2.6 formula calculation

1） Let the user input the content of multiplying any two integers （ Such as “2*3”） Can output “2*3=6”, For example, the operation is ：

Let's finish it step by step . Complete the input first ：

strs = input()

Suppose we enter “2*3”, Now we need to split the two values before and after the asterisk . This can take advantage of the string itself split Function to implement ：

strs = input()
print(strs.split('*'))

The running result is ：

 

  I see ,split Method splits the current string with the characters specified in parentheses , And get the first and second strings , Namely ‘2’ and ‘3’. Two values form a list , Therefore, we can get ：

strs = input()
num = strs.split('*')
print(num[0], num[1])

The running result is ：

You can also use this method to obtain the two separated values ： 

strs = input()
a, b = strs.split('*')
print(a, b)

The running result is ：

  The result is no longer a list , Is the value of two variables of string type .

Please note that , This ‘2’ and ‘3’ All character types , Therefore, direct algebraic operation is not allowed ：

strs = input()
a, b = strs.split('*')
print(a * b)

There is an error in the running result , The error message is that you cannot multiply non integers ：

Therefore, it is necessary to convert integers and perform algebraic operations ：

strs = input()
a, b = strs.split('*')
print(int(a) * int(b))

The running result is ：

This result is the tail of the result we are going to output . Therefore, the previously entered content can be spliced in front of this result , Doesn't it form the final appearance ？

strs = input()
a, b = strs.split('*')
print(strs + int(a) * int(b))

  There is an error in the running result , The error message means that only characters can be connected to characters ：

  The reason is simple ,strs For the string ,int(a) Is an integer , Of course, you can't add it directly . Further improved as ：

strs = input()
a, b = strs.split('*')
print(strs + str(int(a) * int(b)))

The running result is ：

Careful observation shows that , The final output is missing an equals sign , It is revised again as ：

strs = input()
a, b = strs.split('*')
print(strs + '=' + str(int(a) * int(b)))

  The running result is ：

It can also be written as ：

strs = input()
a, b = strs.split('*')
print(a + '*' + b + '=' + str(int(a) * int(b)))

The output content is the same as . This is a bit of a hassle , But the effect is the same . 

In addition to these methods , We can also use another completely different method ：

strs = input()
print(strs + '=' + str(eval(strs)))

If input “2*3”, The running result is ：

How to understand that ？eval Function can directly replace the parameter string , And embedded in the current code . For example, if you enter “2*3”, be eval(strs) Namely 2*3, At this time, there are no quotation marks around its content , So the last line above

print(strs + '=' + str(eval(strs)))

Replace with ：

print(strs + '=' + str(2 * 3)) 

Just the right statement that can be executed directly ！

2） Calculate similarity ：

This is the famous cosine similarity , You can calculate the distance between two vectors . such as ：

  We wrote the first code ：

v11 = 1
v12 = 2
v21 = 3
v22 = 4
fenzi = v11 * v21 + v12 * v22
fenmu = (v11 ** 2 + v12 ** 2) ** 0.5 * (v21 ** 2 + v22 ** 2) ** 0.5
print(fenzi / fenmu)

Output is ： 0.9838699100999074

Although the writing seems very complicated , In fact, it's simple , It is to express various common algebraic operations according to the calculation formula , You can observe carefully .

For square sum square , You can also use math Module to complete ：

import math
v11 = 1
v12 = 2
v21 = 3
v22 = 4
fenzi = v11 * v21 + v12 * v22
fenmu = math.sqrt(math.pow(v11, 2) + math.pow(v12, 2)) * \
        math.sqrt(math.pow(v21, 2) + math.pow(v22, 2))
print(fenzi / fenmu)

The output content is the same as . among ,sqrt Is the square function ,pow Is a power function , The second parameter in parentheses represents square . Please pay attention to one detail , because fenmu The expression is very long , Used \ A newline , This symbol means that the next line is still followed by this code , Just to avoid a long line of content .