Python Projects for Beginners a ten-Week Bootcamp Approach to Python Programming

If-Else Statements

Let’s take it one step further now and add in an else statement. This time we’ll append the string “Even” when the number is divisible by two; otherwise, we’ll append the string “Odd”:

# using if/else statements within list comprehension
nums = [ "Even" if x % 2 == 0 else "Odd" for x in range(10) ] # generates
a list of even/odd strings print(nums)

Go ahead and run the cell. This will output a list of strings that represent the numbers odd or even value. Here we append the string “Even” when the if conditional is True; otherwise, the else statement will be hit and append the string “Odd”. The same representation of code without comprehensions can be found in the following:
>>> nums = [ ] >>> for x in range(10):
>>> if x % 2 == 0:
>>> nums.append("Even") >>> else:
>>> nums.append("Odd")
We’ve reduced the lines of code from six down to one. Comprehensions are great for quick generation of data; however, it becomes more difficult when the conditions are larger. Comprehensions don’t allow for the use of elif statements, only if/else statements.

List Comprehension with Variables

Comprehension is great for generating data from other lists as well. Let’s take a list of numbers and generate a separate list of those numbers squared, using comprehension:

# creating a list of squared numbers from another list of numbers using list comprehension nums = [2, 4, 6, 8]
squared_nums = [ num**2 for num in nums ] # creates a new list of squared numbers based on nums
print(nums)

Go ahead and run the cell. We’ll get an output of [ 4, 16, 36, 64 ]. For this example, we were able to generate the squared numbers by appending the expression “num∗∗2”.
The same representation of code without comprehension would look like the following:
>>> squared_nums = [ ] >>> for num in nums:
>>> squared_nums.append(num**2)
In this example, we were able to reduce the lines needed from three to one.