Lecture 7: Functions. CS1068+ Introductory Programming in Python. Dr Kieran T. Herley 2018/19. Department of Computer Science University College Cork

Transcription

1 Lecture 7: Functions CS1068+ Introductory Programming in Python Dr Kieran T. Herley 2018/19 Department of Computer Science University College Cork

2 Summary Functions in Python. Terminology and execution. Usefulness of functions: readability, modularity and code reuse. Fuitful and nonfruitful functions. 1

3 Functions Mathematics Functions are (typically named) formulae describing how some quantity (result) may be calculated from one or more inputs (arguments) f (x) = 2x 2 + 3x 5 Python A named block of code or subprogram that may be called (executed) from various points in the program. Typically: Takes in some inputs called arguments Executes code block to calculate some result Finishes and returns result to whoever called it 2

4 Some familiar functions Some Python functions we have seen already: len("python") the length of the string max(x, y, z) the largest of the numbers math.sqrt(5) the square root of 5 Syntax: biggest = }{{} max ( x, y, z ) }{{} fun. name arguments Apart from these built-in functions, we can write our own. 3

5 A simple example def square(n): # Return the square of 'n '. return n 2 num = int(input( Please enter a number: )) result = square(num) print( result ) 1 1 Recall x p means x to the power of p 4

6 Terminology: function definition def square(n): # < function definition # Return the square of 'n '. return n 2 # num = int(input( Please enter a number: )) result = square(num) # < function call print ( result ) 5

7 Terminology: function definition def square(n): # < function definition # Return the square of 'n '. return n 2 # num = int(input( Please enter a number: )) result = square(num) # < function call print ( result ) Function header: specifies name and parameters (in parens.) Function body: block of statements; specifies actions to be taken when function is executed statements indented fours spaces wrt def return: Terminates execution of fucntion; passes result back 5

8 Terminology: parameters and arguments def square(n): # < function definition # Return the square of 'n '. return n 2 # num = int(input( Please enter a number: )) result = square(num) # < function call print ( result ) 6

9 Terminology: parameters and arguments def square(n): # < function definition # Return the square of 'n '. return n 2 # num = int(input( Please enter a number: )) result = square(num) # < function call print ( result ) Parameter e.g. n special variable associated with function ; specifies input to function; function body expressed in terms of these variables Argument e.g. 17 actual value supplied when function called; used to initialize corresponding parameter 6

10 Terminology: function call def square(n): # < function definition # Return the square of 'n '. return n 2 # num = int(input( Please enter a number: )) result = square(num) # < function call print ( result ) 7

11 Terminology: function call def square(n): # < function definition # Return the square of 'n '. return n 2 # num = int(input( Please enter a number: )) result = square(num) # < function call print ( result ) Syntax function name and list of arguments in parentheses Meaning (Execution of main program pauses pending completion of function.) Function arguments evaluated and used to initialize function parameters. Function body executes until return encountered. (Execution of main program resumes using 7

12 Function execution Insert animation here. 8

13 Function execution def square(n): # (3) # Return the square of n. return n 2 # (4) num = int(input( Please enter a number: )) # (1) result = square(num) # (2) call, (5) assignment print ( result ) # (6) 9

14 Function execution def square(n): # (3) # Return the square of n. return n 2 # (4) num = int(input( Please enter a number: )) # (1) result = square(num) # (2) call, (5) assignment print ( result ) # (6) 1. Variable num assigned value typed in by user (say 7) 9

15 Function execution def square(n): # (3) # Return the square of n. return n 2 # (4) num = int(input( Please enter a number: )) # (1) result = square(num) # (2) call, (5) assignment print ( result ) # (6) 1. Variable num assigned value typed in by user (say 7) 2. Evaluation of RHS of assignment begins; function square called with argument 7 3. Function square starts; ( main program sleeps until square returns); argument value 7 used to initialize parameter n 9

16 Function execution def square(n): # (3) # Return the square of n. return n 2 # (4) num = int(input( Please enter a number: )) # (1) result = square(num) # (2) call, (5) assignment print ( result ) # (6) 1. Variable num assigned value typed in by user (say 7) 2. Evaluation of RHS of assignment begins; function square called with argument 7 3. Function square starts; ( main program sleeps until square returns); argument value 7 used to initialize parameter n 4. Expression n 2 evaluated (result 49); square finishes and returns value 49 9

17 Function execution def square(n): # (3) # Return the square of n. return n 2 # (4) num = int(input( Please enter a number: )) # (1) result = square(num) # (2) call, (5) assignment print ( result ) # (6) 1. Variable num assigned value typed in by user (say 7) 2. Evaluation of RHS of assignment begins; function square called with argument 7 3. Function square starts; ( main program sleeps until square returns); argument value 7 used to initialize parameter n 4. Expression n 2 evaluated (result 49); square finishes and returns value main program reawakens and assigns 49 to result 9

18 Function execution def square(n): # (3) # Return the square of n. return n 2 # (4) num = int(input( Please enter a number: )) # (1) result = square(num) # (2) call, (5) assignment print ( result ) # (6) 1. Variable num assigned value typed in by user (say 7) 2. Evaluation of RHS of assignment begins; function square called with argument 7 3. Function square starts; ( main program sleeps until square returns); argument value 7 used to initialize parameter n 4. Expression n 2 evaluated (result 49); square finishes and returns value main program reawakens and assigns 49 to result 9

19 Why Bother with Functions?

20 Functions enhance readability def celsius to fahr (temp c): # Return Fahrenheit equivalent of given temperature. temp f = temp c 9 / return temp f temp = float(input( Enter temp (Celsius): )) print ( Equivalent in Fahrenheit is, celsius to fahr (temp)) Function should have header comment describing succinctly but precisely what service function provides. Note comment describes what function does not how. Programmer ought to be able to use a function based solely on description provided by comment. 10

21 Functions enhance modularity and reuse Modularity Complex programs often structured as set of well-defined functions, each addressing some subtask of the overall task. Reuse May be able to reuse functions in other programs. 11

22 Temperature conversion table def celsius to fahr ( celsius ): # Return Fahrenheit equivalent to given temperature. fahr = celsius 9 / return fahr for temp in range( 10, 50, 5): print(temp, C =, celsius to fahr (temp), F ) What does this do? Note: function can be called many times, possibly from different parts of the program. 12

23 Fruitful and Non-fruitful Function

24 Functions: fruitful and non-fruitful Not all functions produce a result, other just do things. Example: def triangle (sym, n): # Print triangluar pattern of character 'sym' # of size 'n '. for row in range(n): for col in range(row+1): print (sym, end = ) print () triangle ( X, 1) for level in range(2, 7, 2): triangle (., level ) Imagine invisible return None at end of function body. 13

25 Functions: fruitful and non-fruitful Not all functions produce a result, other just do things. Example: def triangle (sym, n): # Print triangluar pattern of character 'sym' # of size 'n '. for row in range(n): for col in range(row+1): print (sym, end = ) print () triangle ( X, 1) for level in range(2, 7, 2): triangle (., level ) X Imagine invisible return None at end of function body. 13

26 Named Arguments

27 Named arguments def calc amount( principal, rate, num years): # Return final amount from amount 'principal' invested for ' num y # at rate ' rate ' (percentage). amount = principal for year in range(num years): amount = amount (1 + rate/100) return amount # Arguments need to correspond to parameters in order left to right ; # Need to remember which is which. value = calc amount(10000, 5, 10) # Can use parameter names in function call ; order doesn't matter. value = calc amount(num years = 10, rate = 5, principal = 10000) 14

28 Back Material

29 Notes and Acknowledgements Reading Code Acknowledgements 15