for loop

6.12 for loop

SYNTAX

for (initialization ; test ; increment) {
     statements ;
}

OR

for (initialization ; test ; increment)
     statement ;

PROGRAM FLOW

  1. The initialization is first executed. This is typically something like int i=0, which creates a new variable with initial value 0, to act as a counter. Variables that you declare in this part of the for loop cease to exist after the execution of the loop is completed. Multiple, comma separated, expressions are allowed in the initialization section. But declaration expressions may not be mixed with other expressions.
  2. The boolean expression test is then evaluated. This is typically something like i<10. Multiple, comma separated, expressions are not allowed. If test evaluates to true, flow contiues to step 3. Otherwise the loop exits.
  3. The statement(s) are executed.
  4. Then the statement increment is executed. It is typically something like i++ , which increments i by one or i+=2 , which increments i by two. Multiple, comma separated, expressions are allowed in the increment section.
  5. Flow returns to step 2.

EXAMPLE

The following example computes an approximate value for the integral of f(x) from x1 to x2 using nsteps steps of the trapezoidal rule. 
        double dx = (x2-x1)/nsteps ;
        double x = x1+dx ;
        double integral = 0 ;
        
        for (int i=1 ; i< nsteps ; i++, x+= dx) {
             integral += f(x) ;
        }     

        integral = dx*(integral +( f(x1)+f(x2) )/2) ;

COMMON ERRORS

Putting a semicolon after the closing ) as in 
        for (initialization ; test ; increment) ;
             statement ;
terminates the for loop immediatley. statement is NOT PART OF THE LOOP. If you find you are plagued by this problem, try always using the multiple statement format even if you have just one statement in the loop body.
Next 6.13 if-else
David Maxwell, who is still writing this, would like to hear your comments and suggestions.