Repeating Actions with Loops
OverviewTeaching: 30 min
Exercises: 10 minQuestions
How can I do the same operations on many different values?Objectives
Explain what a
forloops 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
An example task that we might want to repeat is accessing numbers in a list, which we will do by printing each number on a line of its own.
odds = [1, 3, 5, 7]
In Python, a list is basically an ordered collection of elements, and every
element has a unique number associated with it — its index. This means that
we can access elements in a list using their indices.
For example, we can get the first number in the list
odds. One way to print each number is to use four
print(odds) print(odds) print(odds) print(odds)
1 3 5 7
This is a bad approach for three reasons:
Not scalable. Imagine you need to print a list that has hundreds of elements. It might be easier to type them in manually.
Difficult to maintain. If we want to decorate each printed element with an asterisk or any other character, we would have to change four lines of code. While this might not be a problem for small lists, it would definitely be a problem for longer ones.
Fragile. If we use it with a list that has more elements than what we initially envisioned, it will only display part of the list’s elements. A shorter list, on the other hand, will cause an error because it will be trying to display elements of the list that do not exist.
odds = [1, 3, 5] print(odds) print(odds) print(odds) print(odds)
1 3 5
--------------------------------------------------------------------------- IndexError Traceback (most recent call last) <ipython-input-3-7974b6cdaf14> in <module>() 3 print(odds) 4 print(odds) ----> 5 print(odds) IndexError: list index out of range
Here’s a better approach: a for loop
odds = [1, 3, 5, 7] for num in odds: print(num)
1 3 5 7
This is shorter — certainly shorter than something that prints every number in a hundred-number list — and more robust as well:
odds = [1, 3, 5, 7, 9, 11] for num in odds: print(num)
1 3 5 7 9 11
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, such as print
Using the odds example above, the loop might look like this:
where each number (
num) in the variable
odds is looped through and printed one number after
another. The other numbers in the diagram denote which loop cycle the number was printed in (1
being the first loop cycle, and 6 being the final loop cycle).
We can call the loop variable 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
numas a mnemonic; it is short for ‘number’. We can choose any name we want for variables. We might just as easily have chosen the name
bananafor the loop variable, as long as we use the same name when we invoke the variable inside the loop:
odds = [1, 3, 5, 7, 9, 11] for banana in odds: print(banana)
1 3 5 7 9 11
It is a good idea to choose variable names that are meaningful, otherwise it would be more difficult to understand what the loop is doing.
Here’s another loop that repeatedly updates a variable:
length = 0 names = ['Curie', 'Darwin', 'Turing'] for value in names: length = length + 1 print('There are', length, 'names in the list.')
There are 3 names in the list.
It’s worth tracing the execution of this little program step by step.
Since there are three names in
the statement on line 4 will be executed three times.
The first time around,
length is zero (the value assigned to it on line 1)
The statement adds 1 to the old value of
length to refer to that new value.
The next time around,
length is 1,
length is updated to be 2.
After one more update,
length is 3;
since there is nothing left in
names for Python to process,
the loop finishes
Note that a loop variable is a variable that is 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:
name = 'Rosalind' for name in ['Curie', 'Darwin', 'Turing']: print(name) print('after the loop, name is', name)
Curie Darwin Turing after the loop, name is Turing
Note also that finding the length of an object is such a common operation
that Python actually has a built-in function to do it called
print(len([0, 1, 2, 3]))
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
rangethat generates a sequence of numbers.
rangecan accept 1, 2, or 3 parameters.
- If one parameter is given,
rangegenerates a sequence of that length, starting at zero and incrementing by 1. For example,
range(3)produces the numbers
0, 1, 2.
- If two parameters are given,
rangestarts at the first and ends just before the second, incrementing by one. For example,
2, 3, 4.
rangeis given 3 parameters, it starts at the first one, ends just before the second one, and increments by the third one. For example,
range(3, 10, 2)produces
3, 5, 7, 9.
range, write a loop that uses
rangeto print the first 3 natural numbers:
1 2 3
for number in range(1, 4): print(number)
Understanding the loops
Given the following loop:
word = 'oxygen' for char in word: print(char)
How many times is the body of the loop executed?
- 3 times
- 4 times
- 5 times
- 6 times
The body of the loop is executed 6 times.
Computing Powers With Loops
Exponentiation is built into Python:
print(5 ** 3)
Write a loop that calculates the same result as
5 ** 3using multiplication (and without exponentiation).
result = 1 for number in range(0, 3): result = result * 5 print(result)
Summing a list
Write a loop that calculates the sum of elements in a list by adding each element and printing the final value, so
[124, 402, 36]prints 562
numbers = [124, 402, 36] summed = 0 for num in numbers: summed = summed + num print(summed)
Computing the Value of a Polynomial
The built-in function
enumeratetakes 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 idx, val in enumerate(a_list): # Do something using idx and val
The code above loops through
a_list, assigning the index to
idxand the value to
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 coefs = [2, 4, 3] y = coefs * x**0 + coefs * x**1 + coefs * x**2 print(y)
Write a loop using
enumerate(coefs)which computes the value
yof any polynomial, given
y = 0 for idx, coef in enumerate(coefs): y = y + coef * x**idx
for variable in sequenceto process the elements of a sequence one at a time.
The body of a
forloop must be indented.
len(thing)to determine the length of something that contains other values.