Python Loops | jcchouinard.com

Hi everyone, my name is Jean-Christophe Chouinard. In this beginner Python tutorial

we will learn about Python loops. There are two types of loops in Python, the for loop and the

while loop. Those loops are control flow statements that you can use to repeat a block of code again

and again and again and again. The while loop will loop recursively as long as the condition is true

so it can loop infinitely. The for loop allows you to give you a sequence on which to iterate over

So let's look at the first type of loop. So the while loop executes until the condition

becomes false. So it will loop forever as long as the condition is true. So we could say i equal 10

or i equal 0 and we could say while i is less than 5, print i and at each iteration we could

say i plus equal one so we would add one to the to to i element so we can see that it looped

until the condition was met and then stopped one thing to be aware of is that a while loop

runs as long as the condition is true so if you say while true and you said print hello for example

and then you run this it will run forever as long as the code is not stopped so here i'm

interrupting it but it would have printed for as long as the condition is true and in this case that

was forever. So in order to prevent that we could say use the break statement so we could say while

true print hello and in this case if we use the break statement it will loop only once because

we decided to break it after the first iteration so it printed it only once. One last concept I

want to introduce with the while loop is the else statement. So the while loop operates a bit like

the if else statement and you can also do it. So let's take our previous loop that we have seen

and what we want to know is whenever it breaks we want to print a statement that says i is greater

than five. So if we do this we can then return something. So it did the entire loop and when it

break broke it returned the print statement. Next we have the for loop so in this case the for loop

is iterating through a sequence so while the while loop was always running as long as the condition

is true the for loop can allow you to give gates on which it should loops on so for example we

could say for i in and we could a sequence which is a list this case and we can say print i

Running this, we can see that it went through each element of the sequence and it printed it

You can loop other types, for example you can loop a string

So we could say for letter in hello, then we could say print letter

So you don't have to loop through a list, you can loop through any type of sequence in Python

Again we can use the break statement in the for loop so for example if we take again this loop

that we had we could say if letter equals equals e then we could say break the loop so as print each

element when the letter equals it then you break it. So the break statement stopped the loop early

and we ended up with the statement. So we have just seen how the break statement

stopped the loop early. The continue is also another control flow statement that can be used

in order to skip to the next situation in the loop. So for example what we could say is we could say

for letter in python and we can say okay if the letter equals equal h then we could say continue

So in this case the loop doesn't stop we say print letter it doesn't break it just

skip to the next iteration so when we look here it printed each letter apart from the h

So we already saw that the for loop just iterate through a sequence of elements. So in this case

the range function is returned the sequence of elements. So you could say for i in range

5 and we could say print i. So then what we will do is you will print each element from

0 to 5 using the range function. You can also use the range parameters in order to increment. So you

could say for x in range and then we could say print x. So here what will happen is you we will

start with the 2 and then we will end at 20 and we will increment by 2. So if we use that for loop

we see that okay two four six eight ten blah blah blah until we reach 20 So we can also nest loop within loop so you can also create a loop inside of another loop and the only way the simple way to do this is you do the for i in and you could say abc and then by

convention what you'll do is you'll move to the next letter so if you're inside it you would say

for j in def and then you could print i j so what will happen here is it will loop through each a's

and then for each a will loop through the def and then it will move to the b's def and the c's def

so this looping nested looping technique is very useful in python to do things such as looping

through a nested list. So if you want to print each of these elements, for example, you could use the

nested for loop by saying for i in or for nesting in list and then you could say for i in nest

and then you could print i. So what will happen here is you'll print each of the nested element

because without that you would have just did a simple loop and have done for listen nest and

you would have printed the entire list. If you want to loop through dictionaries now there are

many ways you can do it but you can use the dictionaries method to do so. So if we have that

dictionary if we print that my dict if we return that to the the console we see that we have a

dictionary and we can use the myDictKeys method in order to just get the keys. So we can see that

this method will return a list of the keys from the dictionaries or you could use the myDict

values. Let's just look at the the methods for now but this method would return the values

instead of the keys or alternatively you can use the items method to return the output as a tuple

So in this case you're creating a list of tuples for which we can loop. So if you look at this

pattern you'll see that it's similar to looping through nested items. So let's look at how we can

print keys for example. Using the for loop we can print all the keys of the dictionaries by

looking at for x in my dict and keys. We've seen that the keys method return all the keys from the

dictionary so then if we print x we'll see that we're printing each key. We can also print the

values from it using the keys method so you could say okay my dict x because in this case what we're

doing is we're selecting the key from the dictionary so here we're printing the values from the

dictionaries. So far I hope this is not too confusing if you've never done essentially what

this is is if I'm taking my dictionary and I'm selecting name from it I'll have the value right

so in this case it's exactly what it does but it's just looping through each value. The alternative

way to do this is to use them the values method on the dictionary so you could say for x in my dict

values then instead of having to select it we're just printing x this time and we're again returning

all the values so if you want to print keys of values then it would be very complex to say okay

for key in my dict keys and then you would say okay for

So print key, and then you would say myDictKey. So then you would, in this way, what you'll do is you would print the key values, right

