Module 5: Conditionals

Objectives

 

By the end of this module, for simple programs, you will be able to:

 

A simple example

 

Consider this program:

 

In-Class Exercise 1: Add an additional println right below the "Hey, ..." println. Compile and execute the program. Then, change the value of y to 6 and compile/execute. What is the output?
 

About the if-statement:

 

If-else

 

Consider this program:

  • If the if-condition (x>y) evaluates to true, the if-block executes:

  • Otherwise, the else block executes:

 

We can combine an else clause with an added if-clause:

  • If the if condition is true, only the if-block executes:

  • If not, the else-if condition is evaluated:

  • If that's true, the appropriate block is executed:

  • If the second if-condition is false, the last else-clause executes:

 

Comparison operators:

  • These are Java's comparison operators: 
           (x < y)                // Strictly less than.
       (x <= y)               // Less than or equal to.
       (x > y)                // Strictly greater than.
       (x >= y)               // Greater than or equal to.
       (x == y)               // Equal to.
       (x != y)               // Not equal to.

  • Thus, in the above example, we could have written:

 

In-Class Exercise 2: Are there any circumstances under which the last block executes?
 

  • Here, logic dictates that only some blocks can possibly execute.

  • Thus, sometimes it is not necessary to write some blocks of code if they'll never execute.
 

In-Class Exercise 3: Suppose we want to identify whether x has the largest value among x, y and z. Why doesn't the following program work?

Can you alter the program to make it work?
 

Nested conditionals

 

Consider this program:

  • First, the outer if condition is evaluated:

  • Since that's true here, we get inside the if block:

  • The next thing to be executed is the inner if condition:

  • Since that's true here, we get inside that if-statement's if-block:

  • Note how execution goes from there:

 

In-Class Exercise 4: Modify the above program so that it prints out, appropriately, one of "a is the smallest", "b is the smallest" or "c is the smallest", depending on the actual values of a, b and c. Try different values of these variables to make sure your program is working correctly.
 

Combining conditions

 

Consider this program:

  • The if condition combines two comparisons:

  • The combination uses the Boolean operator &&

    • Read && as "AND".

  • The "AND" operator works like this:

    • The overall combination evaluates to true only when BOTH the individual conditions are true.
 

In-Class Exercise 5: Let's go back to this program:

Use a two-subclause if-statement to identify whether a is the smallest of the three.
 

In-Class Exercise 6: Now extend this idea to identify which of the three variables has the smallest value.
 

The OR operator:

  • Here, the || operator should be read as "OR".

  • If either subclause is true, the whole is true:

 

The NOT operator:

 

In-Class Exercise 7: Modify the above to determine whether all three variables have different values.
 

The NOT operator can be applied to a larger clause made of sub-clauses:

  • Here, the inner clauses are first evaluated, and the result is "flipped" to see if the NOT clause turns out to be true:

 

Boolean variables

 

What is a Boolean variable?

  • An int variable takes integer values like: 12, 256, or -9.

  • A double variable takes on values like 3.141, -14.0, or 2.718.

  • A boolean variables takes on one of only two values: true or false.
 

An example:

  • boolean, true and false are reserved words.

  • Note: true and false are not in quotes.
 

Boolean operators:

  • Two binary operators: && and ||

  • One unary operators: !

  • An example:

  • To understand the && operator:

    Let's see what c could be based on all possible combinations of a and b values:
    a b c = a && b
    false false false
    false true false
    true true true
    true false false

  • In short, the result of an AND operation is true only when both operands are true.

  • Similarly, let's look at the || operator:

    Let's see what c could be based on all possible combinations of a and b values:
    a b c = a || b
    false false false
    false true true
    true true true
    true false true

  • In short, the result of an OR operation is false only when both operands are false.

  • The ! operator:
    a !a
    true false
    false true

  • In short, the NOT operator flips the value.

  • Note: the above table describes some of the "rules" for logic.
 

Reading and writing

 

First, reading:

  • When seeing

    say to yourself:

    • "Here's an if-block that may or may not execute."
    • "What follows the if-block (the second println) always executes".

  • Read aloud the if-construct as: "if x > y then ... (the first println)".

  • Similarly when seeing a larger if-else, focus on the structure:

    Say to yourself:

    • "Only one of the three code blocks will execute."
    • "The second condition is evaluated only if the first one is false."
    • "We reach the third block only if both the previous two conditions fail."

  • When seeing a complex condition like

    break it up into a hierarchical view:

 

In-Class Exercise 8: Draw a hierarchical picture that matches the conditional below.

Then, evaluate it to see whether the result is true or false. Write down the true/false values at intermediate levels of the hierarchy.
 

Writing:

  • Remember to type in the matching right bracket immediately after typing in the left one.
  • Use spacing and layout consistent with the above example.
 

When things go wrong

 

 

In-Class Exercise 9: Identify the four errors in this piece of code:

 

In-Class Exercise 10: Fix the errors in the if condition to make this print "Success".

First, try to find the problems without compiling. Then, fix the code and see if you were right while reading.
 

More Exercises...

 

In-Class Exercise 11: Create a program to output health information based on BMI. Use at least two variables: mass and height.

mass(lb) x 703
--------------------
height(in)2

BMI stands for Body Mass Index. This is a numerical value of your weight in relation to your height. BMIs are good indicators of healthy or unhealthy weights for adult men and women, regardless of body frame size.

  • A BMI of 30 or higher indicated obesity.
  • A BMI of greater than 25, but less than 30 is considered overweight.
  • A BMI of less than 25, but greater than 18.5 indicates a healthy weight.
  • A BMI less than 18.5 is considered underweight.
Do the following exercises:
  • Write a solution that uses logical binary operators (e.g. && and ||) and if, else, and/or else if.
  • Write a solution that uses logical binary operators (e.g. && and ||) but no else/else if.
  • Write a solution that uses nested conditionals (you can use if and else), and no logical binary operators.
  • Draw a control flow diagram for each program.
Which version was the most difficult to understand? Which was the simplest? Why?