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Now that you've seen how list comprehension works,

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it's time to learn about another type of comprehension, dictionary

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comprehensions.

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This is also super useful because it allows us to create a new dictionary from

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the values in a list or in a dictionary. For example,

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we could write out our keywords like this.

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We're going to create a new dictionary called new_dict,

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and it's going to be created using this curly bracket now instead of square

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brackets. And inside the comprehension, we get to set the new_key,

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the new_value, and we get to loop through items in a list.

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So any sort of iterable; a list, a range, a string, anything you want.

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And this is the simplest form of dictionary comprehension.

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Now remember that a dictionary comprehension is just a way of creating a

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dictionary using this shortened syntax.

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We could take this one step further.

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We could also create a new dictionary based on the values in an existing

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dictionary.

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So we could take that dictionary and then get hold of all of the items inside

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that dictionary, and then split it into a key and a value.

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So now, we're looping through each of the keys and each of the values in all of

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the items from the dictionary,

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and we can use these key and value variables to create a new_key or a new_value.

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And to go one step further for completion sake, we can, of course,

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as always add our condition at the very end,

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just like we did before with list comprehensions.

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So let's see how this code works in practice.

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I'm going to take my previous list of names

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and then I'm going to click on this button to refresh my console so that all of

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the previous variables get deleted.

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So now I'm going to create a new list of names.

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So I'm going to use the previous names I had before, and this is our list here.

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Now, the next thing I'm going to do is I'm going to create a dictionary where I

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generate a random score for each of these names.

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So let's imagine we had a bunch of students and we wanted to create some random

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scores for each of them so that they would get a random score between 1 and

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100. Now for the teachers out there,

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I really hope this is not what you do when you generate the students' scores.

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But essentially we want to create a dictionary

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which looks something like this. Each of the students in this list,

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so let's start out with Alex. We want our dictionary to have a key,

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which is the name of the student, and then a value,

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which is their random score.

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And we want to continue through that list of names until we've generated a

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random score for each of them. That's the goal.

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And now we have to use our dictionary comprehension.

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Remember, when we're creating a new list using lists comprehension,

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we're using square brackets. But now we're creating a dictionary

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so we're going to use curly brackets. Now, inside these curly brackets,

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I'm going to put down my usual list of keywords.

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So it's going to be a new_key:

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new_value that we're going to be adding to our new dictionary.

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And then I'm going to do

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a for item in list. In this case,

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our list or our iterable is going to be all lists of names.

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Now, even though in my keywords I always use lists,

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it's just easier to imagine. But it's actually, in fact,

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any sort of iterable like a range or a string or a tuple.

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Once we've got our list of names,

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we're going to name each of those names something,

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and I'm just going to call it student.

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So we're looping through all the students in our list of names here.

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And for each of those students,

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I'm going to create the student as the new key,

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but the value is going to be a new, random number.

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So in order to create a random number,

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I'm going to have to first import the random module.

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So I've cut out my previous code, imported random,

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and let's paste that code back in.

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So now we can replace the value for each of these dictionary entries with random

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.randint,

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and we can generate a random number between 1 and 100.

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So now when I hit enter, you'll see our new students_scores

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dictionary being created

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and you can see we've got all of our students in our list of names, Alex, Beth,

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Caroline, et cetera.

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And each of them now have a new random score.

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So it seems like everybody did pretty well other than Eleanor,

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I guess. Random score generating seems to work. Now,

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the next step,

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we're going to take this a little bit further because this was quite simple.

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We just did what we did before, which is looping through a list.

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But now I want to loop through a dictionary. Luckily,

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we've now created this brand, sparkling, new dictionary, our students_scores,

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and I want to use that dictionary in my next dictionary comprehension.

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So the next thing I want to do is to be able to create a dictionary called

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passed_students. And this is going to be a dictionary

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which looks through the dictionary of student_scores,

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so imagine you have an entire high school where everybody's scores are logged

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and you look at everybody who's got a score of 60 or over.

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And then we're going to say, well,

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those people have passed and everybody else has failed that year.

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Essentially we should loop through this entire dictionary of items,

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figure out which of these have a value that's equal to 60 or over 60,

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and then we're gonna add those items back into this new dictionary

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of passed_students.

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So essentially, we'd probably end up with, um, Beth,

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with Caroline, with Dave and with Freddy.

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So it will still look pretty much the same as our students_scores dictionary.

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So it would have something like Beth and then colon her score,

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which is 72. But just the fact that she's included in this dictionary,

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it means that she has passed.

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And we will end up with this new dictionary where everybody is a passed student

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and bear scores.

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Have a think about how you might do this. And if you're up for the challenge,

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try pausing the video and seeing if you can complete this.

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As always, we know that we're going to create a new dictionary, so curly braces,

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and then it's going to be our new_key: new_value.

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And then it's going to be for, and now,

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because we are looping through this dictionary and we want to get hold of each

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of the values,

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we're going to use the method that we saw in our previous slides.

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So we're going to get hold of each of the keys and the value from our

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dictionary. And then after the in keyword,

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it's the thing that we actually want to loop through, which is our dictionary.

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Once we've got our dictionary,

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then we need to call the items method on that dictionary.

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So it's items with a set of parentheses,

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not just the items as an attribute.

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So now that we've got our keywords down,

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let's convert it to our particular case.

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So our dictionary is called students_scores,

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let's make sure I spell it correctly.

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So students_scores.items gets all of the items in that dictionary.

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And then for each of the keys

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which is going to be students, and for each of the values

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which is going to be their score, we're going to loop through this dictionary.

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The new key is just going to be the student's name and the new value is just

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going to be their score.

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But what we need to do is we need to check using an if statement,

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if the student that we're currently looping through has passed.

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So the test is basically looking at the score and seeing if it is greater than

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or equal to 60. And if that is the case,

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then we're going to take this student as the new key,

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their score as the new value, and place it inside this new dictionary.

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So now if I go ahead and hit enter,

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then you can see we've got our new dictionary being created,

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which only contains all the students who have passed. So it's Beth, Caroline,

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Dave, and Freddy, and we've lost Alex and Eleanor.

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So in order to get more practice at this,

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I've prepared some more coding exercises for you on dictionary comprehension.

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So head over to the next lesson and we'll get started.