A better way to do this would be to use the items method just as what we've seen

And what we would do is instead of saying for key here, we would say for key value

and then it would iterate through each of them. similarly but in a much simpler way. In Python there is a concept of iterators. An iterator

in Python is an object that it can be iterated upon. So by definition we can use a for loop to

iterate through an iterator. So one of the examples of this is an enumerate function

returns an object that can be iterated upon. So if we just return the object here what we'll return

is an object for which we can loop and if we want to get the first element we could use the next

function for example to go this. A better way to loop through that iterator however is to use the

for loop. So for example we could say we can use the enemy rate list here and we could say for index

letter in n rate list equals print f index and return the index here and say letter

This is a good way to just show what is the index of your element

Otherwise what you would have needed to do would have been to say okay for so we could have

instantiate i equals 0 for letter in list print i letter and then we would have done i plus equal

1. So that would have been a second way to do the same thing as what we just did so the

intramarator is very useful for you to do this kind of logic. Next we will loop through pandas

data frame. So if you've never seen what a pandas data frame is, it's essentially what's built on

NumPy and it creates and uses arrays to create table like structure. So for example if I run this

pandas data frame here what I have is a pandas with the column I gave it, the index I gave it

and the values associated with each element. So if we want to loop through each element in the

pandas data frame for example what we'll need is to use a for loop again so we can say for column

in data frame and then we can print the column so that's how you loop through each of the column to

print them off otherwise you can use the iterose method so if you do df iterose

you will end up with a generator object and from that generator object you can use the

next method and say okay let's run this it and we return the area object and then what we'll do

is we'll get the first row with each of the column using the df iter row. We can start

integrating this with the for loop so we can say for index row in df iter rows and what we could

do is say okay let's print the index first and run this. So what we'll see is we're printing each

of the index here. We can also print each row. So we can take this iterator object and we can start

splitting it into each of the elements and then we can decide to print the type of the row. For

example, if we run this again, so what we'll see is that we are running the index here, we're running

the row so we're printing each column and for example for each of the column we could say okay

just like one element so column one here double quotes like this

and here you can say that I wrote So we can see here I selected the element from column 1 here this way

So there are many ways so you can select each index from the data frame

You can select an entire row. You can select one element from a row using the row and column

And you can also show for example the type which is Cpandas series here

So let's do that again just for simple more clarity. So we can say for index, row in df, either rows

And then we can say print the index. And I just want to loop through one of the columns this time

And I just want to print the row for the column one

and then we print this and we can see that it looped through each of the element but just for

one column this time. Cool, so now we have learned while loop, for loops, we've learned how to use

generator objects to loop, we've learned how to loop to pandas data frame. Let's dive into slightly

more complex concepts and one of these is how do you break multiple loops because if I take this

loop here. I will range through the value from 0 to 9 and then I will range a loop from each a, b

and c and then I will print ig. What I want to do here is I want to break the entire loop as long as

soon as I reach the letter b. So I just use the if j equal b then break. If I run this I would have

assumed that it would have broke the entire thing, right? But what it did is it looped to the I value, AB, and then broke the second loop

So we never printed C That not what I wanted I want the entire loop to break If you want to do this we don break using the break statement What we would do instead is we would use a function to say for define breakout

and we can put this entire thing inside the function and instead of using the break statement

I would have used the return statement. I won't dive too much into what this is, but using the

the return statement will immediately exist the function. So it will stop the function as soon as

you return the statement. So in this case if you want a loop multiple loops, break multiple loops

you just enclose it into a function and if you run this then you have the correct behavior where

the entire loop broke when you reached the letter B. Finally, I want to teach you one thing is that

for loops can become very very big because you start using for loop and then you end up with

stuff like for i in and then for j in and then for l in and then for blah blah blah. This can

become very complex to read in code. So it is good to know how you can avoid

for loops when you can. There are three ways that you can, three main ways that

you can avoid for loops is one you can use list comprehension, the other you can

use some functions plus the lambda function. So for example if I have this

for loop it will print each value it will print a list where it had appended the values in range 10

If you want to convert this into a list comprehension instead what you would do here

is you would say okay you use the square brackets i4i in range 10. And in this case you can print

ls2 and just say print ls2 and this list comprehension does the same thing as what this loop does

The second technique that you can use to avoid a for loop is to use the map function along with

the lambda. So if I'm taking that list what I'm doing here is I'm taking a list from range 10 but

I'm doubling each element and append it to a list so I end up with 0, 2, 4, 6, 8 with even numbers

Instead of doing this you could have said okay list2 equal and you use the map method and in the

map method you first need to provide a function and then what is the iterator you use on. So in

this case you use range 10 similar to what we've done it here and here in the function we replace

this by lambda x x times 2 and if you print that second list again you will see that you end up

with a map object so instead of printing this what you want to do is return the list element

of all that and you can see that we have the same result without using a for loop this time

the last way to avoid for loops that I want to introduce is getting using the filter method

again with the lambda function so here what i have is a list and i will just want

append the even numbers so here i have in range 10 and i use the mod

modulus operator and one when the modulus is equal zero then i append i

and the result is the fact that i'm just appending even number here

If I want to do the same thing without a for loop this time I would use the filter method

the filter function, sorry. And the filter function works similarly to the map function

What you want to prove give is one function here and you want to put a your iterator here on second

argument and what we'll do is we'll replace the function by lambda x x

modulus 2 equals equal 0. So essentially we're doing the same thing as what we were doing with

the if and we're do applying the filter and if we print even 2 then we will show again an object

instead so we have to put it as a list instead and doing this we end up with the same result

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