CONTROL FLOW CREATING ALTERNATE PATHS OF EXECUTION USING: BRANCHING WHILE LOOPS FOR LOOPS

Transcription

1 CONROL LOW CREAING ALERNAE PAHS O EXECUION USING: BRANCHING WHILE LOOPS OR LOOPS

2 CREAING ALERNAE PAHS O EXECUION Up until now our programs are sequential and execute all lines of code from top to bottom. It is impossible to write complex and interesting programs without being able to run specific lines of code under different circumstances and sometimes loop through the same code multiple times. We will use Conditional Statements to enable our programs to branch. Branching is a fundamental concept in computer programming that allows the program to make a decision to take one path or another depending on the current conditions.

3 CONDIIONAL SAEMENS A compound statement made up of a condition to be evaluated as rue or alse and then an action to be performed if the condition is rue. Conditional Statement orm HEADER: IRS SAEMEN SECOND SAEMEN LAS SAEMEN NO PAR O SAEMEN (no indentation) Example if (10 > 5): print( condition is true ) print( this is part of block ) print( that makes up the body ) print( of the conditional statement ) print( this executes regardless of condition ) Conditional statements start with a header on a new line ending with a : and the statements to be executed if the header is evaluated as rue are all indented to create a block of statements that make up the body of the conditional statement.

4 HEADER O CONDIIONAL SAEMEN Header contains two parts: Part 1: he form of the conditional statement. Examples are: if, elif, else, while, and for. Part 2: Boolean expression or Condition that is evaluated as either rue or alse.

5 I SAEMEN he simplest form of a conditional statement is the if statement. he keyword if is followed by a Boolean Expression. If the expression is rue, then the next indented line(s) that make up the block or body are executed. If the expression is evaluated as alse then the block is skipped and the next nonindented statement is executed. his structure is often referred to as an I-HEN statement.

6 HE PASSWORD PROGRAM

7 HE PASSWORD PROGRAM (CON.) If the password is equal to secret then the two lines in the body are executed and Access Granted is displayed.

8 BOOLEAN EXPRESSION (CONDIION) Conditions can contain boolean variables that already contain rue or alse and/or the condition can contain a comparison operator that evaluates mathematical expressions as rue or alse.) Condition containing boolean myvar1 = rue myvar2 = alse if (myvar1): print( this will execute ) if (myvar2): print( this will NO execute ) Condition containing mathematical evaluation numive = 5 numen = 10 if (numen == 2 * numive): #(10 == 2*5) print( this will execute ) if (numen == numive + numen): #(10 == ) print( this will NO execute )

9 COMPARISON OPERAORS Comparison operators can compare numbers to numbers and strings to strings. (cannot compare numerical values directly strings)

10 BLOCKS Indentation is used to create blocks. Blocks are one or more consecutive lines with the same indentation level. Blocks not only group lines of code visually to make it easier to read but also logically as they form a single section of code that will be run together. here is no correct way to indent code in Python and either tabs or a number of spaces is acceptable as long as you are consistent. he creator of Python uses fours spaces.

11 ELSE CLAUSE he if statement does a good job of selectively executing a block of code if a condition is rue but what if we want our program to do something different if the condition is alse? he else clause allows us to create an alternate path of execution for our program if the proceeding conditions are alse. he else does not have a condition and must be followed by a colon :. When an if statement has an else clause, the else clause must be in the same block as the if statement. his means that both the if and the associated else need to have the same indentation level.

12 HE PASSWORD PROGRAM REVISIED

13 HE PASSWORD PROGRAM REVISIED (CON) If the password is equal to secret then the two lines in the body of the if statement are executed and Access Granted is displayed. Else the password is not equal to secret then the two lines in the body of the else statement are executed and Access Denied is displayed

14 ELI CLAUSE (AKA ELSE-I ) Sometimes we need our programs to check a sequence of possible conditions and respond to them differently. An if statement with elif clause(s) can contain multiple conditions to be evaluated as rue or alse. A chain of if elif elif elif else statements will be evaluated until either a rue comparison or the else statement is reached. Once a comparison is evaluated as rue and the body of the statement is executed, then the rest of the chain is skipped. he elif clause allows us to create alternate paths of execution for many different anticipated conditions if the proceeding conditions are alse. he elif must have a condition to be checked followed by a colon : (just like an if). When an if statement has an elif clause, the elif clause must be in the same block as the if statement. his means that both the if and the associated elif need to have the same indentation level.

15 HE MOOD SURVEY PROGRAM

16 HE MOOD SURVEY PROGRAM (CON) his program utilizes a block of 1 if, 2 elif, and an else. Because they are part of the same block of code (same indentation level) exactly one of the conditional statements will execute. As soon as one of the boolean expressions is evaluated as rue and its associated body is executed, the rest of the if-elif-else structure is skipped. As demonstrated in this program, else is often used as a catch all that responds to all unexpected values. In this case, any moodnum that is not 1, 2, or 3.

