Math2220 Lect5

while loop

while (condition)
{

     statement1;

     statement2;


     .............

     statement_last;

}

How it works?

When the computer encounters the while line, it tests the condition.
If the condition is true:

statement1 to statement_last will be performed.
after all the statements are performed, the computer goes back the the while line and do the same thing all over again

if the condition is false:

computer skips all the statements
it goes to the line immediately after the close brace }

i.e. statements will be performed as long as the condition is true.
if there is only one statement in the loop, the braces can be omited.

Example

#include<iostream>
using namespace std;
void main() {
    int n=1;
    while (n < 10) {
        cout << n << " is strictly smaller than 10.\n";
        n++;
    }

    char line[100];
    cin.getline(line,100);
}

Sample output

1 is strictly smaller than 10.
2 is strictly smaller than 10.
3 is strictly smaller than 10.
4 is strictly smaller than 10.
5 is strictly smaller than 10.
6 is strictly smaller than 10.
7 is strictly smaller than 10.
8 is strictly smaller than 10.
9 is strictly smaller than 10.

Example

#include<iostream>
using namespace std;

void main() {
    int countDown;

    cout << "How many greetings do you want? ";
    cin >> countDown;
    
    
   while(countDown > 0) {
        cout << "Hello ";
        countDown--;
    }
    

    cout << endl;
    cout << "That's all!\n";

    char line[100];
    cin.getline(line,100);
        cin.getline(line,100);
}

Sample output

How many greetings do you want? 3
Hello Hello Hello
That's all!

How many greetings do you want? 0

That's all!

Example

#include<iostream>
using namespace std;
void main() {
    char ch;
    int counter=0;
    cout << "Please enter a character (enter Q or q to quit): ";
    cin >> ch;
    counter++;
    while((ch!='Q') && (ch!='q'))
    {
        cout << "Please enter a character (enter Q or q to quit): ";
        cin >> ch;
        counter++;
    }
    cout << "Your entered " << counter << " characters.\n";


    char line[100];
    cin.getline(line,100);
    cin.getline(line,100);
}

Sample output

Please enter a character (enter Q or q to quit): A
Please enter a character (enter Q or q to quit): B
Please enter a character (enter Q or q to quit): C
Please enter a character (enter Q or q to quit): d
Please enter a character (enter Q or q to quit): e
Please enter a character (enter Q or q to quit): k
Please enter a character (enter Q or q to quit): l
Please enter a character (enter Q or q to quit): q
Your entered 8 characters.

do-while loop

do {

    statement1;


    statement2;

    ..........

    statement_last;

} while (condition);

Don't forget the last semicolon!!!

How it works?

the computer performs all the statements first.
when it reaches the while line. It tests the condition.
if the condition is true, it performs the statements again
if the condition is false, it goes to the next line.
if there is only one statement in the loop, the braces can be omited.

Example

#include<iostream>
using namespace std;

void main() {
    int countDown;

    cout << "How many greetings do you want? ";
    cin >> countDown;
    do {
        cout << "Hello ";
        countDown--;
     } while(countDown > 0);
     // Don't forget the last semicolon ; 

    cout << endl;
    cout << "That's all!\n";

    char line[100];
    cin.getline(line,100);
    cin.getline(line,100);
}

Sample output

How many greetings do you want? 3
Hello Hello Hello
That's all!

How many greetings do you want? 0
Hello
That's all!

Example

#include<iostream>
using namespace std;
void main() {
    char ch;
    int counter=0;

    do {
        cout << "Please enter a character (enter Q or q to quit): ";
        cin >> ch;
        counter++;
    }while((ch!='Q') && (ch!='q')); 
    // Don't forget the last semicolon ; 

    cout << "Your entered " << counter << " characters.\n";

    char line[100];
    cin.getline(line,100);
    cin.getline(line,100);
}

Sample output

Please enter a character (enter Q or q to quit): A
Please enter a character (enter Q or q to quit): B
Please enter a character (enter Q or q to quit): C
Please enter a character (enter Q or q to quit): d
Please enter a character (enter Q or q to quit): e
Please enter a character (enter Q or q to quit): k
Please enter a character (enter Q or q to quit): l
Please enter a character (enter Q or q to quit): q
Your entered 8 characters.

Example

/*
Enter your score (within 0 to 100).
The program will find out your grade.
A: 90-100
B: 80-90
C: 65-80
D: 50-65
*/
#include<iostream>
using namespace std;

void  main() {
    double score;
    char again; // y or n
       do {
        cout << "Please enter your score: ";
        cin >> score;
        if(score> 100 || score < 0) {
            cout << "Invalid score.\n";
            cout << "Please check the score again.\n";
        } else if (score >= 90) {
            cout << "Your grade is A.\n";
        } else if (score >= 80) {
            cout << "Your grade is B.\n";
        } else if (score >= 65) {
            cout << "Your grade is C.\n";
        } else if(score >= 50) {
            cout << "Your grade is D.\n";
        } else {
            cout << "You fail!\n";
        }
        cout << "Continue(y/n)? ";
        cin >> again;
    } while (again=='y' || again=='Y');

    char line[100];
    cin.getline(line,100);
    cin.getline(line,100);
}

Sample output

Please enter your score: 53
Your grade is D.
Continue(y/n)? y
Please enter your score: 78
Your grade is C.
Continue(y/n)? y
Please enter your score: 120
Invalid score.
Please check the score again.
Continue(y/n)? y
Please enter your score: 92
Your grade is A.
Continue(y/n)?n

Difference between while and do-while

in while loop, the condition is checked before the loop is executed.
in do-while loop, the loop body is at least executed once.

Pitfall : Using = in place of ==

Question:

What is the output of the following program if I enter 20 ?
Notice that I use age=21 instead of age==21

Example

#include<iostream>
using namespace std;

void main() {
    int age;
    cout << "How old are you? ";
    cin >> age;
    if(age=21) {
        cout << "You are 21.\n";
    } else {
        cout << "You are not 21.\n";
    }

    char line[100];
    cin.getline(line,100); 
    cin.getline(line,100);
}

Sample output

How old are you? 20
You are 21.

What's wrong?

there is no compiling error because a bool can be represented by an integer
age=21 is an arithmetic expression. It returns the value of right hand side (i.e. 21).
a non zero number means true
be very careful! don't forget to use == in conditions! (unless you know exactly what you are doing)