Programming with Python

Repeating Actions with Loops and Making Choices

Deep down, most people want to learn how to program so they can automate the things they don’t want to do themselves. To do that, we’ll have to teach the computer how to repeat actions.

For example, we might need to access each item in a list. One way to do this would be to use a series of print statements:

odds = [1, 3, 5, 7]
print odds[0]
print odds[1]
print odds[2]
print odds[3]

1
3
5
7

This is a bad approach for two reasons:

• It doesn’t scale: if we want to print the characters in a string or the items in a list that’s hundreds of values long, it would take a very, very long time.

• It’s fragile: if we replace the list of odd values above with a longer list, it will only print part of the data, and if we give it a shorter list, it produces an error because we’re asking for values that don’t exist.

odds = [1, 3, 5]
print odds[0]
print odds[1]
print odds[2]
print odds[3]

1
3
5

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-20-543712197ba5> in <module>()
      3 print odds[1]
      4 print odds[2]
----> 5 print odds[3]

IndexError: list index out of range

A better approach is to automate the task with a for loop:

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

l
e
a
d

This is shorter than writing individual print statements for each character, and more robust as well:

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

o
x
y
g
e
n

The improved version of our script uses a for loop to repeat an operation — in this case, printing — once for each item in a collection. The general form of a for 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 anything we want to run inside the loop. Unlike many other languages, there is no command to end a loop (e.g. end); what is indented after the for statement belongs to the loop.

Here’s another loop that repeatedly updates a variable:

word = 'oxygen'
length = 0
for char in word:
    length = length + 1
print 'There are', length, 'characters in the word', word

There are 6 characters in the word oxygen

It’s worth tracing the execution of this little program step by step. Since there are six characters in ‘oxygen’, the statement on line 4 will be executed six times. The first time around, length is zero (the value assigned to it on line 1) and char is ‘o’. The statement adds 1 to the previous value of length, producing 1, and updates length to refer to that new value. The next time around, char is ‘x’ and length is 1, so length is updated to be 2. After four more updates, length is 6; since there is nothing left in ‘oxygen’ for Python to access, the loop finishes and the print statement on line 5 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 it can be over-written like any other variable:

letter = 'z'
for letter in 'abc':
    print letter
print 'after the loop, letter is', letter

a
b
c
after the loop, letter is c

print 5**3
125

Making Choices

When analyzing data, we’ll often want to automatically detect differences between values and take different actions on the data depending on some conditions. Here, we’ll learn how to write code that runs only when certain conditions are true.

Conditionals

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

num = 42
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 when the test is false:

num = 42
print 'before conditional...'
if num > 100:
    print num, '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. We do this because the single equals sign is used to assign values to variables.

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 tests are true'
else:
    print 'at least one test is false'

at least one test 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'
else:
    print 'neither test is true'

at least one test is true

if 4 > 5:
    print 'A'
elif 4 == 5:
    print 'B'
elif 4 < 5:
    print 'C'

if 5 => 5:
    print 'A'
elif 5 == 5:
    print 'B'
elif 5 < 5:
    print 'C'

if True:
     print 'True is true'
if False:
     print 'False is 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' 

Combining loops and conditionals

Imagine that (for some strange reason) you want to write some code that:

• produces a list of 100 random integers between -10 and 10
• sums the positive and negative numbers in the list separately

Writing complex code is best done by working on individual parts. Let’s start by producing the list of random integers. Python has a library for producing random numbers called random. One of the functions in this library is randint, which produces random integers between some bounds:

import random

print random.randint(-10,10)

7

This command produces a single random number. We can run it repeatedly by writing a for loop and building a list:

randomNumbers = [] # creates an empty list

for num in range(100):
    randomNumbers.append(random.randint(-10,10))
    
print len(randomNumbers)

100

To finish our task, we write a for loop that steps through the list of random integers and determines, using conditionals, if the values are positive or negative:

sumPositive = 0
sumNegative = 0

for num in randomNumbers:
    
    if num > 0:        
        sumPositive = sumPositive + num
        
    else:        
        sumNegative = sumNegative + num
        
print sumPositive, sumNegative

262 -290

x = 1  # original value
x += 1 # add one to x, assigning result back to x
x *= 3 # multiply x by 3
print(x)

    6