Class 21: Control Flow: Loops

Held: Monday, 28 February 2011

Summary: We consider the three main forms of looping in C.

Related Pages:

• EBoard.
• Reading: K&R 3.5-3.6.

Notes:

• I'll reserve time at the start of class for questions on Assignment 5.
• I've posted some answers to your questions on the loop reading and we'll discuss more in class.
• Questions on loops reading missing from: AA, RC, DG, MH, JJ, EO, SR, MT.
• Tonight's reading: K&R 3.7-3.8.
• There've been a few questions lately on the ordering of topics, so I thought we'd discuss them.
• We'll continue today's lab in tomorrow's class.
• EC: Thursday's CS Extra: FPGAs.
• EC: Friday's CS Table: GPU programming in CUDA, revisited.

Overview:

• Loop Basics.
• The Comma Operator.
• Selecting a Loop Structure.
• itoa.
• Lab.

Loop Basics

• The standard loop: A while loop.
• Form

  while (TEST)
    BODY;
  

• Meaning:
  • Evaluate the test
  • If the test does not hold (that is, has a value of 0), you're done
  • Otherwise, evaluate the body and go back to the start.
• The For loop

  for (INIT; TEST; INCREMENT)
    BODY;
  

• An alternate syntax for

  INIT;
  while (TEST)
    {
      BODY;
      INCREMENT;
    }
  

• The Do-While Loop

  do 
    BODY;
  while (TEST);
  

• An alternate syntax for

  BODY;
  while (TEST)
    BODY;
  

The Comma Operator

• Lets you do multiple operations in the same statement.
• Used primarily for the initializers and increments in for loops.

Selecting A Loop Structure

• Given that we have three looping mechanisms (four, if you count recursion; five if you count infinite loops with breaks), how do you choose which one to use?
• Typically:
  • The one that requires you to write the least code.
  • The one that seems clearest to read.
• If you're counting in your loop, a for loop makes sense.
• If you're not sure how many times to do a step, a while loop makes sense.
• If you have to do the body at least once, a do-while loop makes sense.

Some sample problems, which we'll try to write using each kind of loop.

• Print out the numbers from 1 to 10.
• Prompt the user for an action and do the action until the user enters 'q'.
• Print the squares of the numbers read from input.

The itoa function

• There were some questions on itoa, so we'll go over it a bit, as well as alternative mechanisms for implementing it.

Lab

• Begin the lab.
• We'll continue it tomorrow.