Python3 Loop Statements

This chapter will introduce the use of Python loop statements.

The loop statements in Python are for and while.

The control structure diagram of Python loop statements is as follows:

Loop Control Keywords and Methods

Loop Control
Keyword / Function Description Example
for Iterative loop, used to traverse sequences or iterable objects for i in list:
while Conditional loop, continues executing while the condition is True while x > 0:
break Immediately terminate the current loop break
continue Skip the remaining code of this loop and proceed to the next iteration continue
else (loop) Executed when the loop ends normally (not terminated by break) for i in range(3): ... else: ...
pass Placeholder statement in a loop (no operation) for i in range(5): pass
range() Generates integer sequences, often used with for loops range(0, 5)
enumerate() Obtains the index and value simultaneously during iteration for i, v in enumerate(list):

while Loop

The general form of the while statement in Python:

while 判断条件(condition)：
    执行语句(statements)……

The execution flow chart is as follows:

Execution Gif demonstration:

Also pay attention to the colon and indentation. In addition, there is no do..while loop in Python.

The following example uses while to calculate the sum of 1 to 100:

Example

#!/usr/bin/env python3 n = 100 sum = 0 counter = 1 while counter <= n: sum = sum + counter counter += 1 print("The sum from 1 to %d is: %d" % (n,sum))

The execution result is as follows:

1 到 100 之和为: 5050

Infinite Loop

We can implement an infinite loop by setting the conditional expression to never be false, as in the following example:

Example

#!/usr/bin/python3 var = 1 while var == 1 : # The expression is always true num = int(input("Enter a number:")) print ("The number you entered is:", num) print ("Good bye!")

After executing the above script, the output result is as follows:

输入一个数字  :5
你输入的数字是:  5
输入一个数字  :

You can use CTRL+Cto exit the current infinite loop.

Infinite loops are very useful for real-time requests from clients on servers.

while Loop Using else Statement

If the condition statement after while is false, the else statement block is executed.

The syntax format is as follows:

while <expr>:
    <statement(s)>
else:
    <additional_statement(s)>

If the expr condition is true, the statement(s) block is executed; if it is false, additional_statement(s) is executed.

Loop to output numbers and compare their sizes:

Example

#!/usr/bin/python3 count = 0 while count < 5: print (count, "less than 5") count = count + 1 else: print (count, "greater than or equal to 5")

After executing the above script, the output result is as follows:

0  小于 5
1  小于 5
2  小于 5
3  小于 5
4  小于 5
5  大于或等于 5

Simple Statement Group

Similar to the syntax of the if statement, if your while loop body contains only one statement, you can write that statement on the same line as the while, as shown below:

Example

#!/usr/bin/python flag = 1 while (flag): print ('Welcome to Example Tutorial!') print ("Good bye!")

Note:You can use CTRL+C to interrupt the above infinite loop.

After executing the above script, the output result is as follows:

Welcome to Example Tutorial!
Welcome to Example Tutorial!
Welcome to Example Tutorial!
Welcome to Example Tutorial!
Welcome to Example Tutorial!
……

for Statement

Python for loop can iterate over any iterable object, such as a list or a string.

The general format of the for loop is as follows:

for <variable> in <sequence>: <statements> else: <statements>

Flow chart:

Python for loop example:

Example

#!/usr/bin/python3 sites = ["Baidu", "Google","Example","Taobao"] for site in sites: print(site)

The output result of the above code execution is:

Baidu
Google
Example
Taobao

It can also be used to print each character in a string:

Example

#!/usr/bin/python3 word = 'example' for letter in word: print(letter)

The output result of the above code execution is:

r
u
n
o
o
b

Integer range values can be used together with the range() function:

Example

#!/usr/bin/python3 # All numbers from 1 to 5: for number in range(1, 6): print(number)

The output result of the above code execution is:

1
2
3
4
5

for...else

In Python, the for...else statement is used to execute a block of code after the loop ends.

The syntax format is as follows:

for item in iterable:
    # 循环主体
else:
    # 循环结束后执行的代码

When the loop finishes executing (that is, after all elements in the iterable have been traversed), the code in the else clause will be executed. If a break statement is encountered during the loop, the loop will be interrupted, and the else clause will not be executed at this point.

Example

for x in range(6):
  print(x)
else:
  print("Finally finished!")

After executing the script, the output result is:

0
1
2
3
4
5
Finally finished!

The following for example uses the break statement. The break statement is used to jump out of the current loop body, and the else clause will not be executed:

Example

#!/usr/bin/python3 sites = ["Baidu", "Google","Example","Taobao"] for site in sites: if site == "Example": print("Example Tutorial!") break print("Loop data" + site) else: print("No loop data!") print("Loop completed!")