17 HE GUESS GAME PROGRAM he Guess Game Project

18 LOOPING Loops are all around us in our everyday lives. Even the directions on shampoo bottles will often say, While hair is dirty: Wash, Rinse, Repeat. Loops are really just repetitive tasks. In the shampoo case we are directed to Wash and Rinse repeatedly as long as the condition of having dirty hair is rue. Other examples would include: While the player has guesses left in a guess game, allow the player to continue guessing While the program user is entering invalid passwords, continue to ask for the password While the timer still has time left, continue running the program Looping allows a program to continue running through the same block of code over and over again as long as a condition is rue.

19 IERAION Iteration is the process of controlling how many times you repetitively execute a block of code. One pass through a block of code is considered one iteration. o trace through the iterations refers to observing the changes to values of variables as each subsequent iteration occurs.

20 WHILE AND OR LOOPS While loops are used when the number of iterations is unpredictable and variable. Example: If our program is going to continue to ask a player of a guess game to make guesses until the player guesses correctly then we have no idea how many rounds of the game will be needed. A while loop is ideal for this situation. or loops are used when the number of iterations is predictable and constant. Example: If our program is designed to use the length of a string to determine how many iterations of a loop is required. Since the string will have a known length we will know exactly how many iterations we will need. A for loop is ideal for this situation.

21 WHILE LOOP IERAION CONROL Control Variable the variable that is being modified in the body of the loop which is checked in the condition. Sentinel Variable the variable containing the sentinel value or escape value to which the control variable is compared. It is the gatekeeper used to determine if another iteration should be executed. lag or Switch A Boolean variable used to control whether a loop should continue or not. sentinelvar = 999 #Sentinel if (inputnum!= sentinelvar): Body must contain code that enables inputnum to eventually have a value of 101. If it doesn t then the loop would run forever. AKA Infinite Loop print( this will run after loop is exited )

22 DUCK, DUCK, GOOSE PROGRAM

23 AVOIDING HE ININIE LOOP An infinite loop locks a program into executed a block of code forever and should be avoided (usually!!!). Usually infinite loops are the result of one of three cases: Case 1: Case 2: Case 3: It is impossible for the condition to become alse. he control variable is never modified and the condition cannot become alse he sentinel variable contains a value that can be missed under specific circumstances.

24 AVOIDING HE ININIE LOOP (CON) Case 2 Case 1 Case 3?

25 HE ININIE LOOP BY DESIGN?!?! Earlier we mentioned that infinite loops should usually be avoided. here is one caveat. he break keyword can be used to exit any block. break only exits the block it is in, not necessarily the loop. In other words, break will leave the block of code at the same indentation level as the break keyword and start executing code at the next lowest indentation level. Generally experienced programmers avoid using break often. It is never necessary.

26 RANGE() OBJEC range() creates a sequence of integers and can be used with 1 to 3 arguments depending on the sequence you wish to create. Syntax: range(start, stop, step) start starting value of sequence (mandatory if step is used) - INCLUSIVE stop mandatory limit to sequence - EXCLUSIVE step optional if elements of sequence not increasing by 1 Examples: range(10) {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} range(5, 10) {5, 6, 7, 8, 9} range(0, 10, 2) {0, 2, 4, 6, 8} NOE:

27 OR LOOP It may be easier to think of the for loop as a for each step through sequence loop. he for loop looks to see how many steps, or iterations, are needed to walk through a sequence. At the start of each iteration of the for loop the value in the sequence for the current step is assigned to the target variable which may be used if desired. Examples:

28 NESED LOOPS A loop inside of another loop. Example 1:

29 NESED LOOPS (CON) Example 2:

30 NESING CONDIIONALS IN LOOPS

31 COMPOUND CONDIIONALS - HE AND SAEMEN and statements combine two or more conditional statement together just like in your Algebra classes. BOH conditions have to be rue for the compound conditional to be rue.

32 COMPOUND CONDIIONALS - HE OR SAEMEN or statements combine two or more conditional statement together just like in your Algebra classes. EIHER condition being rue will result in the compound conditional to being rue.

33 RUH ABLES A B A and B A or B A B (A & B) & (A or B) (A & B) or (A or B) (A or B) or (A or B)!(A & B) & (A or B) A B A and B A or B A B (A & B) & (A or B) (A & B) or (A or B) (A or B) or (A or B)!(A & B) & (A or B)

34 DE MORGAN S LAW De Morgan s Law helps transform negation logic and feels a lot like the Algebraic Distributive Property when applied to compound statements. Here is an informal explanation: "not (A and B)" is the same as "(not A) or (not B) and "not (A or B)" is the same as "(not A) and (not B)" A B!(A & B)!A or!b!(a or B)!A and!b