Python For Loops - Python Tutorial for Absolute Beginners

[MUSIC PLAYING]

MOSH HAMEDANI: There
are times that we

may want to repeat a
task a number of times.

For example, let's say we
send a message to a user.

If that message
cannot be delivered,

perhaps we want to
retry three times.

Now, for simplicity,
let's imagine

this print statement is
equivalent to sending a message.

In a real-world program, to
send a message to a user,

we have to write 5
to 10 lines of code.

Now, if you want to
retry three times,

we don't want to
repeat all that code.

That is ugly.

That's when we use a loop.

We use loops to
create repetition.

So here is how it works.

We start with for, number, in.

We have a built-in
function called range.

Now how many times do we
want to repeat this task?

Let's say three times.

So we call range and
pass 3 as an argument.

Now, similar to
our if statements,

we need to terminate
this line with a colon.

Enter-- we get indentation.

So in this block, we can
write all the statements that

should be repeated three times.

Let's do a print, a
message like attempt.

Save the changes.

Run the program.

So we have attempt
printed three times.

Beautiful.

Now, what is this number?

Let's take a look.

It's a variable of type integer.

So let's pass it as the second
argument to the print function.

Number.

Run the program.

This is what we get--

0, 1, 2.

So here we have a for loop.

This for loop is
executed three times.

In each iteration, number
will have a different value.

Initially it will be 0.

In the second
iteration, it will be 1.

And finally, in the last
iteration, it will be 2.

Now here we can
do something fun.

We can add 1 to this.

Run the program.

And now the messages
that we print

are more meaningful
or more user-friendly,

like attempting number 1,
attempting number 2, and so on.

We can take this
to the next level.

So we can pass another argument.

Here, I'm going to
add an expression.

One more time.

Number plus 1.

So we'll get 1, 2, 3.

Now I want to put this
expression in parentheses.

So let's select this.

Put it in parentheses, and
then multiply it by a dot.

So here we have a string that
is multiplied by a number.

The result will be that string
repeated that number of times.

Let's take a look.

So run the program.

See.

That's pretty cool, isn't it?

Now, let me show
you one more thing

before we finish this lecture.

As you saw, this range
function generates

numbers starting from 0 all
the way up to this number here,

but it doesn't
include this number.

Here we can pass another
argument, say, start from 1

and finish before 4.

With this change, we don't
need to add 1 to number

every time, because in
the first iteration,

this number variable
will be set to 1.

So we can simplify our
code and make it cleaner.

Let's run it one more time.

We get the exact same result. We
can also pass a third argument

as a step.

So I'm going to change
the second argument to 10

and pass 2 as a step.

Look at the result. These
are the numbers we get--

1, 3, 5, and so on.

So pretty useful.

You're going to use
this function a lot

in real-world applications.

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Continuing with the example
from the last lecture,

let's imagine the scenario
where after the first attempt

we can successfully
send the message.

In that case, we want to
jump out of this loop.

We don't want to repeat this
task of sending a message three

times.

Let me show you how
to implement this.

So in this demo, I'm going to
simulate the scenario where

we can successfully
send a message.

So we define a variable,
successful, and set it to true.

Now here after this
print statement,

we'll have an if statement.

If successful, colon, then
perhaps we can print successful.

Now here we want to
jump out of this loop.

For that, we use
the break statement.

Let's run this program
and see what happens.

So there you go.

After the first attempt,
we are successful,

and there are no more attempts.

So once again, I want you
to pay great attention

to the indentation here because
that's one of the common issues

amongst beginners.

So here's for loop.

These two lines are
indented with four spaces,

and they belong to a for loop.

In every iteration, these
two lines will be executed.

Now when we get to line 4,
if this condition is true,

then these two lines
will be executed

because both these lines
are indented below this if

statement.

Now let's take this
program to the next level.

What if we attempt three
times and we still cannot send

an email?

Perhaps we want to display a
different message to the user.

We say, hey, we tried three
times, but it didn't work.

So I'm going to change
successful to false.

Now at the end, here we
can add an else statement.

This is what we call a
false else statement.

What we put under
this else statement

will only be executed
if this loop completes

without an early termination.

So if we never break
out of this loop,

then the else statement
will be executed.

So here we can print a message
like attempted three times

and failed.

So run the program.

See what we get.

Three attempts followed by this
message-- attempted three times

and failed.

In contrast, if we change
successful to true,

because we terminate this loop
using this break statement,

what we have in the else
block will not be executed.

Take a look.

Run the program.

We have one attempt.

Successful.

Done.

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In programming, we have this
concept called nested loops.

So we can put one loop
inside of another loop.

And with this, we can get
some interesting results.

Let me show you.

So I'm going to start with this
loop-- for x in range 5 colon.

Now inside of this loop, I'm
going to add another loop.

So for y in range 3 colon.

