Lists
CMSC 201
Overview
Today we will learn about:
• For loops
Motivation
A lot of times, we have a pattern like this:
a=0
myList = [1, 2, 3]
while a < len(myList):
print(myList[a])
This is a way we can iterate over a list.
For Loops
For loops give us a faster way of doing the same thing!
myList = [1, 2, 3]
for listItem in myList:
print listItem
For Loops
myList = [1, 2, 3]
for listItem in myList:
print listItem
In the for loop, we are declaring a new variable called
“listItem”. The for loop is going to be changing this for us!
The first time through the loop, listItem will be the first
element of the list. The second time through the loop, it
will be the second element, and so on until the list is
done.
Example
Finding the average using for loops:
myList = [1, 2, 3, 4]
sum = 0
for listItem in myList:
sum = listItem + sum
print(sum / len(myList)
A Downside!
What do you think this code does?
myList = [1, 2, 3, 4]
for listItem in myList:
listItem = 4
print(myList)
A Downside!
What do you think this code does?
myList = [1, 2, 3, 4]
for listItem in myList:
listItem = 4
print(myList)
Prints:
[1, 2, 3, 4]
A Downside!
What do you think this code does?
myList = [1, 2, 3, 4]
for listItem in myList:
listItem = 4
print(myList)
Changing listItem DOES NOT CHANGE THE ORIGINAL
LIST!
listItem is just a copy of each element.
For Loop Trick
Frequently we want to do something like this:
a=0
while a < 10:
print(a)
a=a+1
A lot of times, we want loops that count.
For Loop Trick
There is a built in python function that gives you a list of
numbers!
a = range(0, 10)
print(a)
Prints:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
The range function gives you a list of numbers between
the first number (inclusive) and the second number
(exclusive)
For Loop Trick
a = range(0, 10)
print(a)
So now we can create a list of numbers. Any ideas what
we can do with that?
For Loop Trick
for num in range(0, 10):
print(num)
An easier way to make a counting loop!
Prints:
0
1
2
3
4
5
6
7
8
9
For Loop Trick
We can even count by whatever number we want!
for num in range(0, 10, 2):
print(num)
Prints:
0
2
4
5
6
8
Summary:
For loops:
• Use for iterating over lists
• Also use in situations where we want loops to count.
While loops:
• Use when we have some condition that doesn’t fall into
the previous two categories.