05 _ loop statement

01. Three major procedures of the program

• There are three process methods in program development:
  • Sequence--From top down, Sequential Code Execution
  • Branch-- Determines whether to execute the code based on conditionsBranch
  • Loop-- LetDuplicate specific codeRun

02. whileBasic usage of loops

• The function of loop is to makeSpecified CodeRepeated execution
• whileThe most common application scenarios of loops are:Code to be executedFollowSpecified number of times RepeatedRun

2.1 whileBasic statement syntax

Initial Condition setting-usually the while condition of the counter that is executed repeatedly (determine whether the counter has reached the target number of times): When the condition is met, when the condition 1 is met, when the condition 2 is met, the task 3... (Omitted )... processing condition (counter + 1)

Note::

• whileThe statement and indentation areComplete code block

The first WHILE LOOP

Requirement

• Print 5 times Hello Python

#1. define the number of repeated counter I = 1 #2. use the while condition while I <= 5: # print ("Hello Python ") # process counter I I = I + 1 print ("I = % d" % I after the loop ends)

Note: After the loop ends, the value of the previously defined counter condition still exists.

Endless loop

For programmer reasons,ForgotInside the loopModify the judgment condition of a loop, Resulting in continuous execution of the loop and the program cannot be terminated!

2.2 value assignment operator

• In python, use=You can assign values to variables.
• To simplify code writing during arithmetic operations,PythonIt also provides a seriesArithmetic OperatorsCorrespondingValue assignment operator
• Note:Spaces are not allowed in the assignment operator.

| Operator | description | instance |
| = | Simple value assignment operator | C = A + B assigns the result of A + B to C
| + = | Addition and value assignment operator | C + = A is equivalent to C = C +.
|-= | Subtraction value assignment operator | C-= A is equivalent to C = C-.
|= | Multiplication and value assignment operator | C= A is equivalent to C = C *
|/= | Division assignment operator | C/= A is equivalent to C = C/.
| // = | Assign values by Division | C // = A is equivalent to C = C //
| % = | GetModule(Remainder) value assignment operator | C % = A is equivalent to C = C %
|= | Power assignment operator | C= A is equivalent to C = C **

2.3 counting method in Python

There are two common counting methods:

• Natural notation(From1First) -- more in line with human habits
• Program counting(From0Start) -- counting starts from 0 for almost all programming languages.

Therefore, when writing a program, you should try to develop the habit:Unless otherwise specified, the cycle count starts from 0.

2.4 cyclic computing

In program developmentExploitation cycle Repeated computingRequirements

In this case, you can:

1. InwhileDefines a variable aboveStore the final calculation result
2. Inside the loop body, each cycle usesLatest calculation results,UpdatePreviously Defined variables

Example

• Calculate 0 ~ Sum of all numbers between 100

### Calculate 0 ~ Sum of all numbers between 100 #0. result = 0 #1. variable record cycles defining an integer I = 0 #2. start loop while I <= 100: Print (I) # Every loop, add result + = I # To process counter I + = 1 print ("0 ~ Result of the sum of numbers between 100 = % d "% result)

03. Break and continue

breakAndcontinueIs a keyword specifically used in a loop

• break When a condition is met, Exit the loop, and no longer execute subsequent repeated code
• continue When a condition is met, Do not execute subsequent repeated code

breakAndcontinueOnlyCurrent cycleValid

3.1 break

• In the loop Process, IfAfter a condition is met,NoHope againContinue execution, You can usebreakExit Loop

I = 0 while I <10: # exit the loop when a certain condition of break is met, and no repeated code is executed # I = 3 if I = 3: break print (I) I + = 1 print ("over ")

breakValid only for the current cycle

3.2 continue

• In the loop Process, IfAfter a condition is met,NoHopeExecute the loop code, but do not want to exit the loop, You can usecontinue
• That is, in the entire loop,Only some conditions, You do not need to execute the loop code, but other conditions must be executed.

