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TOMMY MACWILLIAM: Let's talk about loops, one of the common

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programming constructs we'll see throughout CS50.

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We'll use loops when we want to perform a

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task more than once.

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For example, we might want to print "hi" 100 times or print

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out all the letters of the alphabet.

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In both of these cases, we have one thing we want to do

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multiple times, print something out.

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However, what we print out each time can vary.

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So we can do something slightly different on each

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iteration, or run through, of the loop.

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We'll see three different types of loops in C, while

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loops, for loops, and do while loops.

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Though these three types of loops have a different syntax,

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the ideas behind them are the same.

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We'll define some block of code within curly braces,

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called the body of the loop, that we want to be executed

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some number of times.

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By changing the values of variables used in the body, we

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can make our loop do something different each time it's run.

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With any loop we write, we'll also need to decide when the

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loop will stop running.

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If we don't do that, then our trusty computer will continue

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to run that loop until we kill the program.

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In Scratch, we could use the repeat n times

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block to create a loop.

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All of the pieces inside a block that said repeat 10

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would be run 10 times.

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And then, we'd move on to the pieces after that loop.

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So our stop condition was simply, this block has been

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run 10 times.

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So let's recreate this in C. In order for Scratch to ensure

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that the pieces within the repeat block are executed

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exactly 10 times, Scratch needs to keep track of each

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execution of the repeat block.

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To keep track of how many times our loop body has been

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executed, let's create a variable called i.

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We'll start i off at zero, since our loop

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hasn't been run yet.

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OK

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Now we'll use the while keyword to start off our loop.

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Now we'll need to figure out when our loop will stop, but

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let's leave this for now and then come back to it.

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All right.

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Inside of our loop, let's just print a message out, like

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"hi." We can use the printf function for this.

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All right.

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So now we'll record that an iteration of the loop body has

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been executed.

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We can do that by adding 1 to, or incrementing, our counter

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variable, i.

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To do that, we can say i is equal to i plus 1, or more

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simply, i plus plus.

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Great.

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So now we can see that each time our loop is run, our

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counter variable goes up by one.

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So we know exactly how many iterations we've run so far.

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After one iteration of the loop, our value

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of i will be 1.

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After two iterations, i will be 2.

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And after 10 iterations, i will be 10.

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So, if we want to run this loop exactly 10 times, then

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this is when we want to stop.

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So we want to run this loop while i is less than 10, and

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that's exactly what we'll write, while i

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is less than 10.

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This condition looks just like the conditions we used in if

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else blocks.

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After the body of our loop has been executed, our program

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will jump back up to the loops condition.

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If the condition is true, then the body of the

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loop will be run again.

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If the condition is no longer true, then our loop won't be

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run anymore and will move on to the next line of code

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underneath our loop.

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All right.

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So let's take a look at a second type of

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loop, the for loop.

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Next to the while keyword, in parentheses, we had one thing,

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the condition that needed to be true for the

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loop body to be run.

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That means we had to create our counter variable outside

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of the loop and remember to increment it at some point

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inside of the loop.

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The header for our for loop, on the other hand, has three

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parts, each of which will be separated with a semicolon.

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In our first third, we can declare any counter or helper

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variables we'd like to use in our loop.

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In practice, this can be really helpful.

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We really don't need that variable, i, after our while

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loop has run, so we really shouldn't have to declare it

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outside of the loop.

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The second third will be the condition that must be true

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for the body to be executed again, just like

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in our while loop.

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In our last third, we can run a statement that will be

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executed after each iteration of the loop, so we don't have

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to build it into the loop body.

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So let's write a for loop that counts down from 5 to 1.

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We'll start with the for keyword.

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We can create a counter variable first, which we'll

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set to 5 this time since we're counting down,

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followed by a semicolon.

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Next is our condition, which we'll come back to.

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Third, we'd like to decrement our counter variable after

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each iteration of the loop.

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So rather than saying i plus plus, we'll say i minus minus.

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All right.

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So we want the loop body to run while i is still

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greater than 0.

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In the body of the loop, let's print out the value of i.

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To do so, we'll use the printf function, using the %d

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placeholder.

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Remember, that placeholder will be replaced with the

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value of i.

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Finally, let's add a statement after our for loop.

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When we run this loop, i will start off at 5,

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so 5 will be printed.

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Once i gets to 0, the continuation condition, i is

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greater than 0, will no longer hold.

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So our loop will stop executing, and we'll see the

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statement after the loop.

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So let's run this code.

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First, we'll compile a

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countdown.c with make countdown.

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Now, we can run this code with ./countdown.

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In both while loops and for loops, our continuation

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condition will be checked before the body

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of the loop is executed.

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That means that, if our condition is not initially

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true, then the body of our loop will never be run.

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So it's sometimes useful to check the condition after the

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body of the loop rather than before it.

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So let's write a loop to prompt the user for a number

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until a positive number is supplied.

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If the user inputs a negative number, we'll want to ask them

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for another number.

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So we'll want this prompt to be inside the

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body off the loop.

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However, when the loop is run for the first time, the user

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hasn't given us the number yet.

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So it doesn't make sense to check if it's positive.

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Instead, we'll want to check the number after the body of

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the loop is run.

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We can do this with a do while loop.

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First, we'll create a variable, n, that will hold

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the user's input.

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Now we'll use the do keyword, followed by curly braces that

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will start off the body of our loop.

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In the body, we can prompt the user for a number with the

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GetInt function.

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Now, we'll want the body of this loop to execute again if

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the user typed a negative number, so we'll say while n

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is less than 0.

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Notice the semicolon here after the while statement.

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So let's run this code.

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First, we'll compile this with make positive.

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Now we can run the program with ./positive.

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If we give this program a negative number, like negative

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4, then we'll be prompted again for a number since the

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condition of our do while loop was true.

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Once we give a positive number, like 8, then the

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condition of our do while loop will no longer be true.

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So the loop will not be executed again.

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And that's it for the three types of loops we'll use in C.

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My name is Tommy, and this is CS50.