Chapter 3 Types, Operators, and Expressions; While and For Reference: Brooks, Chapter 3 2 3)

• 
  A common programming mistake when one uses a while construct is to either forget the initialization of the variables or to forget the increment of the loop index
  
• 
  The for construct puts both of those things at the top
  
• 
  Programmers are less likely to forget these things
  
• 
  A for loop is simply a more “general” while loop
  

• 
  Here is the version which uses a while loop
  

prev1 = prev2 = 1; /*first two numbers*/

sum = 2; /*sum of the first two numbers*/

count = 2; /*calculate 3rd number on*/

while (count++ < N)

{

curr = prev1 + prev2;

prev1 = prev2; /*advance prev1*/

prev2 = curr; /*advance prev2*/

}

• 
  Here it is again, but using a for loop
  

prev1 = prev2 = 1; /*first two numbers*/

for (sum = 2, count = 2; count < N; count++)

{

curr = prev1 + prev2;

prev1 = prev2; /*advance prev1*/

prev2 = curr; /*advance prev2*/

}

• 
  In this example, we use the comma operator to form a longer, more complex initialization expression
  
• 
  We could also use it for create longer next expressions
  
• 
  The logic of the for loop is slightly different here, but we also got rid of rather tricky code that the while loop was using
  
• 
  The while version post-incremented a count whose initial value was 2
  
• 
  The number of iterations is N-2
  
• 
  The for version starts with count=3 and continues until it is N, thus iteration (N-3)+1 times, i.e. N+2 times
  

· The for statement in C is designed to be a lot more general (and potentially more powerful) than Algol-like languages.

· The first optional expression within the for loop is the initialization

· It is done before the loop starts

· The second optional expression is the test condition

· It is performed at the beginning of each iteration

· If it is nonzero (true), the body of the loop is performed

· If it is zero (false), the for loop terminates

· The third optional expression is an expression which will be performed as the last statement in the loop

· In this context, the semicolons are expression separators

The FOR statement (4)
example -- infinite loop

for ( ;; ) /* while (1) */ { statements }

expr1: i = 1

expr2: i <= n

expr3: i++
The FOR statement (5)
· The for statement is similar to the Pascal for loop or the Fortran DO statement

· However, in C, both the index and limit can be altered within the loop

· This type of thing is not a good idea…

· The loop index and limit are changed by an outsider

· The behavior can be very unpredictable