CS: Loops and conditionals, pt. 2

Sorry this post is coming a bit later than I originally intended. Let's briefly review what we talked about last time before diving into the new material.

Loops and conditionals allow us to control which instructions the computer processor is going to execute and when. Loops let us execute some instructions again, even after we already have executed them. Last time we saw the example of a while() loop that executed the code to add 1 to x and store it in x, and this loop executed 10 times.

Today we're talking about using if() statements to decide if we want to skip over a block of code. If the conditional statement inside the if() statement is true, then the block of code that follows the if() statement is executed. If the conditional statement inside the if() statement is false, then the block of code that follows the if() statement is skipped (it is not executed). Look at this code snippet:

x=1
if(x>2)
{
x=0
}

After the execution of this example the value of x will still be 1 (the value that was set at the beginning of the example). Since x>2 is false (because x is 1, and 1 is not greater than 2), we skipped over the assignment statement that would set x to 0 (x=0). Let's look at an example where the conditional statement evaluates to true:

x=4
if(x>2)
{
x=0
}

Here x starts out as 4, but since 4>2 is true, we execute the block following the if() statement. This block tells us to set x to 0. So at the end of this code snippet, x's final value will be 0.

We can see that if() statements allow us to skip blocks of code that we would only want to execute if the conditional statement of the if() statement is true. Let's look at a more complex example that combines a while() loop, an if() statement and a break:

x=0
y=0
while(x+y<10)
{
if(x<5)
{
y=y+2
}
if(y>4)
{
break
}
x=x+1
}

This example has if() statements inside a while loop. Let's walk through what happens here. First of all, both x and y are set to 0, and then we look at the while loop. Since x+y=0 right now, and 0<10, we will execute the while loop at least once, so let's jump into it. The first thing we run into inside the while loop's block is an if() statement, which asks if x is less than 5. This is true right now, so let's execute that if() statement's block, which increases y by 2. At this point, x is still 0 and y is now 2. Now we look at the next if() statement, which asks if y is greater than 4. This is not true, so we'll skip that block for now. Next we add 1 to x and store the result in x. We've now gone through the loop one time, and x is 1 and y is 2.

What do you do when you finish a run through a while() loop? Go back to the top and test to see if you need to do it all over again. We go back to the top and test x+y<10. Right now x+y is 3, which is definitely less than 10, so we should execute the while() loop's block again. Again we run into the if(x<5) statement, and again this is true, so we execute y=y+2, so now y is 4. Next we test to see if(y>4). Since y is exactly equal to 4, it is *not* greater than 4, so this conditional statement is still false, so we skip the contents of that block. Finally we execute x=x+1, and we're done with our second run of that loop. At this point, x=2 and y=4.

We again go back to the top of the loop, x+y is still less than 10, so we execute the loop one more time. if(x<5) is still true, so we execute y=y+2 again, making y equal to 6 now. This time when we execute the if(y>4) statement, y *is* greater than 4, so we execute the break statement. A break statement means "you're done with this loop now, so exit it immediately without doing anything else on your way out." After executing the break we're now outside the loop and we're done with our code snippet. The final score has x=2 and y=6. Notice that if it weren't for the break statement, since x+y<10 is still true we would still be going through our while() loop until such a time as when x+y is greater than 10.

You might need to read through the examples here a few times. It's substantially more complicated than anything I've presented lately, so you should take your time and go slowly. Make sure you get it.

Next time I'm probably going to talk more about consumer advice. Some time in the future I'll talk a little bit about electricity, just because someone was curious, but I remind you that I am not really qualified to explain it. Good luck to me writing that and you reading that.