And then in our second loop, I'm
going to add a print statement.

Here we can use formatted
strings to display coordinates.

Remember formatted string, so
we have f followed by quotes.

Now here we add parentheses
for our coordinate.

First, we want to display x
and then comma followed by y.

Let's run this program
and see what happens.

There you go.

Pretty cool, isn't it.

So we get 0 and 0,
0 and 1, 0 and 2.

Then we get 1 and 0, 1
and 1, 1 and 2, and so on.

Now let me explain how exactly
Python interpreter executes

this code.

So here we have two loops.

This is what we
call the outer loop,

and this is the inner loop.

So the execution of our
program starts here.

In the first iteration
of this loop, x is 0.

Now we get to this
statement, which

is a child of this for
statement because it's

indented four times.

This statement itself is a loop.

So what we have
inside of this loop

will be executed three times.

In the first iteration, x
is 0 because we're still

in the first iteration
of the outer loop.

And y is also 0 because we
are in the first iteration

of the inner loop.

That is why we get 0 and 0.

Now we go to the second
iteration of this inner loop.

In this iteration, y will
be 1, whereas x is still 0.

That is why we get 0 and 1.

And similarly, in the third
iteration of our inner loop,

we'll get 0 and 2
in the terminal.

Now we're done with the
execution of the inner loop.

So the control moves
back to our outer loop.

Here will be in the
second iteration.

So x will be 1.

And then we start here again.

So we have to execute this
inner loop three times.

In the first iteration,
y will be 0, and x is 1.

So here we have 1 and 0.

Then we'll get 1
and 1, and 1 and 2.

You got the point.

So this is all
about nested loops.

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So you have learned
how to use for loops

to repeat one or more
statements in your programs.

Now let's dive
deeper and see what

this range function returns.

So earlier, you learned about
the built-in type function.

With this function, we can
get the type of an object.

So if I pass 5 here and run this
program, this is what we get.

So the type of this number or
this object is int or integer.

Now let's look at
the type of the value

that we get from
the range function.

So as an argument, we
pass range of a number.

Let's run this program.

So this range function returns
an object of type range.

So in Python, we have primitive
types like numbers, strings,

and Booleans.

But we also have complex types.

Range is an example of one
of those complex types.

Throughout this
course, you're going

to learn about a lot
of other complex types.

Now what is interesting
about this range object

is that it's iterable, which
means we can iterate over

it or use it in a for loop.

That is why we can
write code like this.

So this range function
returns a range object

which is iterable, which
means we can iterate over it.

In each iteration, x will
have a different value.

Now, range objects are not the
only iterable objects in Python.

Strings are also iterable.

So here we can add a
string like Python.

Now in each
iteration, x will hold

one character in this string.

Let me show you.

So print, x, and I'm going to
delete these two lines here.

Let's run this program.

So in each iteration, we'll
get one character and print it.

We have another complex
type called list which we

use to store a list of objects.

So we add square brackets.

This indicates a list.

Now we can add a list of
numbers or a list of strings,

like a list of names.

You will learn about
lists later in the course.

So let's run this one more time.

As you can see, we can
iterate over lists.

In each iteration, we'll
get one object in this list.

Now, later in the
course, I will show you

how to create your own custom
objects that are iterable.

For example, you will learn
how to write code like this--

for item in shopping
cart, print item.

So shopping cart is
going to be a custom

object that you will create.

It's not going to be an
integer or a string or Boolean.

It's a custom object.

It has a different
structure, and we

will make it iterable so we
can use it in a for loop.

And in each
iteration, we can get

one item in the shopping cart
and print it on the terminal.

[MUSIC PLAYING]

All right, time for an exercise.

I want you to write a program
to display the even numbers

between 1 to 10.

So when you run this program,
you should see 2, 4, 6, and 8.

And after these, I want
you to print this message--

We have four even numbers.

Now here is a quick hint
before you get started.

You should call the range
function with 1 and 10.

Do not use the third argument,
which is called step.

So basically, I want you to
iterate over all the numbers

between 1 to 10.

Check if each number
is an even number

and then print it
on the terminal.

So pause the video.

Spend two minutes
on this exercise.

When you're done, come back.

Continue watching.

So we start with a for loop--

for number in range
1 to 10 colon.

We check to see if the remainder
of division of this number by 2

equals 0.

So if number modulus 2 equals
0, then we print this number.

Now let's run this program.

So we get 2, 4, 6, 8.

Beautiful!

Now to count the even numbers,
we need a separate variable.

So let's call that count.

Initially, we set it to 0.

Now in this if block, every
time we find an even number,

we need to increment count.

So we set count plus equals 1.

And finally, after for loop, we
can print a formatted string.

We have count even numbers.

Let's run the program.

And here's the result.

Hi, guys.

Thank you for watching
this tutorial.

My name is Mosh
Hamedani, and I have

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