The concept of comprehensions allows us to build sequences from other sequences. In an easy, quick and elegant way. Let's start working with list comprehensions. Because they are perhaps the most widely used, they are actually a quick alternative to something we've already seen and used thus far. Let's say we have a list and we want to iterate over it and perform some sort of action on each of its elements, we would use a for loop right, let's build a list of all the elements in the range generated by a range of 10 having each element raised to the power of two. First, let's define an empty list.
This is where the results are going to be appended. So list one equals an empty list. Now it's time to iterate over a range of 10 do the math and then append the results to list one. So for I am range of 10 result equals i to the power of two, and then list one dot append of result. Okay. Now let's see list one nice, we got the result we were looking for.
But what if we could write this whole block of code right here on a single line. Python allows you to do this using a list comprehension. Let's see it in action. So let's see we have lists two, equals open and close square brackets. And we have x to the power of two for x in a range of 10. Enter.
So we have created a list to then in between square brackets. Because we are trying to build a list, we first write the action to be performed on each element, x to the power of two. And then on the same line without any colons commas or indentation, we write the loop structure as we would in the office. fashioned way. Pretty cool, I think now let's see the result, it should be the same list as list one. So list two.
And indeed it is. But what if you want to also add a conditional statement inside this looping construct, let's say you want only the numbers greater than five to be raised to the power of two, the solution is to enter the if statement right after the fourth statement at the end of the list comprehension. Let's do this. Let's consider lists three equals x to the power of two for x in range of 10. So nothing new thus far space if x is greater than five. So as you can see, we have the force statement in the middle.
On the left side, we have the action to be performed. And on the right side, we have the conditional statement. Again, no commas or colons are needed. Let's see the new list now. list three. Great, just what we were looking for.
Now let's see set and dictionary comprehensions. The way to build and write them is the same as for list comprehensions. The only difference being the enclosing character for the comprehension for both set and dictionary comprehensions, we use the curly braces as enclosing characters instead of square brackets. Let's better see an example for each of them starting with set comprehensions. So let's consider set one equals x to the power of two for x in range of 10. Okay, now let's see set one and type of sets.
One, of course is class set. So indeed, we built a set using a set comprehension. Cool. Now let's do the same for dictionaries. Let's remember that dictionary elements are actually key value pairs, where the key is separated from the value by a colon. So this is the format we should use for dictionary comprehensions as well.
So let's consider dict one equals x colon x to the power of two for x in range of 10. Okay, let's see dict one and type of dict one is class dict. Now we have a dictionary built from scratch using a dictionary comprehension. Of course, you can also have nested for loops or if statements, and you can do many other types of operations or iterations inside the comprehension structure. I just wanted you to learn a basic set of operations so that you get familiar with the comprehension concept itself. I think that list set and dictionary comprehensions can be a useful and elegant way to write some code, especially when you want to perform simple for loops and save some space within your program.
So keep them in mind. I can almost guarantee that you will need them at some point in your programming adventure.