After executing the script, when the loop reaches "Example", it will jump out of the loop body:

循环数据 Baidu
循环数据 Google
Example!
完成循环!

range() Function

If you need to iterate over a sequence of numbers, you can use the built-in range() function. It will generate a sequence of numbers, for example:

Example

>>>for i in range(5): ... print(i) ... 0 1 2 3 4

You can also use range() to specify values in an interval:

Example

>>>for i in range(5,9) : print(i) 5 6 7 8 >>>

You can also make range() start at a specified number and specify a different increment (which can even be negative, sometimes also called 'step'):

Example

>>>for i in range(0, 10, 3) : print(i) 0 3 6 9 >>>

Negative numbers:

Example

>>>for i in range(-10, -100, -30) : print(i) -10 -40 -70 >>>

You can combine the range() and len() functions to iterate over the indices of a sequence, as shown below:

Example

>>>a = ['Google', 'Baidu', 'Example', 'Taobao', 'QQ'] >>> for i in range(len(a)): ... print(i, a[i]) ... 0 Google 1 Baidu 2 Example 3 Taobao 4 QQ >>>

You can also use the range() function to create a list:

Example

>>>list(range(5)) [0, 1, 2, 3, 4] >>>

For more on the usage of the range() function, refer to:https://www.example.com/python3/python3-func-range.html

break and continue Statements and else Clauses in Loops

break execution flow chart:

continue execution flow chart:

while statement code execution process:

for statement code execution process:

breakThe break statement can jump out of the for and while loop bodies. If you terminate from a for or while loop, any corresponding loop else block will not be executed.

continueThe continue statement is used to tell Python to skip the remaining statements in the current loop block, and then proceed to the next round of the loop.

Example

Using break in while:

Example

n = 5
while n > 0:
    n -= 1
    if n == 2:
        break
    print(n)
print('Loop ended.')

The output result is:

4
3
循环结束。

Using continue in while:

Example

n = 5
while n > 0:
    n -= 1
    if n == 2:
        continue
    print(n)
print('Loop ended.')

The output result is:

4
3
1
0
循环结束。

More examples are as follows:

Example

#!/usr/bin/python3 for letter in 'Example': # First example if letter == 'b': break print ('Current letter is:', letter) var = 10 # Second example while var > 0: print ('Current variable value is:', var) var = var -1 if var == 5: break print ("Good bye!")

The output result of the above script is:

当前字母为 : R
当前字母为 : u
当前字母为 : n
当前字母为 : o
当前字母为 : o
当前变量值为 : 10
当前变量值为 : 9
当前变量值为 : 8
当前变量值为 : 7
当前变量值为 : 6
Good bye!

The following example loops through the string Example and skips the output when the letter o is encountered:

Example

#!/usr/bin/python3 for letter in 'Example': # First example if letter == 'o': # Skip output when the letter is o continue print ('Current letter:', letter) var = 10 # Second example while var > 0: var = var -1 if var == 5: # Skip output when the variable is 5 continue print ('Current variable value:', var) print ("Good bye!")

The output result of the above script is:

当前字母 : R
当前字母 : u
当前字母 : n
当前字母 : b
当前变量值 : 9
当前变量值 : 8
当前变量值 : 7
当前变量值 : 6
当前变量值 : 4
当前变量值 : 3
当前变量值 : 2
当前变量值 : 1
当前变量值 : 0
Good bye!

Loop statements can have an else clause; it is executed when the loop terminates by exhausting the list (with a for loop) or when the condition becomes false (with a while loop), but it is not executed when the loop is terminated by break.

The following example is a loop example for finding prime numbers:

Example

#!/usr/bin/python3 for n in range(2, 10): for x in range(2, n): if n % x == 0: print(n, 'Equals', x, '*', n//x) break else: # No element found in the loop print(n, 'is a prime number')

The output of executing the above script is:

2  是质数
3  是质数
4 等于 2 * 2
5  是质数
6 等于 2 * 3
7  是质数
8 等于 2 * 4
9 等于 3 * 3

pass Statement

Python pass is a null statement, used to maintain the integrity of the program structure.

pass does nothing; it is generally used as a placeholder statement, as in the following example

Example

>>>while True: ... pass # Wait for keyboard interrupt (Ctrl+C)

The smallest class:

Example

>>>class MyEmptyClass: ... pass

The following example executes the pass statement block when the letter is o:

Example

#!/usr/bin/python3 for letter in 'Example': if letter == 'o': pass print ('Execute pass block') print ('Current letter :', letter) print ("Good bye!")

The output of executing the above script is:

当前字母 : R
当前字母 : u
当前字母 : n
执行 pass 块
当前字母 : o
执行 pass 块
当前字母 : o
当前字母 : b
Good bye!
Other extensions