How to understand python's for loop

How to understand python’s for loop?

for loop

for loop can be used to traverse an object (traversal: in layman's terms, it means to visit the first element to the last element in this loop in sequence). The basic structure of the for loop is as follows:

Look at this case specifically:

Design a function to create 10 texts on the desktop and name them with numbers from 1-10.

  def text_create():
     path = '/Users/duwangdan/Desktop/'
     for text_name in range(1,11):
          # 1-10的范围需要用到range函数
          with open (path + str(text_name) + '.txt','w') as text:
              # with...as的用法正文内会详细介绍
              text.write(str(text_name))
            text.close()
              print('Done')
 
 text_create()

Now let’s understand each line of code.

Line 1: Define a text_create function;

Line 2: Assign the variable path to the desktop path;

Line 3: Change the value in the range of 1-10 Each number is loaded into the variable text_name in turn, naming one file at a time;

Line 5: Open the txt file located on the desktop, and perform a write operation for each text;

Line 5 Line 7: Name each file in turn;

Line 8: Close the file;

Line 9: Display a Done after performing a naming operation;

Line Line 11: Call function.

The case mentioned "with...as". In Python, the "with...as" syntax is used to replace the traditional "try...finally".

For example: open the test file on the desktop, try to read the file content, and finally close the file.

 file = open('/Users/duwangdan/Desktop/test.txt')
 try:
     data = file.read()
 finally:
     file.close()

Although this code performs well, it is relatively verbose. If it is expressed using "with...as", the code will be more concise.

with open('/Users/duwangdan/Desktop/test.txt') as file:
     data = file.read()

In addition to the single-layer loop above, there is also a common loop, which is a nested loop.

For example, use nested loops to implement the multiplication formula.

for i in range(1,10):
     for j in range(1,10):
         print('{} X {} = {}'.format(i,j,i*j))

The outermost loop stores the numbers 1-9 in the variable i in sequence; every time the variable i takes a value, the inner loop stores the numbers 1-9 in the variable j in sequence; Finally, print out the current values ​​of i, j, and i*j.

Starting from Python 2.6, the format function has been added to format strings, which can be achieved through {}.format. In the above case, the values ​​​​of i, j, and i*j are stored in the previous { } respectively, and then formatted to unify the form.

Related recommendations: "Python Tutorial"

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