Python comprehension: list comprehension, dictionary comprehension, set comprehension, filter, map, reduce.

list comprehension

Example: Divide by 3 numbers 0-100

nums=[];
num=[];
for i in range(100):
    if i%2==0:
        nums.append(i);
    else:
        num.append(i);
print(nums);
print(num);

n=[i for i in range(100) if i%3==0]
print(n);

Call functions

def squared(x):
    return x*2;
wq=[];
q=[];
for x in range(30):
    if x%3==0:
        wq.append(squared(x));

q=[squared(y) for y in range(30) if y%3==0]
print(wq);
print(q);

Use () to generate generators
Change the [ ] of the list comprehension to ( ) to get the generator. --yeild

def squared(x):
     return x*2;
q=(squared(y) for y in range(30) if y%3==0);
print(type(q));
print(q);

dictionary comprehension

The usage of dictionary comprehension and list comprehension is similar, except that the square brackets are changed to curly brackets
Example: Quickly replace key and value

mcase={'a':10,'b':34};
for k,v in mcase.items(): #items can be replaced with keys and values, traversing keys and values respectively
    print (k, v);

mk={v:k for k,v in mcase.items()}
print(mk);

set comprehension

They are also similar to list comprehensions. The only difference is that it uses curly braces {}

strings=['a','is','with','if','file','exception'];
l={len(s) for s in strings};
print(l);

practise:

0-9 to the power of 2
```
l=[i*i for i in range(10)];
print(l);
```
Convert word length greater than 3 to uppercase output

names = ['bob','tom','alice','jerry','wendy','smith']
a=[name.upper() for name in names if len(name)>3]
print(a);

Find (x, y) where x is an even number between 0-5 and y is a list of tuples consisting of odd numbers between 0-5

b=[(x,y) for x in range(6)if x%2==0 for y in range(6)if y%2==1];
print (b);

Find the list of 3,6,9 in m

m = [[1,2,3],[4,5,6],[7,8,9]]
c=[row[2] for row in m];
print(c);

Find the list of slashes 1,5,9 in m

m = [[1,2,3],[4,5,6],[7,8,9]];
d=[m[i][i] for i in range(len(m))];
print(d);

Find the product of the matrix and elements in m, n

m = [[1,2,3],[4,5,6],[7,8,9]];
n = [[2,2,2],[3,3,3],[4,4,4]]
e=[m[i][j]*n[i][j]for i in range(len(m)) for j in range(len(m[i]))];
print (s);

Combines the elements of two lists if they are not equal

m = [[1,2,3],[4,5,6],[7,8,9]];
n = [[2,2,2],[4,4,4],[7,7,7]];
L=[(m[i][j],n[i][j]) for i in range(3) for j in range(3)if m[i][j]!=n[i][j] ];
print(L);

filter:

The filter() function includes two parameters, function and list. Whether the function returns true according to the function parameter
to filter the items in the list parameter and finally return a new list
The function is to perform a filtering operation on the specified sequence

foo = [2, 18, 9, 22, 17, 24, 8, 12, 27];
print(list(filter(lambda x:x%3==0,foo)));

map：

The map function will map the specified sequence according to the provided function.
The function is to call the function function for each element in the parameter sequence, and return a list containing the return value of each function function.

foo = [2, 18, 9, 22, 17, 24, 8, 12, 27];
print(list(map(lambda x: x * 2 + 10, foo)));

reduce：

The reduce() function accumulates the elements in the parameter sequence.

import functools,_functools
foo = [1,2,3,4,5];
print(functools.reduce(lambda x,y:x+y,foo));#The calculation process is 1+2+3+4+5=15

Python comprehension: list comprehension, dictionary comprehension, set comprehension, filter, map, reduce.

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