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For Loops

Overview

Teaching: 60 min
Exercises: 30 min
Questions

  • How can I repeat the same operations on every item in a list?

Objectives

  • Understand basic loop syntax

  • Be able to explain the role of the loop variable

  • Be able to write simple loops that operate on strings or lists

For loops: Making the computer repeat things

This is where being able to write your own code gets really powerful! Often, we will need to repeat the same calculation over and over, or repeat the same analysis workflow on multiple different files. Loops are one tool that allows us to repeat things.

Because properly using loops requires careful attention to spacing and indentation, we will work primarily from scripts.

Here is a simple example.

#One way to print every letter in a word
word = 'lead'

print(word[0])
print(word[1])
print(word[2])
print(word[3])

However, this is a very inefficient approach! If the word gets much longer, we are better off typing out the letters ourselves.
Or what if we don’t know how long the word is?

Here is a much better way to do things. Notice that we don’t even have to know the length of the string stored in word2:

#A much better way to print every letter in the word
word2 = 'oxygen'
for letter in word2:
	print("The letter is " + letter)
    
print('Done with the for loop')

This approach is much more efficient: We only have type a few lines of code and it is scalable- we could use the same code for any word of any length.

Loop variables

The general format of a loop is this:

for thing in element: #element is a list, string, etc  
	Do something to thing
	Can be multiple lines
	All of these lines will be executed for each thing in the string/list/whatever

This is not part of the loop (is not indented)
Code written like this will only be executed after the loop is finished

In the example above, thing is the loop variable. It will take on multiple values while the loop is running. In our oxygen example, letter will be ‘o’ the first time through the loop, ‘x’, the second time, and so on, until the last time through when it takes on the value ‘n’.

We can name the loop variable anything we want, such as banana or fuzzyKittens, as long as we refer to it inside the body of the loop!

#A much better way to print every letter in the word
word2 = 'oxygen'
for apple in word2:
	print("The letter is " + apple)
    
print('Done with the for loop')

However, this violates all of our rules about good variable names! In general, you should try to give your loop variables descriptive names that will make it clear what your code is doing.

Looping over a list

Previously, we used this code to convert area in pixels to area in mm2:

"""
Created on Mon Apr 24 21:15:50 2017
@author: erin.doyle
A script to convert measured area in pixels to actual area in mm2
"""

#Set variables and conversion factors
AreaInPixels = 6929 
mm2PerPixel = 0.0277 

#Convert from pixels to mm2
AreaInMM = AreaInPixels*mm2PerPixel

#Print the final area in mm2
print('The area in mm2 is', AreaInMM)

Write a new script that uses a for loop to compute and print the area in mm2 for all of the areas stored in the list areas = [6929.6, 8536.47, 11359.3, 17743.4]

Solution

"""
Created on Mon Apr 24 21:15:50 2017
@author: erin.doyle

A script to convert measured area in pixels to actual area in mm2
"""

#Set variables and conversion factors
areas = [6929.6, 8536.47, 11359.3, 17743.4]
mm2PerPixel = 0.0277

#Convert from pixels to mm2
for area in areas:
     areaInMm = area*mm2PerPixel

     #Print the final area in mm2
     print('area in pixels is: ' + str(area) + '. The area in mm2 is: '  + str(AreaInMm)) #print description + number

Using a loop to update a variable

Sometimes, you may need to set up a variable outside of the loop (for example, a counter), and then update it with each run through the loop:

length = 0
for vowel in 'aeiou':
    length = length + 1

print('There are', length, 'vowels')

It’s worth tracing the execution of this little program step by step. Since there are five characters in ‘aeiou’, the statement on line 3 will be executed five times. The first time around, length is zero (the value assigned to it on line 1) and vowel is ‘a’. The statement adds 1 to the old value of length, producing 1, and updates length to refer to that new value. The next time around, vowel is ‘e’ and length is 1, so length is updated to be 2. The trace below walks you through this process.

  vowel length
Before the loop: n/a 0
During the loop: a 0 + 1 = 1
  e 1 + 1 = 2
  i 2 + 1 = 3
  o 3 + 1 = 4
  u 4 + 1 = 5
After the loop: u 5

After three more updates, length is 5; Since there is nothing left in ‘aeiou’ for Python to process, the loop finishes and the print statement on the last line tells us our final answer.

Note that a loop variable is just a variable that’s being used to record progress in a loop. It still exists after the loop is over, and we can re-use variables previously defined as loop variables as well:

letter = 'z'
for letter in 'abc':
    print(letter)
print('after the loop, letter is', letter)

Exercises

From 1 to N

Python has a built-in function called range() that creates a sequence of numbers. range() can accept 1-3 parameters.

If one parameter is input, range() creates an array of that length, starting at zero and incrementing by 1. If 2 parameters are input, range() starts at the first and ends just before the second, incrementing by one. If range() is passed 3 parameters, it starts at the first number, ends just before the second one, and increments by the third one.

For example, range(3) produces the numbers 0, 1, 2. range(2, 5) produces 2, 3, 4, and range(3, 10, 3) produces 3, 6, 9.

Using range(), write a loop that prints the first 3 natural numbers: 1, 2, 3.

Solution

#Create the range
numbers = range(1,4) #note that we have to add one to the top number

for number in numbers: #for loop: numberS is the range, number is one item
     print(number)

Reverse a String

Knowing that two strings can be concatenated (added together) using the + operator, write a for loop that takes a string and produces a new string with the characters in reverse order, so 'Newton' becomes 'notweN'.

Solution

word = 'Newton'
#Create an empty string that we will add on to
newWord = ''

#Loop through word, adding each letter to the end of the string
for letter in word:
    newWord = letter + newWord #The order of these is important!
    
print(newWord) #Print the final value of newWord

Adding up a range of numbers

Modify your range code from the first exercise to add up all of the even numbers between 1 and 10 (inclusive): 2, 4, 6, 8, 10.

Solution

#Create the range
numbers = range(2,11,2) #note that we have to add one to the top number of the range.  The second 2 is the "step" size.

#Create a variable total to store the sum
total = 0
for number in numbers: #for loop: numberS is the range, number is one item
     total = total + number

#after going through all of the numbers, print the final total
print(total)

Key Points

  • Loops require careful attention to spacing and indentation

  • The loop variable takes on the value of each item in the object we are looping over, one at a time

  • You must refer to the loop variable in the code written under the for loop statement

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