PIC10A Lecture 6

Reading

p61-63. p73-76

Switch

switch(expression) {

     case constant1:

          statements1;

          ..............

           break;

     case constant2:

          statements2;

          ............

          break;

             .

             .

     default:

          default_statements;

}

The expression is evaluated.
The result is compared to constant1, constant2 ...
If the result is equal to constant1, statements1 will be executed. If the result is equal to constant2, statements2 will be executed, etc etc.
If none of the constant is equal to the expression. The default_statements will be executed.
The default part is optional.
Don't forget the break in each case.

#include<iostream>
using namespace std;

int main(){
    double num1, num2; // 2 numbers
    char op; // an operator, +,-,* or /
    cout << "Please enter an arithmetic expression: ";
    cin >> num1 >> op >> num2;

    switch(op) {
	case '+':
	    cout << "The result is " << num1+num2 << ".\n";
	    break;
	case '-':
	    cout << "The result is " << num1-num2 << ".\n";
	    break;
	case '*':
	    cout << "The result is " << num1*num2 << ".\n";
	    break;
	case '/':
	    cout << "The result is " << num1/num2 << ".\n";
	    break;
	default:
	    cout << "Sorry, I don't understand the operator " 
                 << op << ".\n";
   }
   cout << "Bye!\n";
   return 0;
}

Output

Please enter an arithmetic expression: 21.3*16.22
The result is 345.486.
Bye!

Please enter an arithmetic expression: 7%6.2
Sorry, I don't understand the operator %.
Bye!

break

#include<iostream>
using namespace std;

int main() {
    int num ;
        cout << "Input a number: ";
	cin >> num;
	switch(num) {
	    case 1:
		cout << "You input 1.\n";
	    case 2:
		cout << "You input 2.\n";
		break;
	    case 3:
		cout << "You input 3.\n";
	    case 4:
	        cout << "You input 4.\n";
		break;
	}
    return 0;
}

Output

Input a number: 3
You input 3.
You input 4.

There is no compiling error if you forget to include break in each case.
When the user input 3, the codes after the case 3: will be executed till the program reaches break.

it can be used in the following example:

case 'A':
case 'a':
   cout << "Exellent! You grade is A!!!";
break;

for loop

for(initialization; condition; update) {
    statements;
}

Example

#include<iostream>
using namespace std;

int main() {
    int num;
    for(num = 10; num >= 1; num--) {
	cout << num << " bottles of beer on the shelf.\n";
    }
    cout << "No more beer!\n";
    return 0;
}

Output

10 bottles of beer on the shelf.
9 bottles of beer on the shelf.
8 bottles of beer on the shelf.
7 bottles of beer on the shelf.
6 bottles of beer on the shelf.
5 bottles of beer on the shelf.
4 bottles of beer on the shelf.
3 bottles of beer on the shelf.
2 bottles of beer on the shelf.
1 bottles of beer on the shelf.
No more beer!

Example

#include<iostream>
using namespace std;

int main() {
    for(int i=0; i < 5; i++) {
	cout << "Hello ";
    }
    cout << endl;
    return 0;
}

Output

Hello Hello Hello Hello Hello

When the program encounters the for loop. It

executes the initialization. It usually gives the initial value of some variables.
tests the condition.
If the condition is true,
- the program will execute statements.
- then it executes the update and go back to step 2.
If the condition is false:
- the program exits the loop and goes to the line right after the close brace.

Infinite loop

while(true) {
   cout << "Hello!\n";
}

Given a while loop, there is always the danger of having a loop that doesn't stop, i.e. infinite loop.
It is important to make sure that the loop terminate.
You can use Ctrl-C to stop the program.