Control Structures in Python

Control Structures in Python

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So Far …• We can step through the items in a list, a

tuple, a string, or a range– [start:stop:step]

• However, we cannot yet– perform operations (or not) based on some

condition– include multiple instructions for each item in

the sequence.• Control structures in a language allow these

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First, Extending I/O options• File object can be created with open() built-in function.

– open(filename, mode) – mode = “r”, “w”, ”a”, ”r+” mode = “r” assumed if not

specified. (read, write, append, read/write)• File methods: (Selected)

– file.close()– file.flush()– file.read([size]) -- read at most size bytes from the file. If size

omitted, read to end of file– file.readline([size]) – read one line of the file. Optional size

determines maximum number of bytes returned and may produce an incomplete line.

– file.readlines() – read to EOF, returning a list of the file lines– file.write(str) – write str to file. May need flush or close to

complete the file write– writelines(sequence) – writes a sequence, usually a list of strings

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IF …• Simplest control

– Do some specified instructions if a given condition is true

if <condition>: <instruction(s) to execute>

– Extension: Do specified instructions if a condition is true, otherwise do something else.

if <condition>: <instruction(s) to execute if true>else: <instruction(s) to execute if false>

Note: indentation and punctuation are required.

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Examples of IF

seq=raw_input("Enter a DNA sequence: ")pattern=raw_input("Enter the pattern: ")if len(pattern)>len(seq): print "Error: Pattern length exceeds string length"else: print "sequence and pattern: ", seq, " ", pattern

Python’s requirement of indentation to delimit blocks of code – not popular, but not a serious issue after getting used to it.The : at the end of the if clause and the else clause may seem awkward, but they are required parts of the python syntax.

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The Flow Chart

Condition test

Block to execute when condition fails

Block to execute when condition true

False True

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Nested IF

• Sometimes, the block to be executed in one branch (or both branches) contain other if statements or other control structures.

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Condition test

Block to execute when condition true

False True

Each block may contain any combination of instructions, function calls, etc.

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Nested if exampleConsider this example code:

values = range(27,150,4)print valuesx=raw_input("Enter x:")if x in values: print "x in value set", xelse: if x> 50: print "new x greater than 50" else: print "new x less than 50"

Understand what it is supposed to do?

Predict the output, x = 43Run it

Explain what is wrong

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if .. else .. if

• The combination if .. else .. if occurs frequently enough that python (and some other languages) offers a shortcut.

• elif combines the else and if – elif x>50

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values = range(27,150,4)print valuesstrx=raw_input("Enter x:")x=int(strx)# or x=int(raw_input("Enter x: "))if x in values: print "x in value set", xelif x> 50: print "new x greater than 50"else: print "new x less than 50"

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Short spot check

• Usual drill – pairs of students• Left side (as I see you) do Practice 4.20

and 4.24• Right side, do Practice 4.22 and 4.23

• Presenters different from last week.• Look at the other questions so you can ask

questions and be sure to understand both sets. 10 to 15 minutes should do it

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Repetition

• Now we can decide which path to take, let’s add the ability to repeat a block of statements without having to put them in the code several times.

• for loops• while loops• function calls• Loops may be nested, may contain

conditionals in the block, may contain any other legitimate code in the block.

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for …

• Sometimes called an iterative loop.• A loop variable takes on each value in a

specified sequence, executes the body of the code with the current value, repeats for each value.

for <variable> in <sequence>:<block of code to execute>

• Sequence may be a list, a range, a tuple, a string

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Flowchart for for loop

Set iteration variable to initial or next value

variable in range? Loop bodyTrue

False

Skip loop body if initial iteration variable value is out of range.

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Input from a file• Built-in file class– Two ways to input a line from the file:• line=source.readline()• for line in source:

filename=raw_input('File to read: ')source = file(filename) #Access is read-onlyfor line in source: print line

where line and source are local names, source previously defined

Note – no explicit read in the for loop

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Iterating through a string

teststring = "When in the course of human events, \

it becomes necessary ..."countc = 0for c in teststring: if c == "a": countc +=1print "Number of c's in the string: ", countc

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Stepping through a list

cousins=["Mike", "Carol", "Frank", "Ann", "Jim", \"Pat", "Franny”,"Elizabeth", "Richard", "Sue"]steps = range(len(cousins))for step in steps: print cousins[step]+", ",

Output: Mike, Carol, Frank, Ann, Jim, Pat, Franny, Elizabeth, Richard, Sue,

Spot check Exercise: Exactly what is steps (type and content)? Get rid of that last comma.

