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Introduction to Programming with Python

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Repeating Actions with For Loops

Overview

Teaching: 30 min
Exercises: 30 min
Questions

  • How can I do the same operations on many different values?

Objectives

  • Explain what a for loop does.

  • Correctly write for loops to repeat simple calculations.

  • Trace changes to a loop variable as the loop runs.

  • Trace changes to other variables as they are updated by a for loop.

  • Combine for loops with conditionals to process collections of data.

A for loop executes commands once for each value in a collection

Printing a column of letters - the hard way

Let’s say you want to print each letter of the word ‘lead’ on a separate line, underneath each other.

word = 'lead'

With the skills you already have, we could do this by accessing each character in the string using its index. For example, we can get the first character of the word 'lead', by using word[0]. One way to print each character is to use four print statements:

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

l
e
a
d

While this works, it is a bad approach for three reasons:

  1. Not scalable. Imagine you need to print the characters of a string that is hundreds of letters long. It might be easier just to type them in manually.

  2. Difficult to maintain. If we want to decorate each printed character with an asterix or any other character, we would have to change four lines of code. While this might not be a problem for short strings, it would definitely be a problem for longer ones.

  3. Fragile. If we use it with a word that has more characters than what we initially envisioned, it will only display part of the word’s characters. A shorter string, on the other hand, will cause an error because it will be trying to display part of the string that don’t exist.

word = 'tin'
print(word[0])
print(word[1])
print(word[2])
print(word[3])

t
i
n

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-3-7974b6cdaf14> in <module>()
      3 print(word[1])
      4 print(word[2])
----> 5 print(word[3])

IndexError: string index out of range

The alternative way of doing this is to use a for loop:

word = 'lead'
for char in word:
   print(char)


l
e
a
d

This is shorter — certainly shorter than something that prints every character in a hundred-letter string — and more robust as well:

word = 'oxygen'
for char in word:
   print(char)

o
x
y
g
e
n

The improved version uses a for loop to repeat an operation — in this case, printing — once for each thing in a sequence.

The general form of a loop is:

for variable in collection:
    do things using variable

Our example demonstrates all the parts of this general for loop form:

word = 'oxygen'
for char in word:
   print(char)

Here, we have a:

This image conveys how a computer processes and understands this for loop, one step at a time:

loop_image

Each character char in the variable word is looped through and printed one character after another. The numbers in the diagram denote which loop cycle the character was printed in (1 being the first loop, and 6 being the final loop).

We can call the loop variable anything we like, but:

anatomy of a for loop

Consider the following Python code:

for my_int in [1,2,3,4,5,6,7,8,9]:
    print(my_int**2)
  1. What is the variable name?
  2. What is the value of the variable at the end of the loop?
  3. What is the body of the loop?
  4. Which part is the collection?
  5. How many times does the body of the loop get executed?
  6. What is the final value of my_int?

The body of a loop can contain many statements.

Unlike many other languages, there is no command to signify the end of the loop body (e.g. end for); what is indented after the for statement belongs to the loop.

primes = [2, 3, 5]
for p in primes:
    squared = p ** 2
    cubed = p ** 3
    print(p, squared, cubed)

2 4 8
3 9 27
5 25 125

for loop syntax

Which of these are valid for loops?

1.

for word in ["Findable", "Accessible", "Interoperable", "Resuable"]:
  print("FAIR")

2.

for word in ["Findable", "Accessible", "Interoperable", "Resuable"]:
  print(word[0])

3.

for word in ["Findable", "Accessible", "Interoperable", "Resuable"]:
print(word)

Loop Variables

In the example above, the loop variable was given the name word. There is nothing special about this variable name, word was chosen as it makes sense in the context. Just w or even flibble would have worked as well; the important thing is to choose a name that will help you understand your code next time you want to work on it!

What’s in a name?

Consider the for loop from the last exercise:

for word in "Findable, Accessible, Interoperable, Resuable":
    print(word)
  1. Change the variable name to x. How many times do you need to replace word?
  2. Now change it again, to something of your choosing.
  3. Can you think of / find a variable name that doesn’t work?

Reusing variable names

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

Try out this code snippet. Explain the results to your neighbour.

Persistance of loop variables

Run this example for loop code:

length = 0
for vowel in 'aeiou':
   length = length + 1
print('There are', length, 'vowels')

What is the value of vowel now? Why?

