Week 8Week 5

COMP 111COMP 111

Introduction to Computer ScienceIntroduction to Computer Scienceand Object-Oriented Programmingand Object-Oriented Programming

Week 8

Feeling Out of the Loop?Feeling Out of the Loop?

Week 8

Methods - So Far Methods - So Far ……• Execute a sequence of statements

Declarations Assignment statements Return Maybe an output (e.g. System.out.println …)

• Execute in order First to last Until last is executed Or a return Or an error

• And from last week ….

With Expressions!

Week 8

Methods - So Far Methods - So Far ……• Optionally execute a sequence of statement• Alternatively execute

this sequence of statement or that sequence of statement

• With variations Series of comparisons Nested comparisons

With Conditons!

Week 8

But But ……

• While (as humans) we know how to Determine when an interest-bearing

investment reaches a target amount Do long division with remainders Average the scores of any 111 class on Test 1

• We cannot write methods that do these!(or at least not easily and in general!)

Week 8

They All Hinge on RepeatingThey All Hinge on Repeating

Describe how repetition drives … Determining when an interest-bearing

investment reaches a target amount Doing long division with remainders Averaging the scores of any 111 class

on Test 1

Week 8

Digging Deeper Digging Deeper ……

• In each case we repeat something gated by some critical test repeat something

• as long as• until

a test is true!

Week 8

They All Hinge on aThey All Hinge on a TestTest

Describe what key test drives … Determining when an interest-bearing

investment reaches a target amount Doing long division with remainders Averaging the scores of any 111 class

on Test 1

Week 8

Examples from the WorldExamples from the World• As long as the meat thermometer is less than

160 degrees, bake another 5 minutes• While the robot does not sense the wall ahead,

move it forward one unit

statementstatement

test false

truewhile a test is true statement

whatever is next

Week 8

Examples from the WorldExamples from the World• Keep going straight until you reach Putnam street• Continue adding stuff while the weight is less than 50 lbs

statementstatement

testfalse

true

do statementwhile a test is true

go on to next

Week 8

Examples from the WorldExamples from the World• Sum the usage minutes for

each day of the year• Compute 33! (33 factorial)

for (a range of counter values) statement

go on to nextstatementstatement

testcounter

false

true

initializecounter

adjustcounter

Week 8

The Java While LoopThe Java While Loop

• Want to repeat while something is true

statementstatement

test false

true

while (test){ statements}

Must the loop body be executed at least once?

the loopbody

Week 8

ConsiderConsider

• Compute integer division “the hard way” count how many times the divisor can be

subtracted from the dividend

dividenddivisorcount

1340

why stop?

A small “aha” fits this into an iteration framework?

Week 8

start the count at 0;while (the dividend >= than divisor){

subtract the divisor;increment the count;

}

Moving to the Moving to the ““WhileWhile””

• Compute integer division “the hard way” count how many times the divisor can be

subtracted from the dividenddividend

divisorcount

1340

542

941

143

Week 8

Realizing In JavaRealizing In Java

• Want to repeat while something is truepublic int divideBy(int num1, int num2){

int count = 0;int dividend = num1;

while (dividend >= num2){

dividend = dividend - num2;count++;

}return count;

}

Week 8

public int divideBy(int num1, int num2){

int count = 0;int dividend = num1;

while (dividend >= num2){

dividend = dividend - num2;count++;

}return count;

}

You Try It?You Try It?

Trace the value fordivideBy(20,5)

divideBy(3,5)

divideBy(7,7)

dividenddivisorcount 0

Week 8

public int divideBy(int num1, int num2){

int count = 0;int dividend = num1;

while (dividend >= num2){

dividend = dividend - num2;count++;

}return count;

}

The Gotcha!The Gotcha!

Trace the value fordivideBy(5,0)

divideBy(7,-2)

dividenddivisorcount 0

Week 8

public int divideBy(int num1, int num2){

int count = 0;int dividend = num1;

while (dividend > num2){

dividend = dividend - num2;count++;

}return count;

}

Another Gotcha!Another Gotcha!

Off-by-one errors!

dividenddivisorcount 0

Try divideBy(4,2)

BadTest

Week 8

The Java Do LoopThe Java Do Loop

statementstatement

testfalse

true

do{ statements}while (test);

• Always executes the body of the loop• At least once!

the loopbody

Week 8

ConsiderConsider

• Compute mystery math operationpublic int mystery(int m, int n){

int r;

do {

r = n % m;n = m;m = r;

} while (r != 0);

return n;}

mystery(8,20)

r n m 20 8

4 8 40 4 0

Week 8

ConsiderConsider

• Compute mystery math operationpublic int mystery(int m, int n){

int r;

do {

r = n % m;n = m;m = r;

} while (r != 0);

return n;}

mystery(20,8)

r n m 8 20

8 20 84 8 40 4 0

Week 8

ConsiderConsider

• Compute mystery math operationpublic int mystery(int m, int n){

int r;

do {

r = n % m;n = m;m = r;

} while (r != 0);

return n;}

mystery(3,11)

r n m 11 3

2 3 21 2 10 1 0

Week 8

What Is It?What Is It?

