Learn python with socratica [My notes] - part 9- If/Then/Else

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Lesson 11

写代码当时候，避免不了的就是循环结构，如果到flag条件，则执行，如果到跳转条件，则跳转到另一个块继续执行。关键词：if,else,then。下面举一个if-then的例子[ipython 不能运行整个代码文件，所以没有输入输出的实验效果，建议在]：

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# collect string / test length

input = raw_input("Please enter a test string: ")

if len(input)<6: #注意冒号和缩紧
    print("Your string is really short.")
    print("please enter a string with at least 6 characters")

将代码保存为python 文件if_then.py，使用命令行执行：

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>>> python if_then.py
>Please enter a test string:ace # for example
    Your string is too short.
    Please enter a string with at least 6 characters.
    
>Please enter a test string:mission # this will return true, so no if.

下面我们开始一段新的代码：

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# Prompt user to enter number / test if even or odd

input = raw_input("Please enter an integer:")
number = int(input)

if number % 2 == 0:
    print("Your number is even.")
else:
    print("Your number is odd.")

将代码保存为python 文件if_then_2.py，使用命令行执行：

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>>> python if_then_2.py
> Please enter an integer: 17
    Your number is odd.
>>> python if_then_2.py
> Please enter an integer: 50
    Yout number is even.

下面我们做一个小实验：三角形的判断。

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# Scalene triangle: All sides have different lengths.
# Isosceles triangle: Two sides have the same length.
# Equilateral triangle: All sides are equal.

a = int(raw_input("The length of side a = "))
b = int(raw_input("The length of side b = "))
c = int(raw_input("The length of side c = "))

if a != b and b != c and a != c:
    print("This is a scalene triangle.")
elif a == b and b == c: # elif = else if
    print("This is an equilateral triangle.")
else:
    print("This is an isosceles triangle.")

将代码保存为python 文件if_then_3.py，使用命令行执行：

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>>> python if_then_3.py
> The length of side a = 3
> The length of side b = 4
> The length of side c = 5
This is a scalene triangle.

这个实验就是已知三条边的长度，判断三角型的类型：等边三角形，等腰三角形。if/then/else的这种循环结构，需要在实际使用中慢慢掌握。

Youtube source:
https://www.youtube.com/watch?v=bY6m6_IIN94&list=PLi01XoE8jYohWFPpC17Z-wWhPOSuh8Er-