Class 24 — Monday October 19

Reaching an understanding with functions

Functional living — Problem-solving strategy — Embrace this future

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Look both ways

• Back or ahead

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Agenda

• Continue function-based chrestomathics

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Examples

• Module harb.py

• Program inger.py

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• Module olio.py

• Program use_olio.py

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Homework

• Module double.py

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Why functions

• In large-scale digital problem solving, the component tasks are written as independent units of code.

• For clarity, code units are named so that there is no ambiguity as to what unit is being considered.

• In Python we call a named unit of code, a function. The name of function must be an identifier.

• When done properly, functions provide an easier, clearer, and reusable way for the manipulation and calculation of values.

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Module harb.py

Function add()

• Has two parameters a and b. Returns the value of a + b

• If the following code segment from program inger.py is executed

import harb

 

n1, n2 = 3, 14

n3, n4 = 15, 92

 

t1 = harb.add( n1, n2 )

t2 = harb.add( n3, n4 )

 

print( "add(", n1, ",", n2, "):", t1 )

print( "add(", n3, ",", n4, "):", t2 )

then the output should be

add( 3 , 14 ): 17

add( 15 , 92 ): 107

Function negate( x )

• Has one integer parameter x. Returns the additive inverse of x; i.e, -x

n5, n6 = -6, 53

 

i1 = harb.negate( n5 )

i2 = harb.negate( n6 )

 

print( "negate(", n5, "):", i1 )

print( "negate(", n6, "):", i2 )

then the output should be

negate( -6 ): 6

negate( 53 ): -53

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Requirement

• All functions that we developed are to be stored in your CS 1112 folder.

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Terminoloy

• A module is a named collection of functions.

• A module is stored in a Python file (.py) that matches its name.

• Module names are required to be identifiers.

• Access to a module capabilities requires for it to be imported.

import module_name

• A function is a named block of code to carry out some specific task

• Function names are required to be identifiers.

• Functions can have special variables called parameters to store start up values.

• To access a particular function in a module it must be invoked.

module_name . function_name ( arguments )

• The arguments are a list of start up values copied (passed) to the function so that it can carry out its task.

• The arguments are used to initialize the corresponding parameters of the function. The value of the first argument is used to intialize the first parameter, and so on.

• Parameter initialization occurs before the function stores to perform its task.

• A function typically produces a value that is returned to the code that invoked it. Python provides the return statement for this purpose.

return function_result

• For functions that end without return statements, Python supplies the value None

• A function definition looks like the following

def function_name( parameters ) :

  ''' header_comment

  '''

  action

• The keyword def signals that a function is being defined

• The parameters are a parenthesized list of variable names (identifiers)

• When invoked, the number of arguments being passed needs to be the same as the number of parameters.

• The function_name along with the parameters form the function header

• The colon  :  is a separator of the function header from the function and its action

• The header comment (docstring) describes the function

• The action is a non-empty block of statements that run when the function is invoked

• A function invocation causes some computer memory to be set aside for the function carrying its task. The memory is called an activation record or a frame

• The activation record stores values of the parameter variables.

• The actication record stores the values of other variables needed to perform its task.

def add( a, b ) : # function named add(), parameters named a and b

  ''' Returns value of a + b

  '''

 

  result = a + b # compute value of interest

 

  return result # function hands back its result

• When a program starts up, its code is executed. We say that it has the flow of control

• When a program invokes a function, it temporarity passes the flow of control so tha code of the function can run.

• The code making up the action of a function is called a statement block.

• A function completes as soon as it reaches a return statement or when it finished executing its statement block, whatever happens first.

• If a return statement is reached, the return value is determined.

• If there is no expression following keyword return, the return value is None

• If the code complete without reaching a return statement, the return value is None

• Once a return value is set, the execution of the function immediately stops.

• The value of the function invocation – its evaluation – is the return value

• The flow of control goes back to the statement that did the invocation.

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Taxonomy

• The two main kinds of functions are

• Functions that have parameters and return values

• Functions that have parameters and have None as the return value

• There can also be

• Functions that do not have parameters, but do return values

• Functions that do not have parameters and have None as the return value

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Designing functions

• Need to determine what information does a function need on startup.

• What information it is to produced.

• Provide an algorithm for going from one to the other.

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Module olio.py

Function voting_age()

• Does not take any parameters. Returns an integer indicating how old you need to be to vote. For example, the following code segment initializes both x and y to the integer 18.

x = olio.voting_age()

y = olio.voting_age()

Function has_blanks( s )

• Has one parameter s. Returns whether s contains at least one blank character; i.e., returns True if s contains a blank, and returns False otherwise. For example, the following code segment initializes b1 and b2 to True and False respectively.

x = 'CS 1112'

y = 'the_aarvark_said_arf_arf'

b1 = olio.has_blanks( x )

b2 = olio.has_blanks( y )

Function great_seal()

• Does not have any parameters and does not return a value. The function prints text from the Great Seal of the United States, i.e., E Pluribus Unum.

• The code segment

olio.great_seal( )

print()

olio.great_seal( )

print()

produces as output

E Pluribus Unum

E Pluribus Unum

Function a_ing( n )

• Has one parameter n. The function does not perform a return, instead it prints n lines of output. The first line prints a single 'a', the second line prints 'aa', the third line prints 'aaa', and so on. The string operator * should prove useful. For example usage, the code segment

olio.a_ing( 5 )

print()

olio.a_ing( 1 )

print()

olio.a_ing( 3 )

produces as output

a

aa

aaa

aaaa

aaaaa

a

a

aa

aaa

 

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© 2020 Jim Cohoon

Resources from previous semesters are available.