Functions and Modular Design函数与模块化设计

A programmer decomposes a grade-report program into four functions as shown below. The main program uses only four lines.程序员将成绩报告程序分解为如下四个函数。主程序只有四行。

FUNCTION get_scores():
    -- reads 5 scores and returns them as a list
    ...
END FUNCTION

FUNCTION compute_average(scores):
    SET total TO 0
    FOR EACH s IN scores: SET total TO total + s
    RETURN total / 5
END FUNCTION

FUNCTION letter_grade(average):
    IF average >= 90 THEN RETURN "A"
    IF average >= 80 THEN RETURN "B"
    IF average >= 70 THEN RETURN "C"
    IF average >= 60 THEN RETURN "D"
    RETURN "F"
END FUNCTION

FUNCTION display_report(avg, grade):
    OUTPUT "Average: " + avg
    OUTPUT "Grade:   " + grade
END FUNCTION

-- Main program
SET scores TO get_scores()
SET avg TO compute_average(scores)
SET grade TO letter_grade(avg)
display_report(avg, grade)

Study the following two functions and the code that calls them.研究以下两个函数及调用它们的代码。

def double(n):
    return n * 2

def add_and_double(a, b):
    total = a + b
    return double(total)

x = add_and_double(3, 7)
print(x)
print(add_and_double(5, 5))

Study the following program carefully. Note the local variable named result inside each function.仔细研究以下程序。注意每个函数内部名为 result 的局部变量。

def square(n):
    result = n * n
    return result

def cube(n):
    result = n * n * n
    return result

a = square(4)
b = cube(3)
print(a)
print(b)
print(a + b)

The two programs below compute the same result but use different designs. Program A uses a shared global variable; Program B passes data only through parameters and return values.以下两个程序计算相同的结果，但使用了不同的设计。程序 A 使用共享的全局变量；程序 B 仅通过形参和返回值传递数据。

Program A (global state):程序 A（全局状态）：

total = 0

def add_item(price):
    global total
    total = total + price

add_item(15)
add_item(30)
print(total)

Program B (parameter-based):程序 B（基于参数）：

def add_item(running_total, price):
    return running_total + price

t = 0
t = add_item(t, 15)
t = add_item(t, 30)
print(t)