Repeating Actions with Loops
Overview
Teaching: 30 min
Exercises: 30 minQuestions
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.
It is very good coding practise to reuse as much code as posible as generally, the fewer lines of code there are, the fewer bugs you’ll have. Loops are a very good way of doing this where a block of code is repeated a number of times or on each element in a collection. An example task that we might want to repeat is printing each character in a word on a line of its own.
word = "lead"
We can access a character in a 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
This is a bad approach for two reasons:
-
It doesn’t scale: if we want to print the characters in a string that’s hundreds of letters long, we’d be better off just typing them in.
-
It’s fragile: if we give it a longer string, it only prints part of the data, and if we give it a shorter one, it produces an error because we’re asking for characters 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
Here’s a better approach:
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 with variable
Using the oxygen example above, the loop might look like this:
where 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 (char
in this case) anything we like, but
there must be a colon at the end of the line starting the loop, and we must indent anything we
want to run inside the loop. 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.
What’s in a name?
In the example above, the loop variable was given the name
char
as a mnemonic; it is short for ‘character’. We can choose any name we want for variables. We might just as easily have chosen the namebanana
for the loop variable, as long as we use the same name when we invoke the variable inside the loop:word = 'oxygen' for banana in word: print(banana)
o x y g e n
It is a good idea to choose variable names that are meaningful though, otherwise it would be more difficult to understand what the loop is doing.
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.
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)
a
b
c
after the loop, letter is c
Note also that finding the length of a string is such a common operation
that Python actually has a built-in function to do it called len
:
print(len('aeiou'))
5
len
is much faster than any function we could write ourselves,
and much easier to read than a two-line loop;
it will also give us the length of many other things that we haven’t met yet,
so we should always use it when we can.
From 1 to N
Python has a built-in function called
range
that creates a sequence of numbers.range
can accept 1, 2, or 3 parameters.
- If one parameter is given,
range
creates an array of that length, starting at zero and incrementing by 1. For example,range(3)
produces the numbers0, 1, 2
.- If two parameters are given,
range
starts at the first and ends just before the second, incrementing by one. For example,range(2, 5)
produces2, 3, 4
.- If
range
is given 3 parameters, it starts at the first one, ends just before the second one, and increments by the third one. For exmaplerange(3, 10, 2)
produces3, 5, 7, 9
.Using
range
, write a loop that usesrange
to print the first 3 natural numbers:1 2 3
Solution
for i in range(1, 4): print(i)
Turn a String Into a List
Use a for-loop to convert the string “hello” into a list of letters:
["h", "e", "l", "l", "o"]
Hint: You can create an empty list like this:
my_list = []
Solution
my_list = [] for char in "hello": my_list.append(char) print(my_list)
Count Occurances of Letters in a String
Count the number of occurances of each letter in a string. Use a dictionary to store the number of times each letter comes up and the loop variable as the key to that dictionary. You will need to check if the letter has been encountered before, i.e. is it
in
the dictionary? and if not, create it.Solution
my_str = "A long string that contains lots of different letters" letter_freq = {} for char in my_str: if not char in letter_freq: letter_freq[char] = 0 letter_freq[char] += 1 print("Letter frequency: ", letter_freq)
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)
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 with i and x
The code above loops through
xs
, assigning the index toi
and the value tox
.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 valuey
of any polynomial, givenx
andcc
.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.