python - advanced features

slice operator

Python provides the slice (Slice) operator, which is very useful and can easily extract a certain sequence of numbers through slicing. For example, the first 10 numbers:

# The slice slice operator takes the first 10 elements 
L=list(range(0,100 ))
 print (L[:10]) #If the index is 0, it can be omitted 

>>[0, 1, 2, 3, 4, 5, 6, 7 , 8, 9]

L[0:10] means to start from index 0 (index 0 can be omitted) until index 10 (but not including index 10), which is a "left closed right open" interval [0,10), slice operator You can also take the reciprocal element, such as L[-2:-1], to get the second-to-last element 98.

Implementing the default method of strip with the slice operator

# str.strip() Remove the specified characters (strings) at the beginning and the end, the default is a space 
str= '    hello world!    ' 
print (str.strip())

#Use the [:] slice operator to remove the leading and trailing spaces from the string 
n= 0
j =0 #Front index 
k=0 #After index 
while n< len(str):
     if str[n:n+1]!= '  ' :
        j=n
        break
    n=n+1
while n<len(str):
    if str[-n-1:-n]!=' ':
        k=-n
        break
    n=n+1
print(str[j:k])

》hello  world!
》hello  world!

Advanced Edition

Implement the strip method using the [:] slice operator

#Use the [:] slice operator to implement the strip method 
def my_strip(str,chr=' '):
    n = 0 #reset loop index 
    J=0 #front index 
    K=0 #back index 
    Ls=len(str) #input string length 
    L=len(chr) #remove character length #get front index J while n< Ls :
         if str[n:n+L]!= chr:
    
    
            J=n
            break
        n=n+L
    n = 0 #loop index reset 
    #get back index K 
    while n< Ls:
         if str[-nL:Ls-n]!= chr:
            K =Ls- n
             break 
        n =n+ L
     #return slice 
    return str[J:K]

str='0000a000'
chr='000'
print(my_strip(str,chr))

》0a

Simple version

After reading the code of an old man, it is very simple and admirable. The record is as follows:

def trim(str,chr=' '):
    L = len(chr)
     while str[:L] == chr: #If    the first string is a space, delete the first 
        str = str[L:]
     while str[-L:] == chr   : #If The last of the string is a space, delete the last 
        str = str[:- L]
     return str