Programming Basics

To make a computer execute an algorithm, it must be written in a programming language. In this lesson, using Python-style notation, we will learn basic elements common to every language: variables, conditional branches, repetition, arrays, and functions.

Variables and Assignment

A variable is a "named box" in which a value is stored. Putting a value into a variable is called assignment and is represented by "=". Unlike the equals sign in mathematics, the programming "=" means "put the value on the right into the variable on the left."

Conditional Branching (if Statements)

Use an if statement to switch processing according to a condition. When the condition is true, the statements inside if are executed; when it is false, the statements inside else are executed.

score = 72
if score >= 60:
    print("Pass")
else:
    print("Fail")

In this example, score is 72 and "score >= 60" is true, so "Pass" is displayed. Conditions use comparison operators such as == (equal), != (not equal), <, >, <=, and >=.

Repetition (for and while Statements)

Use repetition to perform the same process many times. A for statement repeats a process for a specified number of times or range, while a while statement repeats it as long as a condition is true.

total = 0
for i in range(1, 11):
    total = total + i
print(total)

range(1, 11) means "from 1 through 10" (note that 11 is not included). As i changes through 1, 2, 3, ..., 10, it is added to total, and finally 1 + 2 + ... + 10 = 55 is displayed.

Arrays (Lists) and Functions

Use an array (called a list in Python) to handle multiple values together. Write it as scores = [80, 65, 92], for example, and use scores[0] to retrieve the first element (80). Remember that index numbers start at 0.

A function is a named sequence of processes that can be reused. If you define one as def average(a, b): return (a + b) / 2, you can obtain 70 simply by calling average(80, 60). Functions organize programs and eliminate the need to write the same process repeatedly.

Practice Questions

1. After executing "a = 5" and then "a = a + 3," what is the value of a?
2. When writing "for i in range(5):", how many times is the loop executed? What values does i take?
3. In the program for the sum from 1 to 10, what is displayed if range(1, 11) is changed to range(1, 101)?