Repeating Actions with Loops
Last updated on 2023-07-28 | Edit this page
Estimated time 30 minutes
- How can I do the same operations on many different values?
- Explain what a
- Correctly write
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
In the episode about visualizing data, we wrote Python code that
plots values of interest from our first inflammation dataset
inflammation-01.csv), which revealed some suspicious
features in it.
We have a dozen data sets right now and potentially more on the way if Dr. Maverick can keep up their surprisingly fast clinical trial rate. We want to create plots for all of our data sets with a single statement. To do that, we’ll have to teach the computer how to repeat things.
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.
= [1, 3, 5, 7]odds
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.
= [1, 3, 5] odds 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
= [1, 3, 5, 7] odds 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:
= [1, 3, 5, 7, 9, 11] odds 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); everything
indented after the
for statement belongs to the loop.
In the example above, the loop variable was given the name
num as a mnemonic; it is short for ‘number’. We can choose
any name we want for variables. We might just as easily have chosen the
banana for the loop variable, as long as we use the
same name when we invoke the variable inside the loop:
= [1, 3, 5, 7, 9, 11] odds 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:
= 0 length = ['Curie', 'Darwin', 'Turing'] names for value in names: = length + 1 length 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
names, 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) and
Curie. 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,
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 and the
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:
= 'Rosalind' name 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.
Python has a built-in function called
generates a sequence of numbers.
range can accept 1, 2, or
- 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 prints the first 3
1 2 3
for number in range(1, 4): print(number)
The body of the loop is executed 6 times.
= 1 result for number in range(0, 3): = result * 5 result print(result)
= [124, 402, 36] numbers = 0 summed for num in numbers: = summed + num summed print(summed)
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 idx, val in enumerate(a_list): # Do something using idx and val
The code above loops through
a_list, assigning the index
idx and 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.
= 5 x = [2, 4, 3] coefs = coefs * x**0 + coefs * x**1 + coefs * x**2 y print(y)
Write a loop using
enumerate(coefs) which computes the
y of any polynomial, given
= 0 y for idx, coef in enumerate(coefs): = y + coef * x**idx y