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There are two fundamentals types of loops in Python. There is while loops and for

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loops, and let's start by looking at the while loop.

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So we'll take our whileloop.py and make a little copy of it, and call

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it whileloop-working.

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Go ahead and open that, and you'll see this is a simple Fibonacci series.

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It's a common exercise for while loops.

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So here we are assigning two values to a and b: 0 and 1. The 0 will get assigned

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to a, and the 1 will get assigned to b. We say while b < 50 and we go ahead and

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do things we'll make b grow in a Fibonacci series and we print it each time

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using the print function.

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So if I go ahead and run this and select the Python Run, you'll see here we get

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the Fibonacci series.

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So while loops just work like that. They say while, and then there is a condition.

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And as long as this condition is true, the while loop will continue to execute.

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And as soon as this condition becomes not true, then the while loop will end

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and execution will continue after it if there is any code after it, which in

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this case we don't have.

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Down here I could say print ("Done") and if I save that and run it, you'll see we get Done

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at the end.

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You'll notice that the print ("Done") is not indented. If I were to indent it

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here, then it would become part of the loop.

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But by having it back here, at the same indention level as the while statement

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itself, then it is no longer part of this suite of code that is controlled by the while loop.

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So that's how while loops work.

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Let's take a look at for loops.

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Go ahead and make a copy of forloop.py, and I'll name it forloop-working

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and we'll open that.

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What a for loop does is it works with what's called an iterator.

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An iterator is an object that each time you call it,

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it gives you the next value.

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So in this case, we open a file and it's this lines.txt, which we can see right

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here; it just has five lines of text.

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Then for line in fh, that's the file handle, which is actually an object and it

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has this method readlines, which is an iterator.

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So for each time this fh.readlines is called, it will put the next value in the

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line variable, which will be the next line of text, and then we'll print it.

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So when I run it, we get these lines of text.

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Now, you'll notice that there is an extra line ending, an extra new line, in

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each one of these, and that's because the print function puts a new line at the

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end, and each of these lines actually has a new line at the end of it, so we get two for each one.

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If you hover the mouse in Eclipse, you hover the mouse over the print function,

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you get a little bit of documentation for it.

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You'll notice that there is an end option, and it defaults to having this new line.

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So if I just say end= ' ', empty string like that and I save it and run it,

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now, we don't have that effect anymore.

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So, the for loop works likes this.

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It calls an iterator, and its values get put in this variable here.

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It has a colon, like all the control structures in Python, and the block of code

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underneath it is indented.

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So for each line of readlines, it will take the line, put it in the line variable,

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and then print it, just like we see here.

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So that's how for loops work in Python, and those are the two types of loops that

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we'll see in Python.

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We have while loops, which are traditional, conditional loops, and for loops,

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which work with iterators.