Chapter 4 Loops

Chapter 4 Loops

§4.1 The “while” Loop§4.2 The “do-while” Loop§4.3 The “for” Loop§4.4 Nested loop§4.5 “break” and “continue”§4.6 Simple File Input and Output

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Liang, Introduction to C++ Programming, (c) 2007 Pearson Education, Inc. All rights reserved. 013225445X

Objectives To master the usage of while, do-while, and for

loops statements To understand the flow of control in loops To use Boolean expressions to control loops To know the similarities and differences of three

types of loops To know the use functionality of break and continue To know how to read and write data from/to a file

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§4.1 The “while” Loopwhile (loop-continuation-condition)

{

// loop-body;

Statement(s);

}

int count = 0;while (count < 100) { cout << "Welcome to C++!\n"; count++;}

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int count = 0;

while (count < 2)

{

cout << "Welcome to C++!";

count++;

}

(count < 2) is still true since count is 1

(count < 2) is false since count is 2 now

Increase count by 1count is 2 now

Print Welcome to C++

Trace while Loop

The loop exits. Execute the next statement after the loop.

Initialize count(count < 2) is truePrint Welcome to C++Increase count by 1count is 1 now

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Example: An Advanced Math Learning Tool

The Math subtraction tutor program in Listing 3.6, SubtractionTutor.cpp, generates just one question for each run.

You can use a loop to generate questions repeatedly.

Listing 4.1 gives a program that generates ten questions and reports the number of the correct answers after a student answers all ten questions.

The program also displays the time spent on the test and lists all the questions, as shown in sample output.

SubtractionTutorLoopSubtractionTutorLoop Run

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Controlling a Loop with User Confirmation

char continueLoop = 'Y';while (continueLoop == 'Y') { // Execute body once // Prompt the user for confirmation cout << "Enter Y to continue and N to quit: "; cin >> continueLoop;}

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Ending a Loop with a Sentinel Value Often the number of times a loop is executed is not predetermined. You may use an input value to signify the end of the loop. Such a value is known as a sentinel value.

Write a program that reads and calculates the sum of an unspecified number of integers.

The input 0 signifies the end of the input.

SentinelValueSentinelValue Run

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CautionDon’t use floating-point values for equality

checking in a loop control. Since floating-point values are

approximations, using them could result in imprecise counter values and inaccurate results.

double data = pow(sqrt(2.0), 2) - 2;if (data == 0) cout << "data is zero";else cout << "data is not zero";

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§4.2 The “do-while” Loop

do

{

// Loop body;

Statement(s);

} while (loop-continuation-condition);

Loop Continuation Condition?

true

Statement(s) (loop body)

false

“while”

“do-while”

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ExampleListing 4.3: a program that reads and

calculates the sum of an unspecified number of integers. The input 0 signifies the end of the input.

TestDoWhileTestDoWhile Run

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§4.3 The “for” Loopfor (initial-action; loop-continuation-condition; action-after-each-iteration) { // loop body; Statement(s);}

int i;

for (i = 0; i < 100; i++)

{

cout << "Welcome to C++!\n";

}

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int i;for (i = 0; i < 2; i++) { cout << "Welcome to C++!"; }

Execute adjustment statement i now is 2

Trace for Loop

Declare iExecute initializeri is now 0

(i < 2) is true since i is 0

Print Welcome to C++!Execute adjustment statement i now is 1

(i < 2) is still true since i is 1

Print Welcome to C++(i < 2) is false since i is 2Exit the loop. Execute the next

statement after the loop

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Note All the three control expressions in “for” loop can be omitted But the semicolons can not be omitted

1) sum=0; i=1; for (; i<=100; i++) sum += i;

1) sum=0;for (i=1; ; i++){ if (i<=100) break; sum += i;}

1) sum=0;for (i=1; i<=100; ){ sum += i;

i++;}

4) sum=0; i=1; for (; i<=100;){ sum += i;

i++;}

5) sum=0; i=1; for ( ; ; ){ if (i<=100) break; sum += i;

i++;}

-- If the loop-continuation-condition in a for loop is omitted, it is implicitly true.-- It may result in infinite loop.

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Note The initial-action and action-after-each-iteration can

be a list of comma-separated expressions rarely used in practice

for (int i = 0, j = 0; (i + j < 10); i++, j++) { // Do something}

“,” operator

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“,” Operator“,”: concatenate expressions and the value of

the rightmost expression is the value of the whole comma expression

Example：int main(){

int x, y;x=50;for (int i = 0, j = 0; i + j < 10; i++, j++) {

y=(x=x-5, x/5); cout<<x<<" "<<y<<endl;

}}

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Example: Using for Loops

Problem: Write a program that sums a series that starts with 0.01 and ends with 1.0.

The numbers in the series will increment by 0.01, as follows: 0.01 + 0.02 + 0.03 and so on.

