Intro to Python 4
Week 1 | Lesson 2.3
After this lesson, you will be able to:
- Iterate using for loops
- Iterate using while loops
Before this lesson, you should already be able to:
- If you have time, watch a video on for and while loops so that you're familiar with the topic.
|10 min||Introduction||for and while loops|
|25 min||Demo / Guided Practice||for loops|
|25 min||Demo / Guided Practice||while loops|
|25 min||Independent Practice|
Introduction: for and while loops (10 mins)
The for statement is used to iterate over the elements of a sequence. It's used when you have a piece of code which you want to repeat n number of times. You can use any object (such as strings, arrays, lists, tuples, dict and so on) in a for loop in Python.
The while loop tells the computer to do something as long as a condition is met. A while loop consists of a block of code and a condition. The condition is evaluated, and if the condition is true, the code within the block is executed. This repeats until the condition becomes false.
Demo / Codealong: for loops (25 mins)
Instructor Note: demo code
In iPython notebook type:
# basic for loop example for count in [1, 2, 3]: print(count) print('Yes' * count)
This is a for loop. It has the heading starting with for, followed by a variable name (count in this case), the word in, some sequence, and a final colon. As with function definitions and other heading lines, the colon at the end of the line indicates that a consistently indented block of statements follows to complete the for loop.
Let's try a simple repeat for loop. When you just want to repeat the exact same thing a specific number of times. In that case only the length of the sequence, not the individual elements are important.
In iPython notebook type:
# simple repeat loop for i in range(10): print('Hello')
Let's try a for loop through words. In iPython notebook type:
# using a for loop to go through the letters in a word word = 'computer' for letter in word: print letter
Let's try using a for loop to print out a list. In iPython notebook type:
shuttles = ['columbia', 'endeavour', 'challenger', 'discovery', 'atlantis', 'enterprise', 'pathfinder' ] for s in shuttles: print s
Now, try creating a few for loops on your own.
Check What is the basic syntax for a loop?
Demo / Codealong: while loops (25 mins)
Let's create a simple counter using a while loop. In iPython notebook type:
# create a counter using a while loop count = 0 while count < 5: print count count = count + 1
Let's try another. In iPython notebook type:
# another counter using a while loop a = 0 while a < 10: a = a + 1 print a
And maybe one more... In iPython notebook type:
while True: reply = raw_input('Enter text, [type "stop" to quit]: ') print reply.lower() if reply == 'stop': break
This while loop will stop when the user types "stop".
Remember, a while loop runs until the expression is False. The problem is, sometimes they don't stop. To avoid this, here are some rules to follow:
- Make sure that you use while-loops sparingly. Usually a for-loop is better.
- Review your while statements and make sure that the boolean test will become False at some point.
- When in doubt, print out your test variable at the top and bottom of the while-loop to see what it's doing.
Now, try creating a few while loops on your own.
Check What is a common problem that might occur with while loops? What are ways to avoid this problem?
Independent Practice: Topic (25 minutes)
- Keep practicing for and while loops
Conclusion (5 mins)
- When is a for loop useful?
- When is a while loop useful?
- What are the differences between for and while loops?
- What is a problem that while loops run into?
- What are some things you can do so you don't run into this problem with while loops?