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Python basics 2 - Conditional statements and Loops

Overview

Teaching: 40 min
Exercises: 0 min
Questions

  • How can my programs do different things based on data values?

  • How can I do the same operations on many different values?

Objectives

  • Write conditional statements including if, elif, and else branches.

  • Correctly evaluate expressions containing and and or.

  • Explain what a for loop does.

  • Correctly write for loops 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 for loop.

Conditional Statements

Conditional statements are if-then statements:

if BOOLEAN:
    DO SOMETHING

This means, when the boolean is true, the INDENTED line “DO SOMETHING” will run. If the boolean is false, nothing will happen.
You can add a second level of conditional statement like this:

if BOOLEAN1:
    DO THING1 
    if BOOLEAN2:  
        DO THING2

We determine the level with indentation. For example, in the statement above, line 1 will run no matter what. Line 2 and 3 are at the same level, they will run only if line1’s condition is met. Line 4 will run only if line 3’s condition is met. 

a = 1
b = 3
if (a == 1):
    print("a is one!")
    if (b == 2): 
        print ("b is two!")

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 for assignment (assigning a value to a variable). We can also combine logical 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')

You can also add elif (else if) to add another condition, or add else to cover the rest of the situations.

if BOOLEAN:
    DO THING1
elif BOOLEAN: 
    DO THING2
else: 
    DO SOMETHING ELSE

Challenge 1.1

Consider this code:

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

Which of the following would be printed if you were to run this code? Why did you pick this answer?

  1. A
  2. B
  3. C
  4. B and C

Solution

C gets printed because the first two conditions, 4 > 5 and 4 == 5, are not true, but 4 < 5 is true.

Loops

One of the greatest advantages of programming is that the machines can do repetitive things for you. We will soon be dealing with 90,000 rows of data, and loops will be very helpful for you. An example task that we might want to repeat is getting every elements in a list.

assertions = ["Existence","Rights and Obligations","Completeness","Valuation and Allocation"]

(Look familiar, accountants?)
We can access a element in a list using its index. For example, we can get the first element of the list by using assertions[0]. One way to print each character is to use four print statements:

print(assertions[0])
print(assertions[1])
print(assertions[2])
print(assertions[3])

This is a bad approach because it doesn’t scale: if we want to print the list that has hundreds of elements, we’d be better off just typing them out.

Here’s a better approach:

assertions = ["Existence","Rights and Obligations","Completeness","Valuation and Allocation"]
for assertion in assertions:
    print(assertion)

This is shorter — certainly shorter than something that prints every element in a hundred-element list — and more robust as well.

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 something

We can call the loop variable anything we want, 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); what is indented after the for statement belongs to the loop.

What’s in a name?

In the example above, the loop variable was given the name assertion.
We can choose any name we want for variables. We might just as easily have chosen the name nyan_cat for the loop variable, as long as we use the same name when we invoke the variable inside the loop:

assertions = ["Existence","Rights and Obligations","Completeness","Valuation and Allocation"]
for nyan_cat in assertions:
    print(nyan_cat)

But don’t name all your variables nyan_cat, 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:

length = 0
for char in 'PCAOB':
    length = length + 1
print('There are', length, 'characters')

It’s worth tracing the execution of this little program step by step. Since there are five characters in 'PCAOB', 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 char is 'P'. 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, char is 'C' and length is 1, so length is updated to be 2. After three more updates, length is 5; since there is nothing left in 'PCAOB' 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 the 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 (we will dig into functions later) to do it called len:

print(len('PCAOB'))

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.

Range function

Python has a built-in function called range that creates a sequence of numbers. range can accept 1, 2, or 3 parameters.

Enumerate

This is also a useful way to traverse an iterable sequence (for example, go through everything in a list). 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 i, x in enumerate(interable_sequence):
    # Do something with i and x

Challenge 1.2

Exponentiation is built into Python:

print(5 ** 3)

125

Write a loop that calculates the same result as 5 ** 3 using multiplication (and without exponentiation).

Solution

result = 1
for i in range(0, 3):
   result = result * 5
print(result)

Challenge 1.3

Knowing that two strings can be concatenated using the + operator, write a loop that takes a string and produces a new string called “newstring” with the characters in reverse order, so 'CPA' becomes 'APC'.

oldstring = 'CPA'

Solution

newstring = ''
oldstring = 'CPA'
for char in oldstring:
   newstring = char + newstring
print(newstring)
# (ability power carrier?)

Challenge 1.4

You have a list if lists called a Print out the position and the list in a that contains more than 2 elements AND starts with 1 Hint: use len() to get the length

a = [[1,2,3],[1,2],[1,2],[0,2,3],[1,2,4]]

Solution

a = [[1,2,3],[1,2],[1,2],[0,2,3],[1,2,4]]
for pos, lt in enumerate(a):
    if(len(lt)>2):
        if (lt[0] == 1):
            print (pos, lt)

Key Points

  • Use if condition to start a conditional statement, elif condition to provide additional tests, and else to provide a default.

  • The bodies of the branches of conditional statements must be indented.

  • Use == to test for equality.

  • X and Y is only true if both X and Y are true.

  • X or Y is true if either X or Y, or both, are true.

  • True and False represent truth values.

  • Use for variable in sequence to process the elements of a sequence one at a time.

  • The body of a for loop must be indented.

  • Use len(thing) to determine the length of something that contains other values.

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