Loops

Method for getting a sequence of statements to be repeated.

Many different ways of writing loops.

Concentrate on two: counting loops and sentinel-controlled 
loops.

Counting loops:

set the "count of items processed" to 0
do the following n times (n is a data value)
 	read a number
        square it
        display the number and its square
        add 1 to "count of items processed"

In a counter controlled loop, there is a special program 
variable, the counter, that counts the number of loop 
executions so far. Counter is set to an initial value of 0. 
When this count reaches some prespecified maximum, the 
loop is "exited" - loop repetition stops. The part that is 
repeated (the loop body) is indented in above pseudocode.


Sentinel-controlled loops:

read a data value
while the data value read is not the sentinel
      process the last data value read in some way
      read the next data value

In a sentinel controlled loop, there is an "extra" data value 
called the sentinel. For example, exam scores might be 75, 
80, 90. The data would be entered as 75, 80, 90, -1 where 
-1 is the sentinel. The loop body (indented) continues to 
execute until the sentinel value is read.

Implementing loops:

We will study 2 loop forms in C, while statement and for 
statement.

while statement:

while (condition)
{
	statement1;
	statement2;
	.
	statementn;
}

or

while (condition)
	statement;

1. Evaluate the condition. 
2. If the condition is true, execute the loop body (between 
braces or next statement if no braces). If the condition is 
false, exit the loop.
3. Repeat steps 1 and 2 until loop exit occurs.

Implement a counting loop using a while statement

int count;
double x, xSquared;
int numX;  /* input */

printf("How many items to process>");
scanf("%d", &numX);

count = 0;
while (count < numX)
{
	printf("Enter next number to square>");
	scanf("%lf", &x);
	xSquared = x * x;
	printf("The square of %f is %f\n", x, xSquared);
	count = count + 1;
}

while statement implementation of sentinel loop

int score;
int sentinel = -1;

printf("Enter first score>");
scanf("%d", &score);
while (score != sentinel)
{
	process(score);    /* call function process */
	printf("Enter next score>");
	scanf("%d", &score);
}

Class examples:

Write a loop that finds the sum of the first n integers where 
n is a data item.  Display the final sum. (counting or 
sentinel?)


int   
int 
int

scanf(     );
____ = 0;
while (                     )
{


      ____ = ____ + 1;
}

printf(                      );



Write a loop that reads a series of numbers and calculates 
their product. Display the final product. The calculation 
should stop when the number 0 is read. (counting or 
sentinel?)


double
double
double

_______ = _____;
scanf(    );
while (                   )
{
      _______ = _______ * ________;
      scanf(    );
}




Write a loop that reads up to 10 numbers and calculates 
their product. Display the final product. The calculation 
should stop when the 10th number is read or the number 0 is 
read. (counting or sentinel?)


double
double
double 
int

scanf(     );
______ = ______;
while (                       &&                       )
{
           _______ = _____ *  ______;
           _______ = _____ + _____;
           scanf(     );
}