A requirements statement describes what a program is supposed to do, but not how the program should do it. Consider the following simple requirements statement:

• A class of ten students took a quiz. Their grades (integers in the range 0 – 100) are 98, 76, 71, 87, 83, 90, 57, 79, 82, 94. Determine the class average on the quiz.

Once you know the problem’s requirements, you can begin creating an algorithm to solve it. Then, you can implement that solution as a program.

The algorithm for solving this problem must:

1. Keep a running total of the grades.

2. Calculate the average—the total of the grades divided by the number of grades.

3. Display the result.

For this example, we’ll place the 10 grades in a list. You also could input the grades from a user at the keyboard (as we’ll do in the next example) or read them from a file (as you’ll see how to do in the “Files and Exceptions” chapter). We also show you how to read data from SQL and NoSQL databases in later chapters.

The following pseudocode lists the actions to execute and specifies the order in which they should execute:

Note the mentions of total and grade counter. In Fig. 3.1’s script, the variable total (line 5) stores the grade values’ running total, and grade_counter (line 6) counts the number of grades we’ve processed. We’ll use these to calculate the average. Variables for totaling and counting normally are initialized to zero before they’re used, as we do in lines 5 and 6.

The following script implements the pseudocode algorithm.

 1 # fig03_01.py
 2 """Class average program with sequence-controlled repetition."""
 3
 4 # initialization phase
 5 total = 0  # sum of grades
 6 grade_counter = 0
 7 grades = [98, 76, 71, 87, 83, 90, 57, 79, 82, 94] # list of 10 grades
 8
 9 # processing phase
10 for grade in grades:
11     total += grade  # add current grade to the running total
12     grade_counter += 1  # indicate that one more grade was processed
13
14 # termination phase
15 average = total / grade_counter
16 print(f'Class average is {average}')

Class average is 81.7

grades = [98, 76, 71, 87, 83, 90, 57, 79, 82, 94] # list of 10 grades

Line 16 uses the following simple f-string (short for formatted string) to format this script’s result by inserting the value of average into a string:

f'Class average is {average}'

The letter f before the string’s opening quote indicates it’s an f-string. You specify where to insert values by using placeholders delimited by curly braces ({ and }). The placeholder

{average}

converts the variable average’s value to a string representation, then replaces {average} with that replacement text. Replacement-text expressions may contain values, variables or other expressions, such as calculations or function calls. In line 16, we could have used total / grade_counter in place of average, eliminating the need for line 15.