[PDF] Loops and Conditionals Loops and. Conditionals. HORT 59000

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Loops and ConditionalsHORT 59000Lecture 11Instructor: Kranthi Varala

Relational Operators•These operators compare the value of two 'expressions' and returns a Boolean value.•Bewareofcomparing across data types, especially when reading values in from command line orfiles.

Relational Operators==equalTrue if expressions are equal!=not equalTrue if expressions are not equal>Greater thanTrueifleft is greater than the right=greater than OR equal<=lessthan OR equalisidentityTrue if theleft is the same object as rightincontainsTrue if the object on left is contained in object on right (Useful for finding values in list)

Assignment Operators•A += Bincrease A by value of B•A -= Bdecrease A by value of B •A *= BmultiplyA byB and assign value to A•A /= Bdivide A by B and assign value to A•A **=Braise value of A to the power of B•A %= Bmodulus of A by B, assigned to A•A //= BfloorofA divided by B, assigned to A•String context:•S1+= S2add string on right to the one on left•S1 *= AMake A copies of S1 and concatenate them to S1

Boolean OperatorsCombinestwo or more statements that return a Boolean value.A and BTrue if both A and B are trueA or BTrue if either A or B is truenot Areverse the Boolean given by Axor(A,B)Trueifonly one of A or B is TrueABA and BA or BNot Axor(A,B)TRUETRUETRUETRUEFALSEFALSETRUEFALSEFALSETRUEFALSETRUEFALSETRUEFALSETRUETRUETRUEFALSEFALSEFALSEFALSETRUEFALSE

General Python Syntax rules•End of line is end of statement•Statements at the same indentation level are in the same block (e.g., within a loop or condition)•Endofindentation•Exceptions:•Semi colon ; separates statements on the same line•Single line blocks are allowed without indentation

Branching logic•Used to implement alternate paths for the logic flow.https://upload.wikimedia.org/wikipedia/commons/4/44/LampFlowchart.png

If/elif/else statementsif test1:statement 1eliftest2:statement 2else:statement 3•Both the elifand else blocks are optional.

If/elif/else statements

Lamp flowchart with if/elseAcceptsinputfromuser, as astring

Truth and Boolean tests in Python•All objects in python have an inherent true or false value.•Anynonempty object istrue. •For Integers : Any non-zero number is true•Zero,empty objects and special object 'None' are false.•Comparisonsreturnthe values True or False

Loops/Iterations•Aloop is syntax structure that repeats all the statements within the loop until the exit condition is met.•Statements in a loop aredefined by indenting them relative to the loop start.•Loop ends when indentation ends.•Python has two forms of loops: for loop and while loop.•E.g. >>> for x in range(10)•E.g. >>>while (A==10)

while loops•while condition:statement 1statement 2..•Mostgenericformofloop,thatchecks whether the condition is true at the start of each iteration.•Expects the condition to become false at some point during the iterations of the loop.•Ifcondition is never changed, this creates an 'infinite' loop. i.e.,theprogramwillgetstuckinthisloop for ever.

Example while loops

Altering while loops•Normal loop control will execute all statements in block on every iteration. Loop endsonlywhenexit condition is met.•break statement forces the current loop to exit.•continue statement skips the rest of the block and goes to the next iteration of the loop.•pass statementisaplaceholder forempty blocks.

Altering while loops

for loops•for item in sequence:statement 1statement 2..•Genericiteratorforitems in a ordered sequence such as lists, tuples etc.•On each iteration retrieves one item from thelistandassignsittothevariable specified.•Automatically moves to the next item in the order.•Value of variable maybealteredwithintheforloop,butchange isnotmade in the list.

for loops

Looping over StringsandLists•List is a general sequence object while String is a character sequence object.•Bothcanbeiterated over by a for loop:

Looping over lists with and without index•Looping with an index allows accessing the item within the list and changing it.

Looping over Tuples and Dictionaries

Nested Loops•Loops can be nested just like the if/else statements.•Indentationisagainthekey to creating nested loops.•Ina2 level nested loopwithxiterationsontheouterloopandyiterations in the inner loop:•Allstatements in the outer loop will be executed x times•All statements in the inner loop will be executed x*y times

MultiDimensionalListsmyqDList : [[ltqtutc]t[btjtwth]t[.tl•tlltlq]t[lutlctlbtlj]]lqucbjwh.l•lllqlulclbljmyqDList•lqu•qlu

Lopingn vernSetpsnrirmyqDList : [[ltqtutc]t[btjtwth]t[.tl•tlltlq]t[lutlctlbtlj]]for xinrangevlenvmyqDListeeBinsideList:myqDList[x]lqucbjwh.l•lllqlulclbljmyqDList•lqu•qlu

Lopingn vernSetpsnrirmyqDList : [[ltqtutc]t[btjtwth]t[.tl•tlltlq]t[lutlctlbtlj]]for xinrangevlenvmyqDListeeBfor y in rangevlenvmyqDList[x]eeBprintmyqDList[x][y]lqucbjwh.l•lllqlulclbljmyqDList•lqu•qlu

Arbitrary dimensional ListssubL: [[ÕpÕtÕqÕ]t[ÔrÕtÕsÕ]]myqDList : [[ltqtutÕaÕ]t[btjtwtÕcatÕ]t[.tl•tÕeÕtlq]t[ÕbetaÕtlctlbtsubL]]lquabjwcat.l•elqbetalclbmyqDList•lqupqrs•qlu•l•lLoop1Loop2Loop3

Summary: Conditions and loops•Conditional statements with the proper comparison and booleanoperators allow the creation of alternate execution paths in the code.•Loopsallowrepeatedexecutionofthesame set of statements on all the objects within a sequence.•Usinganindexbasedforloopisbestsuited for making changes to items within a list.•Alwaysensurethatyourexitconditionwillbemet.

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