Download Functions in Python What is a function? Function Calling Why Use

Functions in Python
Functions
What is a function?
Definition
Parameters
Scope
A function is something you can call (possibly with some
parameters, i.e., the things in parentheses), which performs
an action and returns a value.
Return Values
Recursion
Functions in Python
Functions in Python
Functions
Parameters
Scope
Return Values
Recursion
Example
def h e l l o ( name , g r e e t i n g ) :
r e t u r n g r e e t i n g + ” , ” + name
p r i n t ( h e l l o ( ’ Markus ’ , ’ p r i v e t ’ ) )
L435/L555
Dept. of Linguistics, Indiana University
Fall 2014
Define first, then call!
In python, a function must be defined before you can call it.
e.g., define it on line 10, call it on line 15.
(http:
//www.greenteapress.com/thinkpython/html/thinkpython004.html)
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Function Calling
Functions in Python
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Why Use Functions?
Functions
Functions
Parameters
We’ve seen functions many times before:
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Built-in functions: int(), type(), ...
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Module-based functions: math.log(),
random.choice(), ...
Parameters
Scope
Scope
Return Values
Return Values
Recursion
Recursion
Functions are extremely useful because:
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They make code reusable
I
They make a program more structured & more
readable, especially when it gets longer
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They make it is easier to work with several programmers
We have also seen function composition
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int(input("Enter a number:"))
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The output of the inner function (e.g., input()) is
passed as the input to the outer function (e.g., int())
Functions in Python
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Functions Calling Functions
Functions in Python
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Parameters (Arguments)
Functions
Functions
Parameters
def p r i n t l y r i c s ( ) :
p r i n t ( ” Cause t h e p l a y e r s gonna p l a y . . . ” )
p r i n t ( ” And t h e h a t e r s gonna hate . . . ” )
Functions in Python
Parameters
Scope
Scope
Return Values
Return Values
Recursion
Recursion
Definition
Parameters (also known as arguments) are inputs to
functions.
def r e p e a t l y r i c s ( ) :
print lyrics ()
print lyrics ()
Example
When you use the min() function, you pass the function a
list as a parameter
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repeat lyrics ()
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e.g., min([8,6,7]) returns 6
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Local Scope
Functions in Python
Functions
Local scope
Functions in Python
Functions
Parameters
Variables and parameters in functions have local scope.
Parameters
Scope
Scope
Return Values
Return Values
Recursion
Mutable types
def change name ( name ) :
name = ’ The Thamesmen ’
name = ’ The New O r i g i n a l s ’
change name ( name )
p r i n t ( name )
# the value of t h i s
Local Scope (2)
Recursion
Mutable data structures change in functions.
def change ( l i s ) :
l i s . extend ( [ ’ shake ’ , ’ i t ’ , ’ o f f ’ ] )
’ name ’ i s unchanged
def again ( ) :
mypi = 3.11
p r i n t ( mypi )
# t h i s g i v e s an e r r o r :
m y l i s t = [ ” I ’m” , ’ j u s t ’ , ’ gonna ’ , ’ shake ’ ]
change ( m y l i s t )
print ( mylist )
’ mypi ’ i s u n d e f i n ed
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3 Types of Parameters
Functions in Python
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Parameter Types
Functions
Functions
Parameters
positional Positional parameters must be entered in the
correct order
hello (name, greeting)
Functions in Python
Definition
Scope
Return Values
Any kind of variable can be passed to a function (string,
integer, float, list, dict, tuple, object). Your function must use
these as the right type though.
Recursion
keyword Keyword parameters can be entered in any order
hello (greeting=’Ni Hao’, name=’Taylor’)
Parameters
Scope
Return Values
Recursion
Example
collected Parameters can also be collected by a function,
allowing the user to input any number of
parameters to the function
def s o r t P e o p l e ( people ) :
r e t u r n sorted ( people )
def h e l l o 2 ( * c ol l e ct e d P a r a m s ) :
print ( ” Intermediate value : ” , collectedParams )
r e t u r n ’ ’ . j o i n ( [ s t r ( x ) f o r x i n c o l l e c t e d P a r am s ] )
p r i n t ( h e l l o 2 ( ’ shake ’ , ’ x ’ , 4 ) )
s p i n a l T a r p = ’ N i g e l , David , and Derek ’
print ( sortPeople ( spinalTarp ) )
s p i n a l T a p = [ ’ N i g e l ’ , ’ David ’ , ’ Derek ’ ]
print ( sortPeople ( spinalTap ) )
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Parameter Types (2)
Functions in Python
Default Values
Functions
Functions
Parameters
Comment your code!
You must know/remember which types work for a function,
so it makes sense to add comments that specify the types of
the parameters and of the return value.
Functions in Python
Parameters
Scope
Scope
Return Values
Return Values
Recursion
Recursion
Parameters can be assigned a default value, used only if no
value is passed in
def myadd (D, key , v a l u e = 1 ) :
i f key i n D:
D[ key ] += v a l u e
else :
D[ key ] = v a l u e
Example
# f u n c t i o n sortPeople s o r t s the i n p u t & r e t u r n s i t
# i n p u t : people − l i s t o r s t r i n g
# output : l i s t
#
( l i s t of characters i f input= s t r i n g )
def s o r t P e o p l e ( people ) :
r e t u r n sorted ( people )
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Return Values
Functions in Python
Tip on Printing
Functions
Functions
Parameters
Definition
Parameters are inputs to functions. Return values are
outputs.
Functions in Python
Parameters
Scope
Scope
Avoid the following
Return Values
Recursion
Printing out stuff in functions (unless debugging)
Return Values
Recursion
def h e l l o ( ) :
print ( ” hello , world ” )
Multiple return values
To return more than one value, put them in a tuple
Instead, do the following
def rhymes ( ) :
x= ’ break ’
y= ’ f a k e ’
return ( x , y )
f o o =rhymes ( )
one , two=rhymes ( )
Returning stuff in functions and printing later
def h e l l o ( ) :
return ( ” hello , world ” )
print ( hello ( ) )
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Recursion
Functions in Python
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Fibonacci numbers
Functions in Python
Iterative version
Functions
Functions
Parameters
Parameters
def f i b i t e r ( n ) :
f minus2 = 0
f minus1 = 1
Scope
A function call (other) functions
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Return Values
Recursion
Recursion is when a function calls itself
def countdown ( n ) :
i f n <= 0 :
print ( ’ Blastoff ! ’ )
else :
print (n)
countdown ( n − 1)
Scope
Return Values
Recursion
i f n == 1 :
f i = 0
e l i f n == 2 :
f i = 1
else :
f o r i i n range ( 2 , n ) :
f i = f minus2 + f minus1
countdown ( 5 )
f minus2 = f minus1
f minus1 = f i
return f i
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Fibonacci numbers
Functions in Python
Recursion notes
Functions in Python
Iterative version
Functions
Functions
Parameters
Parameters
Scope
Scope
Return Values
Return Values
Recursion
Recursion
A few points regarding recursion:
def f i b r e c u r ( n ) :
i f n == 1 :
return 0
e l i f n == 2 :
return 1
else :
r e t u r n f i b r e c u r ( n − 1) + f i b r e c u r ( n − 2)
1. Whatever parameters are passed must move towards
some completion, e.g., integers get smaller (n-1)
2. Recursive functions have two parts:
2.1 Base case(s): what to do when you reach the “bottom”
(e.g., if n == 1)
2.2 Recursive case: what to do in moving from one value
to another
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