Building programs with Python

Repeating Actions with Loops

In the last lesson, we wrote some code that plots some values of interest from our first inflammation dataset, and reveals some suspicious features in it, such as from inflammation-01.csv

but we have a dozen data sets right now and more on the way. We want to create plots for all our data sets with a single statement. To do that, we’ll have to teach the computer how to repeat things.

Suppose we want to print each character in the word “lead” on a line of its own. One way is to use four print statements:

word = 'lead'
print word[0]
print word[1]
print word[2]
print word[3]

l
e
a
d

but that’s a bad approach for two reasons:

1. 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.

2. 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

Traceback (most recent call last):
  File "loop_test.py", line 7, in <module>
    print word[3]
IndexError: string index out of range

Here’s a better approach:

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

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

The improved version of print_characters uses a for loop to repeat an operation—in this case, printing—once for each thing in a collection. The general form of a loop is:

for variable in collection:
    do things with variable

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

Here’s another loop that repeatedly updates a variable:

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. 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

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')

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.

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