#4: While Loop Reading: Chapter3

Transcription

1 CS 130R: Programming in Python #4: While Loop Reading: Chapter3

2 Contents Constants While loop Infinite loop Break Continue

3 Constants Similar to variables, but their names are with capital letters and their value should not be changed throughout the program. NAME = John AGE = 22 Advantages: Makes the program clearer Save re-typing, especially useful when they hold long values NAME = John Sebastian Junior II Smith-Simpson

4 WHILE loop A loop construct in Python allows you to repeat a block of code several times. Two types of loops: while - used when a condition is to be met, or if you want a piece of code to repeat forever for - when you have a piece of code which you want to repeat a certain (known) number of times

5 WHILE loop To write an efficient loop structure you must ask yourself 3 questions: Where do I start? When do I end? How do I go from one iteration of the loop to the next?

6 Syntax: WHILE loop variable initialization while (condition on variable):... loop_body modify variable value... A while loop tests its condition before the body of the loop is executed.

7 WHILE loop while is mostly used where a user's input is required n = input("please enter 'hello':") while n!= 'hello': n = input("please enter 'hello':") print( Loop done! )

8 WHILE loop Example of counter up: counter = 0 while counter < 10: print(counter) counter = counter + 1 print( Final counter after loop:, counter) Final counter after loop: 10

9 WHILE loop Counting down: x = 10 while x!= 0: print x =, x x = x - 1 print "And now the loop has ended And now the loop has ended

10 WHILE loop Counting down with step of 4: x = 20 while x!= 0: print x =, x x = x - 4 print "And now the loop has ended And now the loop has ended

11 WHILE loop Infinite loop a loop that never ends Infinite loop is a big programming mistake!!! Example: counter = 0 while counter < 10: print(counter) print( Final counter after loop:, counter) Notice that counter never changes its value, so it will always be equal to 0 => infinite loop

12 WHILE loop Counting down with step of 3: x = 20 while x!= 0: print x =, x x = x - 3 print "And now the loop has ended Never reaches x = 0 to terminate the condition => infinite loop

13 Break Statement break statement in Python terminates the current loop and resumes execution at the next statement after the loop. Can be used inside WHILE and FOR loops. It is included in the body of an if statement, when you want to skip the execution of loop_body from a certain case/condition on.

14 Break Statement Syntax: while (condition1) : statement_1 statement_2... if condition2 : break statement_10

15 Break Statement var = 10 while var > 0: print 'Current variable value :', var var = var -2 if var == 4: break print "Good bye!" Current variable value : 10 Current variable value : 8 Current variable value : 6 Good bye!

16 Break Statement while True: s = input('enter something : ') if s == 'quit': break print 'Length of the string is', len(s) print 'Done Enter something : Programming is fun Length of the string is 18 Enter something : When the work is done Length of the string is 21 Enter something : quit Done

17 Continue Statement The continue statement rejects all the remaining statements in the current iteration of the loop and moves the control back to the top of the loop. The continue statement can be used in both while and for loops. It is included in the body of an if statement, when you want to skip the execution of loop_body for a certain case/condition.

18 Continue Statement Syntax: while (condition1) : statement_1 statement_2... if condition2 : continue statement_7

19 Continue Statement var = 10 while var > 0: var = var -2 if var == 4: continue print 'Current variable value :', var print "Good bye! Current variable value : 8 Current variable value : 6 Current variable value : 2 Current variable value : 0 Good bye!

20 Loops, Break and Continue Note: Break and continue statements are considered not so good programming statements and should be used only when strictly necessary. That means always try to have loops without break and continue!

21 While while input()!= "stop": print("type 'stop' to stop the program") Warning: If some whitespaces are before or after "stop", we can remove them using the string method strip() that removes whitespaces from the string: while input().strip()!= "stop": print("type 'stop' to stop the program")

22 For loops Next lecture