While Loops

A computer program often needs to do a great deal of computing. For example, if you have a picture, and you want to convert it to its negative image by changing the colors. Then you need to change every pixel (dot) in the picture.

A while loop is one way to do something over and over. You generally write a while loop as follows.

  (initialization)
  while(condition)
  {
    statements
  }

where you put in appropriate things for the parts in red. the statements inside the braces are called the loop body.

Note. The parentheses around the condition are required. If you omit them, then you will get an error.

Warning. Do not put a semicolon at the end of the line that starts while. You will create a loop that never stops.

Example

Suppose that you want to make variable sum hold 1 + 2 + ... + 100. The following accumulates the sum, by starting it at 0, then adding 1, then adding 2, etc.

  int k, sum;
  sum = 0;
  k = 1;
  while(k <= 100) 
  {
    sum = sum + k;
    k = k + 1;
  }

Remarks

The words if and while mean very different things. Do not confuse them. Use if to decide to do one thing or another, or to decide whether to do something at all. Use while to do something over and over.

A while loop does not have an else part. So do not write something like

    while( ...)
    {
      ...
    }
    else 
    {
      ...
    }

If you want to do something when the loop is finished, write it after the loop, not in the loop body. The loop body is for parts that are done over and over. For example, look at the following.
```
     int k, r;
     k = 0;
     r = 1;
     while (k != 10) 
     {
       ...
       k = k + 1;
       if(k == 10) r = x;
     }
  
```
Notice that the if statement is made so that statement r = x; will only be done when the loop is done. (If k is 10, the loop will not continue.) A better way to write this is as follows.
```
    int k, r;
    k = 0;
    r = 1;
    while(k != 10) 
    {
      ...
      k = k + 1;
    }
    r = x;
  
```
You can think of a while loop as trying to make its condition false. After all, it keeps going until its condition is false. So, when you end the loop, you normally find that the condition is false. Suppose you run the following silly loop.
```
    int k;
    while(k != 10) 
    {
      k = k + 1;
    }
    System.out.println(k);
  
```
What value is printed? It cannot be 9, since if k where 9, the loop would have continued. The loop keeps going until k is 10. So 10 is printed.

Questions.

Suppose that variable n has already been created, and has type int. Is the following allowed?
```
    n = 1;
    while n < 10
    {
      n = n + 1;
    }
  
```
Answer[21]
Suppose that variable n has already been created, and has type int. Is the following allowed?
```
    n = 1;
    While(n < 10)
    {
      n = n + 1;
    }
  
```
Answer[22]

What does the following print at the System.out.println line?

    int r,s;
    r = 1;
    s = 1;
    while(r != 5) 
    {
      s = s + s + 1;
      r = r + 1;
    }
    System.out.println(s);

(Work it out before you look at the answer[23].)

[Suppose that C is an ordinary boolean expression (one without side-effects). The expressions that we have seen do not have side-effects.] Do the following program fragments always do the same thing, regardless of what which condition is substituted for C or which statement is substituted for S? The first is
```
    while(C)
    {
      S
    }
  
```
The second is
```
    if(C)
    {
      while(C)
      {
        S
      }
    }
  
```
Answer[24]

Main