Welcome back to CS 124!
Next we’ll continue discussing conditionals. Last time we introduced simple conditional expressions and statements, and blocks. This lesson continues on those topics, introducing a few new wrinkles. We’ll show how you can combine conditional expressions and statements, and discuss how blocks introduce scope to our variables. So let’s get started!
Previously we discussed how you can use conditional operators (
!=) to create conditional expressions.
For example, to test whether the value stored in the variable
speed is less than 70, we’d use the conditional expression
speed < 70.
Conditional expressions evaluate to
So… great! But what if we want to determine if speed is both less than 80 and greater than 70? Here’s how we would do this:
The example above introduces some new syntax, so let’s look at it carefully.
The example above introduces compound conditional expressions.
They are created from combining multiple individual conditional expressions using two new operators: and (
&&) and or (
Both and (
&&) and or (
||) combine two conditional expressions, one on either side of the operator:
&&) evaluates to true if both the conditional expression on the right and the left are
||) evaluates to true if either the conditional expression on the right or the left is
Let’s look at how they work by example:
By combining simple conditional expressions using
||, we can express arbitrary decision-making logic.
For example, if we want to determine if the value of a variable was both greater than 10, less than or equal to 20, and not equal to both 14 and 15:
We’re typically going to keep our compound conditional expressions as simple as possible. But when evaluating a compound conditional expression, Java follows a few rules:
Some of this may seem complicated or confusing! But don’t worry—99% of the conditional expressions you’ll find in real programs are extremely simple. If you find yourself writing something fairly complex in CS 124, please ask for help. There may be a simpler way!
So we can combine multiple conditional expressions together using
|| to create more complex decision-making logic.
We can also create compound conditional statements to put our conditional expressions to use.
Here’s an example.
Let’s go through it slowly and carefully.
Our code is starting to look more complicated.
But if you break it down line by line, it’s still just made up of the same simple building blocks.
Here’s how Java evaluates an
ifbranch where the conditional expressions evaluates to
true, it enters that branch and executes it
if else), it always enters that block
Last time we introduced the idea of a block of code. We needed blocks so that we could set off the code that should or should not be executed by a conditional statement. And we’ll be using them again later—both to indicate what part of a program should be repeated, and when we start to create reusable pieces of logic called functions.
But there is another important aspect of blocks that we need to discuss. Let’s introduce it using an example:
Try to run the code above. What happens? An error occurs! Why? Because variables are not available outside of the block in which they were declared!
The part of a program in which a variable is available is known as its scope. For the variables that we’ve been using so far, their scope is limited to the block in which they were declared. Note that they can be accessed in blocks declared inside their block, just not outside of their block. That’s hard to explain in words. So let’s talk it through.
Need more practice? Head over to the practice page.