University of Washington, CSE 142

Lab Resources: Assertions

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Basic lab instructions

Mouse over highlighted words if you're not sure what they mean!
Talk to your classmates for help.
You may want to bring your textbook to future labs to look up syntax and examples.
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Complete as much of the lab as you can within the allotted time. You don't need to keep working on these exercises after you leave.
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Assertions

Assertion problems test your ability to reason through code. For example, consider the following:

public static void mystery(int x, int y) {
    int z = 0;

    // Point A
    while (x != y) {
        // Point B
        z++;

        if (x > y) {
            // Point C
            x = x / 10;
        } else {
            // Point D
            y = y / 10;
        }
    }
    // Point E	    
    System.out.println(x + " " + y + " " + z);
}

Assertions: Addressing Point A

For each point, we want to figure out whether z == 0, x > y, and x == y are always true, sometimes true, or never true. Let's first think about Point A. We know a few things about Point A:

x > y, x == y:
x and y are equal to whatever was passed in as a parameters (any integers!) Are there any restrictions on the values that x and y could hold?
z == 0:
z = 0 executes immediately before Point A. Is it possible for z == 0 to be false at A?

Assertions: Addressing Point A

	`x > y`	`z == 0`	`x == y`
Point A	SOMETIMES	ALWAYS	SOMETIMES

public static void mystery(int x, int y) {
    int z = 0;

    // Point A
    while (x != y) {
        // Point B
        z++;

        if (x > y) {
            // Point C
            x = x / 10;
        } else {
            // Point D
            y = y / 10;
        }
    }
    // Point E
    System.out.println(x + " " + y + " " + z);
}

Assertions: Engaging Point E

Now let's skip to Point E, where we're going to need to do a little bit more reasoning.

x == y:
This code has a while loop that loops while x != y. This means that when we break out of the loop, x == y. Since Point E is after the while loop, what do we know about x == y there?
x > y:
If x == y must be true, is it possible for x > y to be true?
z == 0:
Evaluating whether z == 0 is tricky. z is initially equal to 0. However, as soon as we go into the while loop, we execute z++ (which would mean z != 0). So z == 0 is true at Point E if we don't go into the while loop, and untrue if we do go into the while loop. Will we necessarily go through the while loop?

Assertions: Engaging Point E

	`x > y`	`z == 0`	`x == y`
Point E	NEVER	SOMETIMES	ALWAYS

public static void mystery(int x, int y) {
    int z = 0;

    // Point A
    while (x != y) {
        // Point B
        z++;

        if (x > y) {
            // Point C
            x = x / 10;
        } else {
            // Point D
            y = y / 10;
        }
    }
    // Point E
    System.out.println(x + " " + y + " " + z);
}

Assertions: Battling Point B

Okay, now we need to figure out what's going on inside the while loop! Let's take a look at Point B first.

x == y:
We only enter/continue looping through a while loop while its test is true. So we only encounter Point B immediately after verifying that x != y is still true. Can x or y have changed values by Point B?
x > y:
For an assertion to be sometimes true, we just need to be able to think of a case where it's true, and a case where it's false. x and y could have any initial values (since they're just whatever was passed in as parameters). Is there any reason that x > y must be true or false?
z == 0:
z is initially equal to 0, and on our first iteration through the while loop, we hit Point B before we alter z's value. However, after our first pass over Point B, we execute z++ (and z != 0). So will z == 0 always, sometimes, or never be true at Point B?

Assertions: Battling Point B

	`x > y`	`z == 0`	`x == y`
Point B	SOMETIMES	SOMETIMES	NEVER

public static void mystery(int x, int y) {
    int z = 0;

    // Point A
    while (x != y) {
        // Point B
        z++;

        if (x > y) {
            // Point C
            x = x / 10;
        } else {
            // Point D
            y = y / 10;
        }
    }
    // Point E
    System.out.println(x + " " + y + " " + z);
}

Assertions: Calculating Point C

Now let's take a look at Point C!

x > y:
If we reach Point C, we entered the if statement: if (x > y). This means that we will only reach Point C when x > y! Is it possible for x > y to be false at Point C?
x == y:
Since x > y must be true, this means that x != y. Additionaly, to enter/continue looping through the while loop, x != y must be true. x and y don't change values between the while loop test and Point C. Is it possible for x == y to be true at Point C?
z == 0:
To get to Point C, we must pass z++. Is it possible for z == 0 to be true at Point C?

Assertions: Calculating Point C

	`x > y`	`z == 0`	`x == y`
Point C	ALWAYS	NEVER	NEVER

public static void mystery(int x, int y) {
    int z = 0;

    // Point A
    while (x != y) {
        // Point B
        z++;

        if (x > y) {
            // Point C
            x = x / 10;
        } else {
            // Point D
            y = y / 10;
        }
    }
    // Point E
    System.out.println(x + " " + y + " " + z);
}

Assertions: Deducing Point D

Finally, we need to figure out what's going on with Point D!

x > y:
If x > y were true, then we would go into the if branch. But Point D is in the else branch. So what do we know about x > y at Point D?
x == y:
Not going into if (x > y) tells us that x <= y. But can x == y? Recall that to enter/continue looping through the while loop, x != y must be true. Also x and y are not changed between the while loop test and Point D. So what do we know about x == y at Point D?
z == 0:
To get to Point D, we must pass z++. Is it possible for z == 0 to be true when we get to Point D?

Assertions: Deducing Point D

	`x > y`	`z == 0`	`x == y`
Point D	NEVER	NEVER	NEVER

public static void mystery(int x, int y) {
    int z = 0;

    // Point A
    while (x != y) {
        // Point B
        z++;

        if (x > y) {
            // Point C
            x = x / 10;
        } else {
            // Point D
            y = y / 10;
        }
    }
    // Point E
    System.out.println(x + " " + y + " " + z);
}

You're done!

Nice work! Assertions are tough. Make sure to practice some more problems before the midterm!