Remember lists? They look like this:
flowers = ['rose', 'violet', 'buttercup']
For now, let's just create a list and print it out. Our script will look like this:
flowers = ['rose', 'violet', 'buttercup']
print(flowers)
Save this to a new file called loop.py and run it with python loop.py. You should see the list printed out in the terminal.
So far, we've only learned one function: type(). Let's try out another:
flowers = ['rose', 'violet', 'buttercup']
# print(flowers)
list_length = len(flowers)
print(list_length)
The len() function returns the number of items in a list or the number of characters in a string.
Notice that, if you run the code above, you won't see the flowers list printed out. That's because that line has become a comment. If you put a # (hash or pound) at the beginning of a line, that line will be ignored.
A useful property of a list is the list index. This allows you to pick out an item from within the list by a number starting from zero:
print(flowers[1]) # rose
print(flowers[1]) # violet
You can print out the last item in a list using negative numbers:
print(flowers[-1]) # buttercup
What if we want to print out each item in the list separately? For that, we'll need something called a loop:
flowers = ['rose', 'violet', 'buttercup']
# print(flowers)
for flower in flowers:
print("My favorite flower is the", flower)
What's happening here? This kind of loop is called a "for" loop, and tells Python: "for each item in the list, do something." Let's break it down:
for <variable name> in <list name>:
<do something>
Indented code like this is known as a "code block." Python will run the code in the code block once for each item in the list. You can also refer to <variable name> in the block.
You can also perform more complicated operations. Let's tackle one in a challenge.
Here's a list of numbers:
prime_numbers = [2, 3, 5, 7, 11]
Write some code to print out the square of each of these numbers. The solution is here, but you're not allowed to look at it until you've tried to solve it yourself for 3.5 minutes. Seriously. That's 210 seconds.
First, ignore this challenge because it's too hard. Next, look up string formatting on Google and use it to write a loop that gives the following output:
The square of 2 is 4.
The square of 3 is 9.
The square of 5 is 25.
The square of 7 is 49.
The square of 11 is 121.