CSCI 1300
Lecture Notes

9/11/97
Administrative Stuff
Selective Execution
  • Sequential Execution
  • Selective Execution
  • Simple Conditions
  • Compound Conditions
  • Switch Statements

    • Administrative Stuff

  • Web page: http://www.cs.colorado.edu/~sbrandt/CSCI1300.html
  • Assignment #3 details
  • Graded assignments will be handed back in class
  • QUIZ 1 on Tues. Sept 16. - will cover chapters 3 and 4

    • Selective Execution


    Sequential Execution

     C++ programs execute sequentially, line by line, top to bottom

     Every time you run one (with what we know now), it will do exactly the same thing (except for any differences in the data).

     What if we want it to do different things depending on the input?

     Example: What happens if you try to set num_pages to -1 in the card catalog entry?
    The function should print an error message

     What we need is the ability to execute different pieces of code depending on the situation, rather than blindly executing every line.

    Selective Execution

     The "if" statement allows us to execute a piece of code or not, based on the value of the expression we pass to it

     Pseudocode:

    IF "condition" THEN "statement";
    If "condition" is TRUE (non-zero), then "statement" is executed.
    If "condition" is FALSE (zero), then "statement" is not executed.

    Example:
        int age;
     
        cout << "Please enter your age: ";
        cin >> age;
     
        if(age <= 0) {
            cout << "Age must be > 0\n";
            exit(-1);
        }

        cout << "Your age is " << age << "\n";

     The "if" statement also allows us to choose between two alternatives

    Pseudocode:

    IF "condition" THEN "statement1" ELSE "statement2"

     
    If "condition" is TRUE (non-zero), then "statement1" is executed.
    If "condition" is FALSE (zero), then "statement2" is executed.

    Example: Bar doorman function
        int age;
     
        cout << "Please enter your age: ";
        cin >> age;

        if(age <= 0) {
            cout << "Age must be > 0\n";
            exit(-1);
        }

        if(age >= 21) {
            cout << "Welcome\n";
        } else {
            cout << "I'm sorry, you can't drink here\n";
        }

    Even fancier - stringing if-then-elses together
  • If-then-else statements can be strung together
  • Exactly one of the alternatives will be executed

    • Example:

        if(num_credits < 0) {
            cout << "Come on, get real\n";
        } else if (num_credits < 12) {
            cout << "Part-time student\n";
        } else if (num_credits < 18) {
            cout << "Full-time student\n";
        } else {
            cout << "Glutton for punishment\n";
        }

    Conditions

    Logical expressions that evaluate to either TRUE or FALSE.

    Logical expressions are also called boolean expressions after George Boole.
     

    In C++
  • The value 0 is used to represent FALSE
  • The value 1 is used to represent TRUE
  • Any non-zero value also means TRUE


    • Simple Boolean Expressions

        < (is less than)
        > (is greater than)
        == (is equal to)
        != (is not equal to)
        <= (is less than or equal to)
        >= (is greater than or equal to)

    Examples:

    x < 5
    c == 'c'
    age >= 21
    x != 0    // How else could we write this?
    m > n

    Note: These operators don't work for strings.

    Compound Boolean Expressions

     Boolean expressions don't string together like numerical expressions.

     Examples:

    4 + 5 + 6 = 15
    4 < 5 < 6 = ?
    4 < 6 < 5 = ?
    We combine simple boolean expressions with the following operators:
    ! (negation)
    && (logical and)
    || (logical or)
    Examples:
    (4 < 5) && (5 < 6)
    (4 < 6) && (6 < 5)
    (x > y) || !(x > y)


    Switch Statements

    A cleaner way to select among multiple alternatives.

    Given a numeric value x, where we want to execute different blocks of code based on the value of x, we can write:

        switch(x) {
            case 0:
                cout << "Invalid value for x";
                break;

            case 1:
                y = z;
                break;

            case 2:
                y = 1/z;
                break;
     
            default:
                y = 37;
                break;
        }