Creating Objectives
your own functions
Oct 15, 2007
Sprenkle - CS111
1
Why write functions?
Allows you to break up a hard problem into smaller,
more manageable parts Makes your code easier to understand Hides implementation details (abstraction)
Provides interface (input, output)
Makes part of the code reusable so that you:
Only have to type it out once Can debug it all at once Isolates errors Can make changes in one function (maintainability)
Oct 15, 2007
Sprenkle - CS111
2
Functions Function is a black box
Implementation doesn't matter Only care that function generates appropriate output, given appropriate input
Example:
Didn't care how raw_input function was implemented
input prompt raw_input user_input output
Oct 15, 2007
Sprenkle - CS111
We saved output in a variable
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Syntax of Function Definition
Keyword Function Name Input Name/ Parameter
def ftpsToMPH(ftps) :
Body (or function definition)
Function header
SECOND_TO_HOUR = 3600 FEET_TO_MILE = (1.0/5280) result = ftps * SECOND_TO_HOUR * FEET_TO_MILE return result
Keyword: How to give output
Oct 15, 2007
Sprenkle - CS111
4
Where are functions in the code?
Can be defined in script before use (calling
it) Could be in separate module
Import to use in script Example: menu.py Define in modules when functions are reusable for many different programs
Benefits: shorter code (no function defns), isolate
testing of function, write "test driver" scripts to test functions separately from use in script
Oct 15, 2007 Sprenkle - CS111 5
Parameters
The inputs to a function are called
parameters or arguments When calling/using functions, parameters must appear in same order as in the function header
Example: round(x, n)
x is float to round n is integer of decimal places to round to
Oct 15, 2007
Sprenkle - CS111
6
Parameters Formal Parameters are the variables
named in the the function definition. Actual Parameters are the variables or literals that really get used when the function Formal is called. Actual def round(x, n) : roundCelc = round(celc,2) Formal & actual parameters must match in order, number, and type!
Oct 15, 2007 Sprenkle - CS111 7
Practice: Old McDonald
A verse of the song goes
Old McDonald had a farm, E-I-E-I-O And on that farm he had a dog, E-I-E-I-O With a ruff, ruff here And a ruff, ruff there Here a ruff, there a ruff, everywhere a ruff, ruff Old McDonald had a farm, E-I-E-I-O
Write a function to print a verse
Why does it make sense to write a function for the verse? What is input? What is output?
Oct 15, 2007 Sprenkle - CS111 8
Function Output
When the code reaches a statement like
return x x is the output returned to the place where function called and the function stops
For functions that don't have explicit output, return does not have a value with it
return Optional: don't need to have output/return
Oct 15, 2007
Sprenkle - CS111
9
Flow of Control
To input function
num1 gets the value of x num2 gets the value of y def max(num1, num2) :
x=2
result=0 num1 >= num2
False
y = input("Enter ...") z=max(x, y)
Gets replaced with True function's output
result=num1
result=num2
print "The max is", z return result
Oct 15, 2007
Sprenkle - CS111
10
Flow of Control: Using return
def max(num1, num2) : if num1 >= num2 : return num1 return num2
def max(num1, num2) : num1 >= num2
True
x=2 y=6 z = max( x, y )
return num1
return to caller
Oct 15, 2007
Implicit false branch: Only way got here is if the condition was not true return num2
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Sprenkle - CS111
Using return
Use return to "shortcut" function
Return output as soon as know answer Compare efficiency of two functions in binaryToDecimal.py
Oct 15, 2007
Sprenkle - CS111
12
Passing Parameters
Only copies of the actual parameters are
given to the function The actual parameters in the calling code do not change.
Showed example with swap function
Oct 15, 2007
Sprenkle - CS111
13
Program Organization
Functions can go inside of program script
Defined before use
Functions can go inside a separate module
Reduces code in main script Easier to reuse by importing from a module Maintains the "black box"
Oct 15, 2007
Sprenkle - CS111
14
Writing a main function
In many languages, you put the "driver" for
your program in a main function
You can (and should) do this in Python as well
Typically main methods go at the top of your
program
can Readers quickly see what program does
main usually takes no arguments
Example: def main():
Oct 15, 2007
Sprenkle - CS111
15
Using a main Function
Call main() at the bottom of your program Side-effect:
Do not need to define functions before main function main can "see" other functions Note that main is a function that calls other functions
Any function can call other functions
Oct 15, 2007 Sprenkle - CS111
oldmac.py
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Example program with a main()
oldmac.py
Oct 15, 2007
Sprenkle - CS111
17
Function Variables
def main() : x=2 y=6 max = max( x, y ); def max(num1, num2) : max = num1 if num2 >= num1 : max = num2 return max main()
Oct 15, 2007 Sprenkle - CS111 18
Why can we name two variables max?
Function Variables
def main() : x=2 y=6 max = max( x, y ); def max(num1, num2) : max = num1 if num2 >= num1 : max = num2 return max main()
Oct 15, 2007
The stack x main y max
Sprenkle - CS111
2 6
Variable names are like first names
Function names are like last names
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Function Variables
def main() : x=2 y=6 max = max( x, y ); Called the function max, so need def max(num1, num2) : to add its parameters to the stack max = num1 num1 2 max if num2 >= num1 : num2 6 max = num2 x 2 return max main y 6 max main()
Oct 15, 2007 Sprenkle - CS111 20
Function Variables
def main() : x=2 y=6 max = max( x, y ); def max(num1, num2) : max = num1 if num2 >= num1 : max = num2 return max main()
Oct 15, 2007
max
num1 2 num2 6 max 2 2 6
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x main y max
Sprenkle - CS111
Function Variables
def main() : x=2 y=6 max = max( x, y ); def max(num1, num2) : max = num1 if num2 >= num1 : max = num2 return max main()
Oct 15, 2007
max
num1 2 num2 6 max 6 2 6
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x main y max
Sprenkle - CS111
Function Variables
def main() : x=2 y=6 max = max( x, y ); def max(num1, num2) : max = num1 if num2 >= num1 : max = num2 return max main()
Oct 15, 2007
Function max returned, so we no longer have to keep track of its variables on the stack. The lifetime of those variables is over. x main y max
Sprenkle - CS111
2 6 6
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Variable Scope
Functions can have the same parameter and
variable names as other functions
Need to look at the variable's scope to determine which one you're looking at Use the stack to figure out which variable you're using
Scope levels
Local scope (also called function scope)
Can only be seen within the function
Global scope (also called file scope)
Whole program can access More on these later
Oct 15, 2007 Sprenkle - CS111
scope.py
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Practice
What is the output of this program?
Example: user enters 4
def square(n): return n * n def main(): num = input("Enter a number to be squa...