Conditions

Learning Objectives

• Explain the similarities and differences between tuples and lists.
• Write conditional statements including if, elif, and else branches.
• Correctly evaluate expressions containing and and or.

Comparing things

In the first lesson, we saw some mathematical operators. Python is also equipped with comparison operators. All comparison operators return True or False. Note the capitalization of these names.

A big part of programming includes comparing things. What's the easiest thing to compare? Numbers, of course. Let's see how that works:

print(5 > 2)
print(3 < 1)
print(5 > 2 * 2)
print(1 == 1)
print(5 != 2)

True
False
True
True
True

We gave Python some numbers to compare. As you can see, Python can compare not only numbers, but it can also compare method results. Nice, huh?

Give Python two more tasks:

print(6 >= 12 / 2)
print(3 <= 2)

True
False

> and < are easy, but what do >= and <= mean? Read them like this:

• x > y means: x is greater than y
• x < y means: x is less than y
• x <= y means: x is less than or equal to y
• x >= y means: x is greater than or equal to y

Control flow

We can ask Python to take different actions, depending on a condition, with an if statement:

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 and or. 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

while 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

While loops

Now we have a little bit of experience using comparison operators to control the flow of our code, we can take a look at another very common form of loop.

The while statement allows you to repeatedly execute a block of statements as long as a condition is true. A while statement is an example of what is called a looping statement. A while statement can have an optional else clause.

We begin by looking at one of the simplest dynamic programming algorithms, generating a fibonacci sequence.

n = int(input("n = "))
f0 = 0
f1 = 1
while n > 1:
    nxt = f0 + f1
    f0 = f1
    f1 = nxt
    n -= 1
print("Fn =", f1)

This code inputs an integer n for which it computes and prints the nth Fibonacci number.

Here is how the algorithm works in a bit more detail:

• We first initialize (set to their starting values) variables f0 and f1 to 0 and 1 respectively.

• Then we run a loop as long as n > 1. In that loop, we

  • Compute the sum f0 + f1 and save it for later in the variable nxt (short of "next", which we don't use because next is a built-in Python function).

  • We assign f0 = f1 and f1 = nxt. In other words, the new value of f0 is equal to the old >value of f1 and the new value is equal to the sum of old >values of f0 and f1.

  • We reduce n by 1.

Note that the int() function here converts a string into a an integer.

And here is the Fibonacci algorithm we developed wrapped in a function:

From a "regular" code to a function¶

def fibonacci(n):
    """
    Takes one argument `n` and returns the `n`-th Fibonacci number.
    """
    f0 = 0
    f1 = 1
    while n > 1:
        nxt = f0 + f1
        f0 = f1
        f1 = nxt
        n -= 1
    return f1

The function definition opens with the word def, which is followed by the name of the function and a parenthesized list of parameter names. The body of the function - the statements that are executed when it runs - is indented below the definition line.

Let's try running our function. Calling our own function is no different from calling any other function:

fibonacci(10)

55

Challenges

 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')

Resources

DjangoGirls Python Basics: Comparisons video.