Learn python with socratica [My notes] - part 5 - Arithmetic

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Lesson 6 & 7

Experiments in python2

算术运算是编程中非常基础的部分，那么python中是怎么实现的呢？python2和python3在这个方面又有什么样的区别？

首先介绍的是算术运算的基础：

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# Let's begin.

# Numbers: int, long, float, complex
# operation: + - * /

# the first thing to operate numbers is that
# both of the numbers are the same type

# for example
x = 5  #int
y = 5L #long

# you can also get long number in this way
long(5)
5L

# ints are narrower than longs
# longs are wider than ints

# also, floats are wider than longs
# all longs can be converted to floats
x = 28L
y = 28.0
float(28)
28.0

# however, not all floats can be converted to longs
# for example
a = 3.14 # float

# try to convert to long, and you will find:
long(a)
3L

用long强制转换3.14这个float类型的数，得到的并不是同一个数。

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# similarly
x = 1.721 # float

1.721 + 0j
(1.721+0j)

同样的，复数也是包含所有浮点数的。

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complex(1.721+0j)
(1.721+0j)
float(1.721+0j)

TypeErrorTraceback (most recent call last)

<ipython-input-9-00c7ec0f6750> in <module>()
----> 1 float(1.721+0j)

TypeError: can't convert complex to float

现在，我们可以得出结论，ints<longs<floats<complex，这在python算术运算中，也起到决定性的作用。在python中，如果是两个类型不同的数做运算，结果是什么类型呢？我们做个实验一探究竟：

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a = 6 # int
b = 3L # long
c = 4.5 # float
d = 2.1 + 0j # complex

# Rule: widen numbers so they are the same type

# Addition
a + b # int + long
9L

# Subtraction
c - b # float - long
1.5

# Multiplication
a * c # int * float
27.0

# Division
d / c # complex / float
(0.4666666666666667+0j)

# integer division
16 / 5 # get the quotient
3

# get the Reminder
16 % 5
1

# want the accurate result
print(float(16)/5)
print(16/float(5))
3.2
3.2

# error -> 0
2/0

ZeroDivisionErrorTraceback (most recent call last)

<ipython-input-17-e8326a161779> in <module>()
----> 1 2/0

ZeroDivisionError: integer division or modulo by zero

总结一下，在python中算术运算会以wider number为基准，运算结果以wider number的类型为准。在除法运算中，有两个注意点：

  1. 如果分数线两侧都是int型，那么运算得到的是整除后quotient部分，想得到余数，使用取余%操作得到，先得到准确结果，可以把分母或者分子做float类型的强制转换。
  2. 分母不可以为0。

Differences in python3

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16 /5
3.2

算术运算在python3中就这一点不同，分数线两边默认是float类型，如果想要得到整除结果，使用一下的运算符号：

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# no differnce
16%5
1

16 // 5 # 整除
3

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