Control Sructures - Repetition

Repetition Statements

Repetition statements are called loops, and are used to repeat the same code mulitple times in succession.
The number of repetitions is based on criteria defined in the loop structure, usually a true/false expression
The three loop structures in C++ are:
- while loops
- do-while loops
- for loops
Three types of loops are not actually needed, but having the different forms is convenient

`while` and `do-while` loops

Format of while loop:
```
 while (expression) 
    statement 
```

Format of do/while loop:


 do 
    statement 
 while (expression);

The expression in these formats is handled the same as in the if/else statements discussed previously (0 means false, anything else means true)
The "statement" portion is also as in if/else. It can be a single statement or a compund statement (a block { } ).

We could also write the formats as follows (illustrating more visually what they look like when a compound statement makes up the loop "body"):

 // while loop format
 while (expression) 
 {
    statement1;
    statement2;
    // ...

    statementN;
 } 

 // do-while loop format
 do
 {
    statement1;
    statement2;
    // ...

    statementN;
 } while (expression);

HOW THEY WORK
- The expression is a test condition that is evaluated to decide whether the loop should repeat or not.
  - true means run the loop body again.
  - false means quit.
- The while and do/while loops both follow the same basic flowchart -- the only exception is that:
  - In a while loop, the expression is tested first
  - In a do/while loop, the loop "body" is executed first

Examples

Both of the following loops add up all of the numbers between 1 and 50.

 // while loop example 
 // loop body runs 50 times, condition checked 51 times 
 int i = 1, sum = 0; 
 while (i <= 50) 
 { 
    sum += i;		// means:  sum = sum + i; 
    i++; 		// means:  i = i + 1;
 } 

 cout << "Sum of numbers from 1 through 50 is " << sum; 
  

 // do/while loop example 
 // loop body runs 50 times, condition checked 50 times 
 int i = 1, sum = 0; 
 do 
 { 
    sum += i;		// means:  sum = sum + i; 
    i++; 		// means:  i = i + 1;
 } while (i <= 50); 

 cout << "Sum of numbers from 1 through 50 is " << sum;

Example Links

The `for` loop

The for loop is most convenient with counting loops -- i.e. loops that are based on a counting variable, usually a known number of iterations

Format of for loop:


 for (initialCondition; testExpression; iterativeStatement) 
    statement

Remember that the statement can be a single statement or a compound statement (block), so an alternate way to write the format might be:
```
 for (initialCondition; testExpression; iterativeStatement) 
 {
    statement1;
    statement2;
    // ...

    statementN;
 }
```
How it works
- The initialCondition runs once, at the start of the loop
- The testExpression is checked. (This is just like the expression in a while loop). If it's false, quit. If it's true, then:
  - Run the loop body
  - Run the iterativeStatement
  - Go back to the testExpression step and repeat

Examples of `for` loops

Recall this while loop example, which adds the numbers from 1 to 50:

 int i = 1, sum = 0; 
 while (i <= 50) 
 { 
    sum += i; 
    i++; 
 } 

 cout << "Sum of numbers from 1 through 50 is " << sum;

Here's a for loop that does the same job:

 // for loop example 
 // loop body runs 50 times, condition checked 51 times 
 int i, sum = 0; 
 for (i = 1; i <= 50; i++) 
 { 
    sum += i; 
 } 

 cout << "Sum of numbers from 1 through 50 is " << sum;

This loop prints out the word "Hello" 10 times:

  for (int i = 0; i < 10; i++)
    cout << "Hello\n";

So does this one

  for (int i = 1; i <= 10; i++)
    cout << "Hello\n";

A few common types of algorithms using for-loops:
- A counting algorithm
- A summing algorithm -- Adding up something with a loop
- A product algorithm -- Multiplying things with a loop
Examples using nested loops:

Special notes about for loops:

It should be noted that if the control variable is declared inside the for header, it only has scope through the for loop's execution. Once the loop is finished, the variable is out of scope:

    for (int counter = 0; counter < 10; counter++) 
    {
       // loop body    
    } 

    cout << counter;	// illegal.  counter out of scope

This can be avoided by declaring the control variable before the loop itself.

    int counter;    // declaration of control variable 

    for (counter = 0; counter < 10; counter++) 
    {
       // loop body    
    } 

    cout << counter;	// OK.  counter is in scope

For loops also do not have to count one-by-one, or even upward. Examples:
```
    for (i = 100; i > 0; i--)

    for (c = 3; c <= 30; c+=4)
```
The first example gives a loop header that starts counting at 100 and decrements its control variable, counting down to 1 (and quitting when i reaches 0). The second example shows a loop that begins counting at 3 and counts by 4's (the second value of c will be 7, etc).

Special statements: `break` and `continue`

These statements can be used to alter the flow of control in loops, although they are not specifically needed. (Any loop can be made to exit by writing an appropriate test expression).
break: This causes immediate exit from any loop (as well as from switch blocks)
- An example of break
continue: When used in a loop, this statement causes the current loop iteration to end, but the loop then moves on to the next step.
- In a while or do-while loop, the rest of the loop body is skipped, and execution moves on to the test condition
- In a for loop, the rest of the loop body is skipped, and execution moves on to the iterative statement
- An example of continue

Control Sructures - Repetition

Repetition Statements

while and do-while loops