TestSumTestSum Run

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Floating-point Numbers in Relational Expressions

int main() { // Initialize sum double sum = 0; double i=0; // Add 0.01, 0.02, ..., 0.99, 1 to sum for (i = 0.01; i <= 1.0; i = i + 0.01) sum += i; // Display result cout << "The sum is " << sum; return 0;

}

int main(){ // Initialize sum double sum = 0; double prei=0; double i;

// Add 0.01, 0.02, ..., 0.99, 1 to sum for (i = 0.01f; i <= 1.0f; i = i + 0.01f){ sum += i;

prei=i; } // Display result cout << "The sum is " << sum<<endl; cout<<setprecision(10)<<"i is increaded by "<<i-prei<<endl; cout<<"The final i is "<<i<<", and i-1.0 is " <<(i-1.0)<<endl;

return 0;}

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Which Loop to Use?

while (loop-continuation-condition) { // Loop body }

(a)

Equivalent

(b)

for ( ; loop-continuation-condition; ) { // Loop body }

for (initial-action; loop-continuation-condition; action-after-each-iteration) { // Loop body; }

(a)

Equivalent

(b)

initial-action; while (loop-continuation-condition) { // Loop body; action-after-each-iteration; }

while, do-while, and for, are expressively equivalent and mutually conversable.

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Which Loop to Use?“for”:

if the number of repetitions is known.“while”:

if the number of repetitions is unknown“do-while”:

to replace a while loop if the loop body has to be executed before testing the continuation condition.

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§4.4 Nested loopLoop with one outer loop and one or more

inner loops

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Nested Loops

Listing 4.5: a program that uses nested for loops to print a multiplication table.

TestMultiplicationTableTestMultiplicationTable Run

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Loop Example:Finding the Greatest Common Divisor

Problem: Write a program that prompts the user to enter two positive integers and finds their greatest common divisor.

Solution: Let the two input integers be n1 and n2. You can check whether k (for k = 2, 3, 4, and so on) is a common divisor for n1 and n2, until k is greater than n1 or n2.

GreatestCommonDivisorGreatestCommonDivisor RunRun

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Loop Example: Finding the Sales Amount

Problem: You have just started a sales job in a department store. Your pay consists of a base salary and a commission. The base salary is $5,000. The scheme shown below is used to determine the commission rate.

Sales Amount Commission Rate$0.01–$5,000 8 percent$5,000.01–$10,000 10 percent$10,000.01 and above 12 percent

Your goal is to earn $30,000 in a year. Write a program that will find out the minimum amount of sales you have to generate in order to make $30,000.

FindSalesAmountFindSalesAmount RunRun

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Loop Example: Displaying a Pyramid of Numbers

Problem: Write a program that prompts the user to enter an integer from 1 to 15 and displays a pyramid.

For example, if the input integer is 12, the output is shown below.

PrintPyramidPrintPyramid RunRun

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§4.5 “break” and “continue”break:

May only be used inside a loop or a switch statement.

To terminate the current loop/switch immediately and transfer control to the statement immediately following that loop/switch.

sum=0; mark=1; for ( i=0; ; ){ if (i>100) break; sum += i; i++;}if(i>0) avgnum = sum/I;…

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“continue”“continue”:

May only be used inside a loop.To terminate the current iteration of the loop and

proceeds directly to the next. In the case of a for loop it jumps to its action-after-

each-iteration.

sum=0; mark=1; for ( i=0; ; ){ if (i>100) continue; sum += i; i++;}if(i>0) avgnum = sum/I;…

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Examples

Use of “break”

Use of “continue”

TestBreakTestBreak

TestContinueTestContinue

Run

Run

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Loop Example: Displaying Prime Numbers

Problem: Write a program that displays the first 50 prime numbers in five lines, each of which contains 10 numbers.

An integer greater than 1 is prime if its only positive divisor is 1 or itself. For example, 2, 3, 5, and 7 are prime numbers, but 4, 6, 8, and 9 are not.

Solution: The problem can be broken into the following tasks:•For number = 2, 3, 4, 5, 6, ..., test whether the number is prime.

•Determine whether a given number is prime.•Count the prime numbers.•Print each prime number, and print 10 numbers per line.

PrimeNumberPrimeNumber RunRun

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§4.6 Simple File Input and OutputTo read input from the keyboard and write

output to the screen/console?“cin” and “cout”

To read and write data from/to a file?ifstream and ofstream

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Simple File Input and Output

SimpleFileOutputSimpleFileOutput RunRun

SimpleFileInputSimpleFileInput RunRun

To write to a file

To read from a file

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Testing End of a FileListing 4.3 read only three linesHow to determine when to stop reading if you

don’t know how many numbers are in a file?Use the eof() function to detect it.

Listing 4.14 revises Listing 4.13 to read all lines from the file numbers.txt.

TestEndOfFileTestEndOfFile RunRun

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Summary“while” and “do-while” loops“for” loopsDifference between loop stementsUse of “break” and “continue”Simple file I/OTesting of end-of-file

Chapter 4 Loops

Documents

nested loop4

types of loopsto

use functionality of

flow of control

int count

loopbody statements

simple file input

loopwhile loopcontinuation