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Stepping through the list again

• OutputMike, Carol, Frank, Ann, Jim, Pat, Franny, Elizabeth, Richard,

Sue,

cousins=["Mike", "Carol", "Frank", "Ann", "Jim", \"Pat", "Franny”,"Elizabeth", "Richard", "Sue"]for step in range(len(cousins)): print cousins[step]+", ”,

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Stepping through a file# Based on Figure 8.4 of Object Oriented Programming in # Pythonfilename=raw_input("Enter filename: ")source=file(filename)numlines = numwords = numchars = 0for line in source: numlines += 1 numwords += len(line.split()) numchars +=len(line) print lineprint "The file contains ", numlines, " lines, ", numwords, " words, and ", numchars, "characters."source.close()

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Danger – modifying iteration variable

original = ["A","B","C","D","E","F"]for entry in original: print "Current entry: ", entry original.remove(entry) print "List with ", entry, " removed. ", original

original.append(entry) print "List with ", entry, " appended. ", originalprint "List at end of loop:\n ", original

Predict the output

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While loops• for loops are iterated over the elements of

a sequence. You can know exactly how many times the loop will execute by checking the length of the sequence.

• while allows more flexible loop control– repeat the loop as long as a specified condition

is true– danger of loop not terminating– stop when a goal is reached without going

through more steps than needed.

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Silly example

ans=raw_input('Shall we play a game? ')while answer.lower() in ('y', 'yes'): #code for playing game ans= raw_input('Would you like to play again? ')

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Operation of the while

Condition Loop bodyTrue

False

Note – condition is tested before the loop body is ever executed. It is possible that the loop body will not be executed at all.

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Spot check• Exercise 5.8 (Write a program that answers

the following question. Starting with x = 1, how many times can x be doubled before reaching one million or more?

• Exercise 5.10: Write a prgram that allows the user to end=ter words, continuing until the user enters a duplicate. When a duplicate is encountered, print “You already entered” then the duplicated word. Then “You entered n distinct words” where n is the correct number.

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Read while data remains# Based on Figure 8.3 of Object Oriented Programming in #Pythonfilename=raw_input("Enter filename: ")source=file(filename)numlines = numwords = numchars = 0line = source.readline()while line: numlines += 1 numwords += len(line.split()) numchars +=len(line) line = source.readline()print "The file contains ", numlines, " lines, ", \ numwords, " words, and ", numchars, "characters."source.close()

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The set/frozenset containers

• We have seen several ways to store a collection of values.

• All are sequences– list, tuple, string (range produces a list)

• A sequence is an ordered collection of values• A set or frozenset is a collection of unique

values, with no particular order– unique = no duplicates– set is mutable– frozenset is immutable– elements in either are immutable

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List Comprehension• Some actions are common enough to

warrant a shortened version. • result = [expression for identifier in

sequence if condition]– [ ] shows that a list is generated– expression says what the list elements are– identifier iterates through the elements in

the sequence– condition determines whether the

sequence item meets a requirement and is included in the result

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List Comprehension• An example with no condition import nltkfrom nltk.book import text1print text1print "\n", "-----------------------"words=set(text1)print "Count of words in Moby Dick: ", len(sorted(words))print "Count of unique words in Moby Dick: ",\len(set([word.lower() for word in text1]))

• In the highlighted terms,• Expression is ???• Identifier is ???• Sequence is ???

Use of “set” removes duplicates

Example from nltk book

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Functions• Define a collection of statements to be

executed as needed. • Parameters allow modification of the

values used during execution• We have seen a number of built-in

functions– examples --- ??

• Now we will create our own

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Book example

def maxLength(stringSeq): longSoFar = "" #null string -- nothing between quotes for entry in stringSeq: print entry, " length is ", len(entry) if len(entry) > len(longSoFar): longSoFar = entry return longSoFar

ingredients = ['carbonated water', 'caramel color', 'caffeine', 'phosphoric acid',\'sodium saccharin', 'potassium benzoate', 'aspartame', 'potassium citrate',\'citric acid', 'dimethylpolysiloxane', 'natural flavors']concern = maxLength(ingredients)print concern

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Parameters and arguments

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Nested function calls

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Optional parameters• Provide a default value to be used if none is

provided during the call.

def countdown(start = 10): for count in range(start, 0, -1) print count

countdown() #will countdown starting at 10countdown(5) #will countdown starting at 5

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Conditional Statements with Duplicate Code.

#this has duplicate codeif destination == 'Missouri':

subtotal = sum(shoppingCart)tax = subtotal * 0.04225total = subtotal + tax

elif destination == 'Illinois':subtotal = sum(shoppingCart)tax = subtotal * 0.0625total = subtotal + tax

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Conditional Statements: Better than Previous slide

subtotal = sum(shoppingCart)if destination == 'Missouri':

tax = subtotal * 0.04225elif destination == 'Illinois':

tax = subtotal * 0.0625total = subtotal + tax

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Conditional Statements (Avoiding Duplicate Code)

# To improve readability call a function to # lookup tax rates.subtotal = sum(shoppingCart)tax = subtotal * lookupTaxRate(destination)total = subtotal + tax

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Spot check• Practice 5.21 Define a function

sumOfSquares(n) that returns the sum of the first n positive integers,

€

k 2k=1

n

∑