• Can you name the mystery operation mystery(20,8) = 4 mystery(8,20) = 4 mystery(3,11) = 1

Week 8

Another ProblemAnother Problem

What is the sum of thefirst n positive odd integers?

For example• the first 5 positive integers are 1, 3, 5, 7, 9• their sum is 25• easy, but what about sum of first 1000?

Week 8

A StrategyA Strategy What is the sum of the

first n positive odd integers?

• Start with odd number 1 with a sum of 0

• Repeat n times add the odd number to the sum find the next odd number

Week 8

Realizing the StrategyRealizing the Strategy What is the sum of the

first n positive odd integers?• Start

with odd number 1 with a sum of 0

• Repeat n times add the odd number to the sum find the next odd number

start the odd number at 1start the sum at 0;while (less than n times){ add the odd number to the sum; find next odd number;}

Week 8

Moving ForwardMoving Forward What is the sum of the

first n positive odd integers?• Start

with odd number 1 with a sum of 0

• Repeat n times add the odd number to the sum find the next odd number

int oddNum = 1;int sum = 0;

while (less than n times){ sum = sum + oddNum; oddNum = oddNum + 2;}

start the odd number at 1start the sum at 0;while (less than n times){ add the odd number to the sum; find next odd number;}

Week 8

Thinking About CountingThinking About Counting How do you count n times?

start a count at 0

while (count < n){ add 1 to the count;}

int count = 0;

while (count < n){ … count++;}

Note: when n is 5, this counts 0, 1, 2, 3, 4

How would you count 1, 2, 3, 4, 5?

Week 8

Putting It TogetherPutting It Together What is the sum of the

first n positive odd integers?

start the odd number at 1start the sum at 0;while (less than n times){ add the odd number to the sum; find next odd number;}

int count - 0;int oddNum = 1;int sum = 0;

while (count < n){ sum = sum + oddNum; oddNum = oddNum + 2; count++;}

int count = 0;

while (count < n){ … count++;}

Week 8

In a MethodIn a Methodpublic int sumFirstOdds(int n){

int count = 0;int oddNum = 1;int sum = 0;

while (count < n){ sum = sum + oddNum; oddNum = oddNum + 2; count++;}return sum;

}

sumFirstOdds(5)

n count oddNum sum

Week 8

You Try ItYou Try Itpublic int sumFirstOdds(int n){

int count = 0;int oddNum = 1;int sum = 0;

while (count < n){ sum = sum + oddNum; oddNum = oddNum + 2; count++;}return sum;

}

sumFirstOdds(1)

n count oddNum sum

Week 8

CountingCounting Is CommonIs Common

• Counting up, counting down, etc.• Using a counter (or index)

int count = 0;

while (count < n){ … count++;}

Week 8

The Java For LoopThe Java For Loop• Abbreviates the

initialization testing adjusting

of a counter• Implicitly executes them

for (init, testing, adjusting) statement statement

statement

testcounter

false

true

initializecounter

adjustcounter

implicit

implicit

implicit

Week 8

The Java The Java ForFor Loop Loop• Abbreviates the

initialization testing adjusting

of a counter

int count;for (count = 0; count < n; count++){ statements}

statementstatement

count< n

false

true

count = 0;

count++

implicit

implicit

implicit

Week 8

NotesNotes the the ForFor Loop Loop• Note the semicolons• Can declare counter within the for construct

for (int count = 0; count < n; count++){ statements}

Does the for replace the while loop?

Week 8

Using the Using the ForFor Loop Loop

public int sumFirstOdds(int n){

int oddNum = 1;int sum = 0;

for (count = 0; count < n; count++){ sum = sum + oddNum; oddNum = oddNum + 2;}return sum;

}

sumFirstOdds(5)

n count oddNum sum

Week 8

Would This Work?Would This Work?

public int sumFirstOdds(int n){

int oddNum = 1;int sum = 0;

for (count = 1; count <= n; count++){ sum = sum + oddNum; oddNum = oddNum + 2;}return sum;

}

sumFirstOdds(5)

n count oddNum sum