; line


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Errors in coding

ERROR: Using the wrong loop counter variable.

What is the output of the following piece of code?for (int i = 1; i


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Commenting: forloops

Place a comment on complex loops explaining what they do from a conceptualstandpoint, not the mechanics of the syntax.

Bad:

// This loop repeats 10 times, with i from 1 to 10.

for (int i = 1; i


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Mapping loops to numbers

Suppose that we have the following loop:

for (int count = 1; count


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Mapping loops to numbers

Now consider another loop of the same style:

for (int count = 1; count


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Loop number tables

What statement could we write in the body of the loop that would

make the loop print the following output?

2 7 12 17 22

To find the pattern, it can help to make a table.

Each time countgoes up by 1, the number should go up by 5.

But count * 5is too big by 3, so we must subtract 3.

1720174

22

12

7

2

number to print

25

15

10

5

count * 5

225

123

72

21

count * 5 - 3count


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Another perspective: Slope-intercept

-10

-5

0

5

10

15

20

25

-2 0 2 4 6 174

22

12

7

2

number to print (y)

5

3

2

1

count (x)


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Caution: This is algebra, not assignment! Recall: slope-intercept form (y = mx + b)

Slope is defined as rise over run (i.e. rise / run). Since the run isalways 1(we increment along xby 1), we just need to look at the rise.The rise is the difference between the yvalues. Thus, the slope (m) isthe difference between yvalues; in this case, it is +5.

To compute the y-intercept (b), plug in the value of yat x = 1andsolve for b. In this case, y = 2.y = m * x + b

2 = 5 * 1 + b

Then b = -3

So the equation is

y = m * x + by = 5 * x 3

y = 5 * count - 3 174

22

12

7

2

number to print (y)

5

3

2

1

count (x)


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Algebraically, if we always take the value of yatx = 1, then we can solve for bas follows:

y = m * x + b

y1 = m * 1 + b

y1= m + bb = y1m

In other words, to get the y-intercept, just subtract theslope from the first yvalue (b = 2 5 = -3) This gets us the equation

y = m * x + by = 5 * x 3

y = 5 * count 3

(which is exactly the equation from the previous slides)


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Loop table exercise

What statement could we write in the body of the loop that would

make the loop print the following output?

17 13 9 5 1

Let's create the loop table together.

Each time count goes up 1, the number should ...

But this multiple is off by a margin of ...

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20

-12

-8

-4

count * -4

1

9

13

17

count * -4 + 21

54

1

9

13

17

number to print

5

3

2

1

count

for_loop CIS 1068 FOR LOOP PPT

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