Python loop tricks

Python's for loop is one of the most commonly used statements in coder. If you simply loop through the container, you will lose a lot of good experience. Like the following:

for i in range(10):        print(i)

Python provides a lot of techniques for loops, these methods can make the code more concise and beautiful, this time I will take a look.

 

1. Enumerate() function

When traversing a non-numerical sequence, sometimes it is necessary to extract the elements and the index together, and then the enumerate()function can be used .

enumerate()The function accepts a sequence or iterator, and returns a tuple containing the elements and their index values.

>>> seasons = ['Spring', 'Summer', 'Fall', 'Winter']>>> list(enumerate(seasons))[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]

You can also startspecify the starting value of the sequence value by adjusting the parameters:

>>> list(enumerate(seasons, start=1))[(1, 'Spring'), (2, 'Summer'), (3, 'Fall'), (4, 'Winter')]

 

At this time, use the for loop to traverse enumerate()the sequence wrapped by the function to get the elements and their index values:

>>> for i, v in enumerate(['tic', 'tac', 'toe']):... print(i, v)...0 tic1 tac2 toe

 

2. Dictionary item() method

When traversing the dictionary, if you traverse the dictionary object directly, you can only get the keys in the dictionary:

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>>> sample = {'a':1,'b':2,'c':3}>>> for i in sample:... print(i)...abc

 

If you use the dictionary items()method, you can output the key and the corresponding value at the same time:

>>> sample = {'a':1,'b':2,'c':3}>>> for i in sample.items():... print(i)...('a', 1)('b', 2)('c', 3)

 

3. The zip() function

zip()The function receives one or more iterable objects, aggregates the elements corresponding to each iterable object, and returns an iterator of tuples.

>>> x = [1, 2, 3]>>> y = [4, 5, 6]>>> zipped = zip(x, y)>>> list(zipped)[(1, 4), (2, 5), (3, 6)]

 

When looping in two or more sequences at the same time, you can use  zip() functions to match the elements one by one.

>>> color = ['white','blue','black']>>> animal = ['cat','dog','pig']>>> for i in zip(animal,color):... print(i)...('cat', 'white')('dog', 'blue')('pig', 'black')

 

4. sorted() function

When you need to sort the list first and then loop through it, you need to use a sorted()function.

sorted()The function receives an iterable object and returns a sorted list.

In addition, the sorted()function has two parameters: key and reverse

key  specifies a function with a single parameter used  to extract the key for comparison (for example ) from  each element of  iterablekey=str.lower . The default value is  None (Compare elements directly)

 

reverse  is a Boolean value. If set  True, each list element will be sorted in reverse order.

>>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']>>> for f in sorted(set(basket)):... print(f)...applebananaorangepear

 

5. reversed() function

reversed()The function is used to reversely arrange the elements in the iterable object and return a reverse iterator.

>>> list(reversed([2,5,3,9,6]))[6, 9, 3, 5, 2]

The for loop traverses the reversed()iterable object wrapped by the function, and the number can be retrieved in reverse.

>>> for i in reversed([2,5,3,9,6]):... print(i)...69352

 

to sum up:

Five tips for for loops, they are enumerate()、item()、zip()、sorted()、reversed(), these methods and functions not only make the loop more concise, but also can be used in many other codes