Programming with Python
- Write conditional statements including
- Correctly evaluate expressions containing
In our last lesson, we discovered something suspicious was going on in our inflammation data by drawing some plots. How can we use Python to automatically recognize the different features we saw, and take a different action for each? In this lesson, we’ll learn how to write code that runs only when certain conditions are true.
We can ask Python to take different actions, depending on a condition, with an
num = 37 if num > 100: print('greater') else: print('not greater') print('done')
not greater done
The second line of this code uses the keyword
if to tell Python that we want to make a choice. If the test that follows the
if statement is true, the body of the
if (i.e., the lines indented underneath it) are executed. If the test is false, the body of the
else is executed instead. Only one or the other is ever executed:
Conditional statements don’t have to include an
else. If there isn’t one, Python simply does nothing if the test is false:
num = 53 print('before conditional...') if num > 100: print('53 is greater than 100') print('...after conditional')
before conditional... ...after conditional
We can also chain several tests together using
elif, which is short for “else if”. The following Python code uses
elif to print the sign of a number.
num = -3 if num > 0: print(num, "is positive") elif num == 0: print(num, "is zero") else: print(num, "is negative")
"-3 is negative"
One important thing to notice in the code above is that we use a double equals sign
== to test for equality rather than a single equals sign because the latter is used to mean assignment.
We can also combine tests using
and is only true if both parts are true:
if (1 > 0) and (-1 > 0): print('both parts are true') else: print('at least one part is false')
at least one part is false
or is true if at least one part is true:
if (1 < 0) or (-1 < 0): print('at least one test is true')
at least one test is true
Checking our Data
Now that we’ve seen how conditionals work, we can use them to check for the suspicious features we saw in our inflammation data. In the first couple of plots, the maximum inflammation per day seemed to rise like a straight line, one unit per day. We can check for this inside the
for loop we wrote with the following conditional:
if numpy.max(data, axis=0) == 0 and numpy.max(data, axis=0) == 20: print('Suspicious looking maxima!')
We also saw a different problem in the third dataset; the minima per day were all zero (looks like a healthy person snuck into our study). We can also check for this with an
elif numpy.sum(numpy.min(data, axis=0)) == 0: print('Minima add up to zero!')
And if neither of these conditions are true, we can use
else to give the all-clear:
else: print('Seems OK!')
Let’s test that out:
data = numpy.loadtxt(fname='inflammation-01.csv', delimiter=',') if numpy.max(data, axis=0) == 0 and numpy.max(data, axis=0) == 20: print('Suspicious looking maxima!') elif numpy.sum(numpy.min(data, axis=0)) == 0: print('Minima add up to zero!') else: print('Seems OK!')
Suspicious looking maxima!
data = numpy.loadtxt(fname='inflammation-03.csv', delimiter=',') if numpy.max(data, axis=0) == 0 and numpy.max(data, axis=0) == 20: print('Suspicious looking maxima!') elif numpy.sum(numpy.min(data, axis=0)) == 0: print('Minima add up to zero!') else: print('Seems OK!')
Minima add up to zero!
In this way, we have asked Python to do something different depending on the condition of our data. Here we printed messages in all cases, but we could also imagine not using the
else catch-all so that messages are only printed when something is wrong, freeing us from having to manually examine every plot for features we’ve seen before.
How many paths?
Which of the following would be printed if you were to run this code? Why did you pick this answer?
- B and C
if 4 > 5: print('A') elif 4 == 5: print('B') elif 4 < 5: print('C')
What Is Truth?
False are special words in Python called
booleans which represent true and false statements. However, they aren’t the only values in Python that are true and false. In fact, any value can be used in an
elif. After reading and running the code below, explain what the rule is for which values are considered true and which are considered false.
if '': print('empty string is true') if 'word': print('word is true') if : print('empty list is true') if [1, 2, 3]: print('non-empty list is true') if 0: print('zero is true') if 1: print('one is true')
That’s Not Not What I Meant
Sometimes it is useful to check whether some condition is not true. The Boolean operator
not can do this explicitly. After reading and running the code below, write some
if statements that use
not to test the rule you formulated in the previous challenge.
if not '': print('empty string is not true') if not 'word': print('word is not true') if not not True: print('not not True is true')
Write some conditions that print
True if the variable
a is within 10% of the variable
False otherwise. Compare your implementation with your partner’s: do you get the same answer for all possible pairs of numbers?
Python (and most other languages in the C family) provides in-place operators that work like this:
x = 1 # original value x += 1 # add one to x, assigning result back to x x *= 3 # multiply x by 3 print(x)
Write some code that sums the positive and negative numbers in a list separately, using in-place operators. Do you think the result is more or less readable than writing the same without in-place operators?
Tuples and Exchanges
Explain what the overall effect of this code is:
left = 'L' right = 'R' temp = left left = right right = temp
Compare it to:
left, right = right, left
Do they always do the same thing? Which do you find easier to read?
Sorting a List Into Buckets
The folder containing our data files has large data sets whose names start with “inflammation-”, small ones whose names with “small-”, and possibly other files whose sizes we don’t know. Our goal is to sort those files into three lists called
other_files respectively. Add code to the template below to do this. Note that the string method
True if and only if the string it is called on starts with the string passed as an argument.
files = ['inflammation-01.csv', 'myscript.py', 'inflammation-02.csv', 'small-01.csv', 'small-02.csv'] large_files =  small_files =  other_files = 
Your solution should:
- loop over the names of the files
- figure out which group each filename belongs
- append the filename to that list
In the end the three lists should be:
large_files = ['inflammation-01.csv', 'inflammation-02.csv'] small_files = ['small-01.csv', 'small-02.csv'] other_files = ['myscript.py']
Write a loop that counts the number of vowels in a character string.
Test it on a few individual words and full sentences.
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)?