I = 0 while I <10: # When I = 7, you do not want to execute the code that needs to be repeated if I = 7: # Before using continue, you should also modify the counter # Otherwise the endless loop I + = 1 continue # re-executed code print (I) I + = 1

• Note: Usecontinue,Special attention should be paid to the Code for conditional processing., Accidentally appearsEndless loop

continueValid only for the current cycle

04. whileLoop nesting 4.1 loop nesting

• whileNesting is:whileThere iswhile

While condition 1: when conditions are met, things done 1 when conditions are met, things done 2 when conditions are met 3... (Omitted )... while condition 2: when conditions are met, things done 1 when conditions are met, things done 2 when conditions are met 3... (Omitted )... processing Condition 2 processing condition 1

4.2 loop nested drill-step 5 of the 9 th multiplication table: Print stars with nesting

Requirement

• Output five rows consecutively on the console*, The number of asterisks in each row increases sequentially

*  **  ***  ****  *****  

• Print with string *

#1. Define a counter variable. Starting from number 1, the loop will be more convenient ROW = 1 while row <= 5: Print ("*" * row) Row + = 1

Step 2: Print stars with nested loops

Knowledge PointPairprintFunction usage is enhanced

• By default,printAfter the function outputs the content, a line break is automatically added to the end of the content.
• If you do not want to add a line break at the end, you canprintAdded content after function output, end=""

• Where""You can specifyprintAfter the function outputs the content, continue the content you want to display.

• The syntax format is as follows:

# Print ("*", end = "") # simple line feed print ("")

end=""Indicates that the output content to the console will not wrap after it is finished.

Hypothesis Python Not providedString*OperationConcatenated string

Requirement

• Output five rows consecutively on the console*, The number of asterisks in each row increases sequentially

*  **  ***  ****  *****    

Development procedure

• 1> complete simple output of five lines of content
• 2> analyze*What should I do?
  • The number of stars displayed in each row is the same as that of the current row.
  • Nest a small loop to process each rowColumnStar display

Row = 1 while row <= 5: # suppose Python does not provide a string * operation # inside the loop, add another loop, implement the star print Col = 1 while Col <= row: Print ("*", end = ") COL + = 1 # after each row is output, add another line feed print ("") Row + = 1

Step 2: multiplication table

RequirementOutput the 9-9 multiplication table in the following format:

1 * 1 = 1     1 * 2 = 2   2 * 2 = 4     1 * 3 = 3   2 * 3 = 6   3 * 3 = 9     1 * 4 = 4   2 * 4 = 8   3 * 4 = 12  4 * 4 = 16    1 * 5 = 5   2 * 5 = 10  3 * 5 = 15  4 * 5 = 20  5 * 5 = 25    1 * 6 = 6   2 * 6 = 12  3 * 6 = 18  4 * 6 = 24  5 * 6 = 30  6 * 6 = 36    1 * 7 = 7   2 * 7 = 14  3 * 7 = 21  4 * 7 = 28  5 * 7 = 35  6 * 7 = 42  7 * 7 = 49    1 * 8 = 8   2 * 8 = 16  3 * 8 = 24  4 * 8 = 32  5 * 8 = 40  6 * 8 = 48  7 * 8 = 56  8 * 8 = 64    1 * 9 = 9   2 * 9 = 18  3 * 9 = 27  4 * 9 = 36  5 * 9 = 45  6 * 9 = 54  7 * 9 = 63  8 * 9 = 72  9 * 9 = 81  

Development procedure

* 1. Print 9 rows of stars

*   *****          ****          *****                                ******           *******                    ******** *********                         

* 2. Set*Replace with the corresponding row and column to multiply

# Define the start row = 1 # print up to 9 rows while row <= 9: # define the start column Col = 1 # print the maximum row column while Col <= row: # End = "", indicating that the output ends without line breaks # "\ t" can output a tab on the console, align print ("% d * % d = % d" % (COL, row, row * col), end = "\ t") when outputting text ") # Number of columns + 1 Col + = 1 # line feed print ("") # number of rows + 1 row + = 1

Escape characters in strings

• \tOutputTabTo assist in text outputVerticalKeep alignment
• \nOutputLine Break

TabThe function isVerticalAlign text by Column

| Escape Character | description
| \\| Backslash
| \ '| Single quotes
| \ "| Double quotation marks
| \ N | line feed
| \ T | horizontal Tab
| \ R | enter

05 _ loop statement