Sahar Mosleh California State University San MarcosPage 1 While Loop and For Loop.

Sahar Mosleh California State University San Marcos

While Loop

and

For Loop


While

- Programs are very good at performing similar tasks over and over

- In a payroll program, printing thousands of cheques involves a similar activity being repeated thousands of times

- Repetition in programming often called looping

- Repetition in C++ can be handled by the “while” looping construct


While Loops

while (logical expression)Statement;

Next-Statement;

Is logical expression true

Execute statement

Execute next statement

NoYes


While Loops

- Often used a compound statement in a while loop

while (logical expression) {

stmt 1; stmt 2; ……. ……. stmt n;}

As long as the logical expression is true, statements within the block will execute repeatedly(checks before each iteration)


Count Controlled While

As long as I have less than 10 parking tickets, I will continue to park illegally

- This is a count controlled loop- An activity will continue to take place until a specific count is reached

- Could be pseudo-coded as follows:



int ticket_ count = 0;

while (ticket_ count < 10) {

keep parking illegallyif (I get a ticket) {

ticket_count++;}

}

Loop controlled by counterthat must be given an initialvalue

Counter is checked at“top” of loop. If conditiontrue, body of loop isexecuted

Can “nest” constructswithin other constructs



int count = 1;

// Print out “Hello World!” 10 times

while (count <= 10) {

cout << “Hello World!” << endl; count++;

}

- What would happen if count was set to 0 initially?

< See: Example 1>


Event Controlled While

As long as I am not in LA, I will continue to drive

- This is an event controlled loop An activity will continue to take place until a specific event occurs

- Could be pseudo- coded as follows:



bool LA = false;

while (! LA) {keep drivingif (I am in LA) {

LA = true;}

}

Loop controlled by Booleanthat must be given an initialvalue

Boolean is checked at“top” of loop. If conditionis true, body of loop isexecuted



There can be many events that control a while loop– One of the most common is the “end of file”

(EOF) event


End of File

- Input file streams generally have a special EOF character at the very end

- When a program encounters this character, the EOF condition is raised

- This is often an event that stops the loop

- When reading an input file stream like cin or fin, the name of the file stream can be used as if it were a Boolean

- If the last input operation was successful it will be true -- false otherwise


End of File

#include <iostream>#include <fstream>using namespace std;int main(){

ifstream infile;int value;infile. open(“myfile.txt”);

infile >> value;while (infile) {

cout << value << endl;infile >> value;

}}< See: Example 2>

This input statement checks to see if the file is empty

If there is nothing more toread (EOF), then infile willeffectively be false and theloop will soon terminate


End of File#include <iostream>#include <fstream>using namespace std; int main(){

ifstream infile;int value;infile.open(“myfile.txt”);if (! infile)

{cout << “Oh no: (can’t open file!)” << endl; return;}

infile >> value ;while (infile) {

cout << value << endl;infile >> value;

}}

< See: Example 3>

Immediately after the open:infile is true if open successfulinfile is false if open unsuccessful


End of File

- This program can also be written as follows:

ifstream infile;int value;infile.open(“myfile”);

if ( ! infile){cout << “Oh no: (can’t open file!)”; return;

}infile >> value;while ( ! infile.eof() ) {

cout << value;infile >> value;

}

< See: Example 4>


While

- Whiles are often used as “undetermined” loops

- Not sure how long a while loop will last- When will EOF be found?

- Very often, it is known exactly how long a while loop last- Perhaps an idea to use a more deterministic looping construct


While

Recall the while construct:

stmt1;while (expr1) {

stmt1;stmt2;stmt3;stmtn;

}. . .

initialization

Anything to change the Statusof expre1 in the while (ex: increment)

continuation


For

The for repetition construct:

for (expr0; expr1; expr2) {stmt1;stmt2;stmt3;… ;.. ;stmtn;

}

The for loop is controlled by threestatements / expressions

initialization increment continuation


For

- A compact counted loop:

for (count= 1; count <= 10; count++) {stmt1;stmt2;stmt3;…. ;…. ;stmtn;

}

initializationincrementcontinuation

How many times willthis loop execute?


For

When it is known “ahead of time” how manytimes to loop, consider a for loop. For example,

- Adding marks of 100 students - Getting exactly 10 numbers from the user- Reading the first 5 numbers from an input file


For (example)

Write a for loops to print stars as follows:

* ****************************************


For

int i, stars;

// printing the first 5 linesfor (stars= 1; stars<= 9; stars= stars+ 2){ for (i= 1; i<= stars; i++)

cout << ‘* ’;cout << endl;

}

// printing the last 4 linesfor (stars= 7; stars>= 1; stars= stars- 2) {

for (i= 1; i<= stars; i++)cout << ‘* ’;

cout << endl ; }

Sahar Mosleh California State University San MarcosPage 1 While Loop and For Loop.

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