Lecture Overview

• Boolean expressions
• Python statements are organized into blocks. Each block is identified by how much indentation appears at the beginning of the line. Indentation can be spaces or tabs, but it is recommended to never use tabs and always use exactly 4 spaces for indentation.
• Before starting a new block, the previous line must end with a colon.
• The if statement takes a boolean expression and a block of code and only executes the block of code if the expression evaluates to true.
• There are extensions to the if statement using elif and else, see if statement

• The while statement is a loop. It will execute a block of code repeatedly until the boolean expression is false.

• Here is code to compute the nth Fibonacci Number

term = int(input("Compute which Fibonacci number? "))
if term == 0:
    print(0)

elif term == 1:
    print(1)

else:
    f_prev = 0
    f_current = 1
    i = 1
    while i < term:
        temp = f_current + f_prev
        f_prev = f_current
        f_current = temp
        i = i + 1
    print(f_current)

First, the code above is a single if-elif-else statement with three blocks (note the indentation). Only one of these three blocks will be executed depending on the term variable. The first two blocks are straightfoward. Let's look at the third block. Before the while loop we set up three variables.

• i is the number such that we have computed the ith Fibonacci number.
• f_current contains the ith Fibonacci number
• f_prev contains the (i-1)th Fibonacci number

Initially we set up i = 1, f_current = 1 and f_prev = 0. That means we have currently computed the 1st Fibonacci number which is stored in f_current and the previous Fibonacci number is in f_prev. We now will loop until we have computed the termth Fibonacci number. So while i < term we will repeatedly execute the indented block (four statements) under the while statement. These four statements are called the body of the loop.

The body first computes the next (i+1)st Fibonacci number and stores it in temp. It then updates f_current and f_prev so that they hold the (i+1)st Fibonacci number and the ith Fibonacci number and finally increments i by one. Thus after the body of the loop has completed, i is one larger and f_current and f_prev still hold the ith Fibonacci number and (i-1)th Fibonacci number. We execute the body of the loop repeatedly until i == term when we print the result.

Exercises

Edit the above code to compute the Lucas Numbers instead of the Fibonacci numbers.