Comp 151. Control structures.

Transcription

1 Comp 151 Control structures.

2 admin For these slides read chapter 7 Yes out of order.

3 Simple Decisions So far, we ve viewed programs as sequences of instructions that are followed one after the other. While this is a fundamental programming concept, it is not sufficient in itself to solve every problem. We need to be able to alter the sequential flow of a program to suit a particular situation. Python Programming, 2/e 4

4 Simple Decisions Control structures allow us to alter this sequential program flow. In this chapter, we ll learn about decision structures, statements that allow a program to execute different sequences of instructions for different cases, allowing the program to choose an appropriate course of action. Python Programming, 2/e 5

5 Motivation up till now written simple programs storing and manipulating small amounts of data. first line in program executed, then each line after that. now add ability to take more control from computer

6 Making decision sometimes we don't want every line of code to execute don't want to do the same thing every time real life example?

7 Making decision sometimes we don't want every line of code to execute don't want to do the same thing every time real life example? how about if campus is open go to class can do this in programming as well technically called 'selection' decision making Some of you did this in the PAL sections for lab1

8 Selection in python syntax if <something that evals to boolean> : statements here the <> brackets just indicate that whatever is between them is semantic and should be replaced again note indent of statements example value = True if value: print ( yes )

9 Boolean expressions laymans terms: something that evaluates to true/false value in python True False Note capitalization for those of you c++/java refugees what sorts of things give true/false values?

10 Common booleans (in python) equality and inequality == (equality) >, <, >= (greater than or equal) <=!=(not equal) straight negation not eg: value = True if not value: print( something )

11 doing something else for false sometimes you want to do something else when the condition is false if the campus is open go to class otherwise go back to bed. if today == 'Monday': print ( Back to work ) else: print( same old, same old ) else only after if.

12 notice the indenting? code 'blocks' As you know indenting is how python determines code that logically belongs together called a 'code block' in jargon terms can have as many lines as desired in said code block all indenting the same same code block

13 multiple conditions can have multiple conditions use elif keyword if today == 'monday': print ('blah') elif today == 'friday': print ('yup') elif today == 'Sunday': else: print('almost all over') print( same old same old )

14 Finishing conditionals We looked at some simple conditionals last time Comparisons Now lets consider compound conditions In logic we have and or and not X and Y is true iff X is true and Y is true X or Y is true if either X is true or Y is true or both are true. If X is true then not X is false. Python actually uses these words for logical operators

15 Logical Operators II Var1 = hello Var2 = 3 if len(var1)>4 and Var2<12: print( it was true ) else: print( condition was false )

16 Short Circuit Operators Lets take a look at another example Var1 = hello Var2 = 3 if Var2>12 and len(var1)>4 : print( it was true ) else: print( condition was false ) Do you really need to evaluate the len(var1)>4 part? Why or why not?

17 Short Circuit Operators Lets take a look at another example Var1 = hello Var2 = 3 if Var2>12 and len(var1)>4 : else: print( it was true ) print( condition was false ) Do you really need to evaluate the len(var1)>4 part? Why or why not? You don't. Because the first part is false So it doesn't matter what the second part was

18 Short Circuit Operators II Python (and most other languages) Takes advantage of this logical property If the expression to the left of the operator (and/or) completely determines the truth value of the statement Then the expression to the right is never evaluated Left is true for or don't don't need to evaluate right Left is false for and don't don't need to evaluate right Implications?

19 Short Circuit Operators II Python (and most other languages) Takes advantage of this logical property If the expression to the left of the operator (and/or) completely determines the truth value of the statement Then the expression to the right is never evaluated Left is true for or don't don't need to evaluate right Left is false for and don't don't need to evaluate right Implications? Put expensive operations or right for efficiency Potential for errors to be hidden

20 definite and indefinite loops two kinds of loops in programming definite: know how many times to loop when program begins looping indefinite don't know how many times, but know how world will be when done (ends on condition) real world examples of each from class? which is the range looping we did last week?

21 While: python indefinite loop Syntax while <condition>: codeblock as long as the condition is true, code block will be executed again and again lets do one in idle stalker 'friend' then add ifs

22 potential loop problems I Infinite loop you must update the loop variable somewhere in the code block for the loop. i.e. the loop needs to have the chance to get closer to ending each time through. bad: counter = 1 sum = 0 while counter <=5: sum = sum+counter

23 Potential loop problems II Off by one errors want to make sure you loop the right number of times not one too many or one too few code below is supposed to be calculating how much a bank balance is after 10 years of 5% interest. what are problems? balance = input( how much to start?: ) balance = float(balance) years = 0 while years <= 10: balance = balance + balance* 0.05

24 loop problems III problems in that loop: infinite loop questions/problems should years start at 0 or 1? should the condition be < or <= consider simple conditions lets hand execute on board first.

25 Random Module Introduce random.randint Edit stalker friend to use it.

26 Assignment Read chapter 7 And see assignment if Wednesday