Python advanced features --- slices, iterative algorithm, and a list of questions related to the formula

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This link: https://blog.csdn.net/qq_37189082/article/details/100312325

1. The use of the slicing operation, to achieve a trim () function, end to end string of blanks is removed. Be careful not to call the str strip () method.

def trim(s):
    while len(s)>0 and s[:1]==' ':
        s=s[1:] 
    while len(s)>0 and s[-1:]==' ':
        s=s[:-1]
    return s
    
def test_trim():
    assert trim('hello ') == 'hello'
    assert trim(' hello') == 'hello'
    assert trim('hello  ') == 'hello'
    assert trim('  hello') == 'hello'

test_trim()

2. Use an iterative find a list of the minimum and maximum values, and returns a tuple. Iteration is actually for traversal.

def findMinAndMax(L):
    if L==[]:
        return (None,None)
    else:
        min=max=L[0]
        for x in L:
            if x>max:
                max=x
        for y in L:
            if y<min:
                min=y
        return (min,max)

     Do not use an iterative method to achieve

def findMinAndMax(L):
    n = len(L)
    L.sort()
    if n == 0:
        return(None,None)
    else:
        return (L[0],L[n-1])

3. Use formula list, the list of elements of type String Africa was filtered off, and the lower case string.

L1 = ['Hello', 'World', 18, 'Apple', None]

Results: [ 'hello', 'world', 'apple']

L2 = [i.lower() for i in L1 if isinstance(i, str)]