CSCI 1300
Lecture Notes

9/18/97
Administrative Stuff

Repetition
  • For Loops
  • While Loops
  • Do-While Loops

    • Administrative Stuff

    Suggestion: read the programming pointers in the various chapters

    Grading: I will hand back the quiz and assignment #2 on Tuesday

    Review: if statements

    Examples:

      // Check age
  if(age > 100) {
    cout << "Whoa\n";
  }

  // Resident     => low tuition
  // Non-resident => high tuition
  if(resident == 1) {
    tuition = $2000;
    cout << "Resident\n";
  } else {
    tuition = $10000;
    cout << "Non-resident\n";
  }

  // :-)
  if( (month == 1) && (day == 1) ) {
    cout << "Happy New Year\n";
  }

  // f = ma;
  if( m != 0 ) {
    a = f/m;
  } else {
    a = 0;
  }

  // check input
  if( c == 'a') {
    do_something();
  } else if (c == 'b') {
    do_something_else();
  } else {
    go_home();
  }

    Repetition

    Loops of various sorts are used to repeat a set of statements some number of times.

    There are three basic ways to implement loops in C++
  • for loops
  • while loops
  • do-while loops

    • For loops

    For loops are used to repeat a set of statements some specific number of times.

    They are often used for numerical computations where the number of iterations (passes through the loop) are known when the for statement is first executed.

    Example: Summing all of the numbers from 1 to 100

    The tedious, inflexible way - repeating everything over and over
    The cool way - using a for loop
    The cool, flexible way - using a for loop with a user specified limit

    As the above examples show, for loops have 4 parts:
  • An initialization expression
  • A loop condition
  • A step expression
  • The body of the for loop
    • The initialization expression
  • Sets the loop control variable to its initial value
  • The loop control variable is usually, but not always, an integer
  • i is the loop control variable in the above examples
  • i = 0; is the initialization expression in the above examples
    • The loop condition
  • Compares the loop control variable with some limit value
  • The limit value may be a constant (like 100) or a variable (like count)
  • The Loop condition must evaluate to either TRUE or FALSE (remember conditions from last week)
  • If the loop condition evaluates to TRUE
    - The body of the for loop is executed
    - The step expression is executed
    - The loop condition is checked again
  • If the loop condition evaluates to FALSE
    - The for statement ends and the program continues with whatever is after it
  • i <= 100; and i <= count; are the loop conditions in the above example
    • The step expression
  • Modifies the loop control variable before the next execution of the loop condition
  • i++; in the above examples
    • The loop body
  • Actually does the business of whatever the loop is supposed to do each time around
  • sum += 1; in the above examples
    • Note: A break; statement exits the loop immediately

    While Loops

    While loops are a little more general than for loops. In particular, it can be used where the number of iterations of the loop is not known beforehand.

    While statements have 2 parts:
  • The loop condition
  • The loop body
    • These are just like in for loops.

    If the loop condition is TRUE

    The loop body is executed
    The loop condition is evaluated again
    If the loop condition is FALSE
    The loop body is not executed

    Example: echo.cc

    How does it stop?
  • The loop body can change something so the loop condition is FALSE
  • The loop body can execute a break; statement
  • Maybe it never stops
    • Example: cat.cc - prints a text file on the screen

    Example: cat2.cc - prints a text file on the screen

    Do-While Loops

    Do-while loops are very much like while loops, except that the loop body is executed once before the loop condition is ever evaluated. After that, they are exactly the same.

    Example:

        do {
      cout << "This is a test";
    } while(0);