From 1 to N: using range for iteration

Python has a built-in function called range that creates a sequence of numbers.

difference between range and list

Try:

print(range(10))

Is this what you expected? Now try:

print(list(range(10)))

Now run and compare the following code snippets:

for i in range(100000000):
    pass

for i in list(range(100000000)):
    pass
  1. Are the two for loops equivalent?
  2. What do you notice that is different?

For efficency, the elements in a range are produced on the fly:

print('a range is not a list:', range(0, 3))
for number in range(0,3):
    print(number)

a range is not a list: range(0, 3)
0
1
2

range can accept 1, 2, or 3 parameters.

Write a loop that uses range to print the first 3 positive integers:

1
2
3

Solution

for i in range(1, 4):
    print(i)

Even integers

Write a loop that uses range to print all even integers between 0 and 100 inclusive.

Solution

for i in range(0, 101, 2):
    print(i)

The accumulator structure

Here’s another loop that repeatedly updates a variable:

length = 0
for vowel in 'aeiou':
    length = length + 1
print('There are', length, 'vowels')

There are 5 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. 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 line 4 tells us our final answer.

In this example, vowel is the loop variable, but we don’t actually use it in the body of the loop. Despite this, it still exists after the loop has finished.

In this example, we use a different variable, length, within the body of the loop. We first assign length to 0 outside the loop, before it starts. Then, within the loop, we update the value of length, each time we progress through the loop. This kind of pattern, where we initialise a variable to 0, then modify its value throughout a loop, is know as an accumulator.

Practice Accumulating

Fill in the blanks in each of the programs below to produce the indicated result.

# Total length of the strings in the list: ["red", "green", "blue"] => 12
total = 0
for word in ["red", "green", "blue"]:
    ____ = ____ + len(word)
print(total)

# List of word lengths: ["red", "green", "blue"] => [3, 5, 4]
lengths = ____
for word in ["red", "green", "blue"]:
    lengths.____(____)
print(lengths)

# Concatenate all words: ["red", "green", "blue"] => "redgreenblue"
words = ["red", "green", "blue"]
result = ____
for ____ in ____:
    ____
print(result)

# Create acronym: ["red", "green", "blue"] => "RGB"
# write the whole thing

Cumulative Sum

Reorder and properly indent the lines of code below so that they print a list with the cumulative sum of data. The result should be [1, 3, 5, 10].

cumulative = cumulative.append(my_sum)
for number in data:
cumulative = []
my_sum = my_sum + number
print(cumulative)
data = [1,2,2,5]

Computing Powers With Loops

Exponentiation is built into Python:

print(5 ** 3)

125

Write a loop that calculates the same result as 5 ** 3 using multiplication (and without exponentiation).

Solution

result = 1
for i in range(0, 3):
    result = result * 5
print(result)

Reverse a String

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

Solution

newstring = ''
oldstring = 'Newton'
for char in oldstring:
    newstring = char + newstring
print(newstring)

Counting Vowels

  1. Write a loop that counts the number of vowels in a character string.
  2. Test it on a few individual words and full sentences.
  3. Once you are done, compare your solution to your neighbor’s. Did you make the same decisions about how to handle the letter ‘y’ (which some people think is a vowel, and some do not)?

Solution

vowels = 'aeiouAEIOU'
sentence = 'Mary had a little lamb.'
count = 0
for char in sentence:
    if char in vowels:
        count = count + 1

print("The number of vowels in this string is " + str(count))

Computing the Value of a Polynomial

The built-in function enumerate takes a sequence (e.g. a list) and generates a new sequence of the same length. Each element of the new sequence is a pair composed of the index (0, 1, 2,…) and the value from the original sequence:

for i, x in enumerate(xs):
    # Do something using i and x

The code above loops through xs, assigning the index to i and the value to x.

Suppose you have encoded a polynomial as a list of coefficients in the following way: the first element is the constant term, the second element is the coefficient of the linear term, the third is the coefficient of the quadratic term, etc.

x = 5
cc = [2, 4, 3]

y = cc[0] * x**0 + cc[1] * x**1 + cc[2] * x**2
y = 97

Write a loop using enumerate(cc) which computes the value y of any polynomial, given x and cc.

Solution

y = 0
for i, c in enumerate(cc):
    y = y + x**i * c

Key Points

  • Use for variable in sequence to process the elements of a sequence one at a time.

  • The body of a for loop must be indented.

  • Use len(thing) to determine the length of something that contains other values.

  • Use accumulator structures with a for loop to update the value of an existing variable

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