Control loop of program (while loop)

  Infinite loop ：

A condition controlled cyclic operation .

The syntax is as follows ：

while < Conditions >:

< Sentence block >

In the above format while Keywords and colons identify while The beginning and end of a statement , Cyclic conditions and while Keywords are separated by spaces , Statement blocks are indented with while Statement generates Association .

  perform while When the sentence is , If the value of the cycle condition is True, The statement block in the loop is executed , After executing the statement block, judge the loop condition again , So back and forth , Until the value of the cycle condition is false When the loop terminates , Execute the code after the loop .while The execution flow of the statement is as follows ： 

Cycle according to this condition , If this condition holds , Then the following statement will be executed , Judge the condition again after execution , If the conditions are still true , She will continue to execute the following sentence , Come back to judge the cycle .

Repeatedly execute the statement block , Until the conditions are not met .

The code example is shown below ：

a=3
while a>0:
    a=a-1
    print(a)

The operation interface is as follows ： 

Code example 2 is shown below ：

i=1
result=0
while i<=10:
    result+=i  #result=result+i
    i+=1       #i=i+1
print(result)

In the above example, the variable i It's a cyclic factor , Its initial value is 1, It will increase with the number of cycles ; Variable result Is the sum of the requirements , Its initial value is 0. First execution while loop , because i<=10 The value of is True, So the code segment in the loop will be executed , send result The value of is determined by 0 Turn into 0+1,i The value of is determined by 1 Turn into 2; Judge again i<=10, So back and forth , until i The value of a 11 when ,i<=10 The value of a False, The loop ends , Execute the print statement after the loop , Output result Value .

The operation interface is as follows ： 

Cycle control reserved word ：

break and continue

break Jump out and end the current loop , Execute the statement after the loop

continue End the current cycle and continue to execute the subsequent cycles .

break and continue It can be done with for and while Use it in a circular way

continue When the sub loop body executes , If continue Reserved words , Then the contemporary cycle will end and go to the next cycle .

break It means that once this reserved word is encountered, the whole cycle ends .

break The sample code is as follows ：

for word in "Python":
    if word=='o':
        break
    print(word,end="  ")

Traversing this string python, If o, Then stop printing the string , Out of the loop , That is, the cycle ends .

The operation interface is as follows ： 

continue The sample code is as follows ：

for word in "Python":
    if word=='o':
        continue
    print(word,end="  ")

Traversing this string , If o, Don't print this o, Otherwise, print all other characters .

The operation interface is as follows ： 

A nested loop ：

Loops can be nested with each other , And then realize more complex logic . Loop nesting can be divided into while Loop nesting and for A nested loop .

while A nested loop ：

while Loop nesting means while Nested in statement while or for sentence .

The format is as shown below ：

while The loop condition 1：  # The outer loop

Code segment 1

   while The loop condition 2：  # Inner circulation

Code segment 2

perform while Loop nesting , If the cycle conditions of the outer cycle 1 The value of is True, Then execute the code snippet 1, And the circulation conditions of inner circulation 2 Judge ： If cycle condition 2 The value of is True Then execute the code snippet 2, If it's worth False End the inner loop . After the execution of the inner loop, continue to judge the loop conditions of the outer loop 1, So back and forth , Until the cycle condition 1 The value of is False End the outer cycle .

The code example is as follows ：

i=1
while  i<6:
    j=0
    while j<i:
        print("*",end="")
        j+=1
    print()
    i+=1

Variables in the above example i Represents the number of lines of the graph , Variable j For each line “*” The number of . It should be noted that , Printed by the above program “*” Just wrap the line once , So the code is modified in the inner loop print() Function Terminator （ Pass code print("*",end="") take end Parameter default Terminator '\n' Replace with a space character .

The running code is as follows ： 

Format 2 is as follows ：

while The loop condition 1：  # The outer loop

Code segment 1

   for  Loop variable  in  Traversal structure   # Inner circulation

Code segment 2

The code example is as follows ：

i=1
while  i<6:
   j=0
   for j in '012345':
    print("*",end="")
    print(j)
  

First of all, for while Cycle to judge the conditions ,i The initial value of 1, Condition is True, Conduct for loop , The string ‘012345’ Each character in it is iterated , Output one by one , At the same time output j.

The operation interface is as follows ：

for A nested loop ：

 for Loop nesting means for Nested in statement while or for sentence .

The format is as shown below ：

for Loop variable  in Traversal structure ：  # The outer loop

          Code segment 1

       for Loop variable  in Traversal structure ：# Inner circulation

          Code segment 2

perform for Loop nesting , The program first accesses the first element of the loop variable in the outer loop 、 Execute code snippets 1、 Access the first element of the loop variable in the inner loop 、 Execute code snippets 2, Then access the next element of the inner loop 、 Execute code snippets 2, So back and forth , Until the loop variable of the inner loop is accessed, the inner loop ends , Instead, continue to access the next element in the outer loop , End the outer loop after accessing the loop variables of the outer loop . therefore , Every time the outer loop is executed , Will execute an inner loop .

The code example is as follows ：

for i in range(1,6):
    for j in range(i):
        print("*",end='')
    print()

Operation interface ： 

Format 2 is as follows ：

for  Loop variable  in  Traversal structure   # The outer loop

Code segment 1

while The loop condition 1：  # The outer loop

Code segment 2

The code is as follows ：

for i in range(5):
    j=1
    while i>j:
        print("*",end='')
        j+=1
    print()
    i+=1

First, execute the outer loop for loop ,range(5) From 0 Start , Then execute the inner loop ,i=0, After judging ,i The value of is not greater than j Value , That is, the condition is false, Output directly . then i add 1 operation ,i=1, At the same time false, Then the output , cycle , Until all output .

The operation interface is as follows ： 

In loop nesting , If used break sentence , The program is running to break Statement will end the loop of this layer . namely break Only jump out of the current innermost loop .

The code example is as follows ：

S='PYTHON'
while S!="":
    for c in S:
        if c=='T':
            break
        print(c,end='')
    S=S[:-1]

The operation interface is as follows ：

  Advanced use of loops ：

The expansion of the cycle

Circulation and else

The format is as follows ：

for < Loop variable > in < Traversal cycle >:

< Sentence block 1>

else:

< Sentence block 2>

When the cycle is not break When the statement exits , perform else Sentence block .

else The statement block is used as a reward for the normal completion of the loop .

This is a else And exception handling else Similar usage .

The code example is as follows ：

for c in "PYTHON":
    if c=='T':
        continue
    print(c,end="")
else:
    print(" The normal exit ")

The operation interface is as follows ： 

Otherwise, the code example is as follows ：

for c in "PYTHON":
    if c=='T':
        break
    print(c,end="")
else:
    print(" The normal exit ")

The operation interface is as follows ：