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Nesting a for loop within another for
loop is a powerful way to process data

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In fact, that's something that's done a
lot. It sounds like something special, but

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it's just really one block of code
inside another block. So let's see an

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example of that, using for loops to
generate times tables. Now these were used

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to teach children multiplication and
normally went up to 12. So I'm going to

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call my new Python file timestables.py.

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Alright, so in terms of the code, I'm gonna start by typing
for i in range parenthesis 1 comma 13 closing

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parenthesis and a colon. Then on the next
line for j in range parenthesis 1 comma

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13 right parenthesis colon. Then we're going to
print on the line after that, print

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parenthesis double quotes left and right
curly brace with a 0 in the middle, times,

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and left and right curly brace with a 1 in
the middle is and left and right curly braces

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with 2 in the middle. Then closing double
quotes dot format. Then we're gonna type

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j comma i comma i times j and we've got two
closing right parentheses. What I'm going

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to do on the next line then, is print, and
we'll just print a series of dashes

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to separate the output. Ok.

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So what have we done here?
Well firstly, we've got an outer loop

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on line one that goes around 12 times.
Inside that on line two, we've got another

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loop that also goes around 12 times and
it'll do that each time the outer loop loops.

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When i has the value 1, j will loop
through the values from 1 to 12.

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i will then become a 2 and j goes
through 1 to 12 again, and so on.

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So I'm gonna run the program first and then we'll
step it through the debugger. So when we

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run it first, you can see we've got the
output there. We're printing the value of

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j first as, you can see, and you can see
that looping from 1 to 12. The second

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value on each line after times is the value
of i, so in the first table, i is 1.

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When we go around the outer loop again,
i is 2. If we scroll down we see the next set of

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tables there, and that's giving us the
second table, and so on.

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Now the debugger can make, can
help with making sense of this.

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So what I'm going to do is set a breakpoint on
line three, the line that does the

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printing, and then let's run the program
in the debugger. Firstly we can see the

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values of i and j in the variables pane.
Not all IDEs do this, but IntelliJ also

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shows the values in the code pane. You
can see it at the top of the screen at

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the end of lines one and two. That's
helpful because you can focus on the

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code and still see the values changing,
but it also means that I can switch to

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the console tab so you can see the
output being printed and still see the

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values of our variables. So I'm gonna go
ahead and do that. Alright, so firstly,

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let's try this. I'm going to step over
the first line and we get back to

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line two - this is the inner for loop. Step
over again, which I'll do.

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You can see j now has the value of two. When I
step over that line again, we get back to

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line two, ready for j to get its next
value. So I'm going to continue stepping

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through the code until j is 12,

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and there's j 12, as you can see there.
That's the end of the inner loop, so if I

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step over again now, you see we get to go
one more time and then it's hit line four.

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We're still inside the outer loop
but the inner loop has now completed.

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If I step over once more now, we're ready to
go round the outer loop again, and I'm

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going to step over twice more to see i
gets its next value 2 and j is

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starting counting from 1 again. Alright
so there's nothing really else to say

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about this. If you want to continue
stepping through the code until you're

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comfortable with what's happening, then
pause the video now and come back to it

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once you've stepped all the way through
it. What I'm going to do now is clear the

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break point, and use the resume program
button over here to let the program

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finish at normal speed, and you can see
the program is now finished. Alright, so

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let's finish this program now with a
mini challenge.

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Here's your mini-challenge. What happens
if we: 1. Indent the print another four

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spaces? In other words we move it over to the right.
What would happen? and 2. What

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would happen if we move the indent
completely for that line. So work

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out what you'd expect to happen before
you try it. Pause the video if you're

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doing that. When you're ready, come back
and we'll go through that challenge then.

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Alright so welcome back. How did you
get on? Did you work out what the effects

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of those changes would be before running
the code? Firstly, looking at line four,

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it's currently part of the block that's
controlled by the outer for loop. So that

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means that all twelve lines for j get
printed out before we get the row of

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dashes in the output. So if we indent
line four, it'll become part of the inner

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loop and should print out every time
around the loop. So let's go up here and do

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that now. I'm just going to tab over, like
so. If I run the program now,

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you can see we get a row of dashes after
every line showing the multiplications,

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and obviously it's made the output significantly
longer as well. Alright, so we'll close

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down the run window. The second part was
determining what would happen if we

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remove the indentation completely.
Alright, so removing the indentation

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completely will take line four out of
both loops. So that means it should only

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print once, right at the end of the
program. So let me do that.

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So I moved it way over to the left, so in
other words, removed all the indentation for

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that line. If we run the program again now,
you can see what's happened there.

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We've only got one row of dashes,
right at the end of all the output.

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So there's another couple of things we
need to know about for loops but we'll

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look at them in the next few videos.
We've seen the basics of how a for loop

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works, to iterate over some sequence. The
sequence can be a string or a range,

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but Python also has other sequences that we
can iterate over. I'll actually be

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talking about lists in the next section
but I'll introduce them in the next

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video. They'll give you more practice at
working with for loops, as well as a way

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to cover two more things we need to know
about them. So, see you in the next video.