1. CS 1110 Introduction to Programming Spring 2017
Functions (2)
CS 1110
Introduction to Programming
Spring 2017

2. Print versus Return Print Return
Review
Print versus Return
Print
All print statements reached by the function are executed
They are printed to the screen
After a print statement is executed, the execution proceeds to the next statement
Return
A return statement is optional
Only first return statement reached gets run
If no return statement, function returns None
A return ceases execution of a function and returns a value
At most one (the first) return statement that is reached during a particular function call is executed
A return value is not printed, unless a function is printed
print(add(2, 3))
CS 1110

3. Void versus Value-Returning Functions
Review
Void versus Value-Returning Functions
Void functions
Does not return anything
None in Python
Examples
print(str)
random.seed(seed)
random.shuffle()
Value-Return functions
return something
Examples
abs(some_number)
random.randInt(0, 100)
random.sample(some_list)
CS 1110

4. Tracing through Code with Functions
Review
Tracing through Code with Functions
Pass by value  copy of actual value
Pass by reference  copy of the memory address
seven steps for every function call:
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5. Importing Existing Functions
Review
Importing Existing Functions
Be careful when naming your file. If the file name is the same as Python standard objects/libraries, the interpreter gets confused !!
import random up_time = 210 number_of_breakdowns = 16 num_runs = int(input("How many simulation runs? : ")) seed = int(input("Enter a seed (0 for random) : ")) if seed != 0: random.seed(seed) # sets the integer starting value used in generating random numbers sum_MTBF = 0 for run in range(num_runs): likelihood = random.random() MTBF = (up_time * likelihood) / number_of_breakdowns sum_MTBF += MTBF print(likelihood) print("Estimated MTBF over " + str(num_runs) + " simulations = " , sum_MTBF/num_runs)
CS 1110

6. Importing Existing Functions
Review
Importing Existing Functions
Assuming the add() function and other functions are saved in a file called math_lib.py
From … import * allows us to import all functions from math_lib.py into the current file
Call functions from other files
Can use add() without defining it here
Python imports some standard functions, such as str() and len() automatically; others need the import statement
Put at the top of the current file
from math_lib import * add(2, 3) print(add(4, 5)) print(add(1, -1))
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7. Calling Functions from Functions
def main(): print('The sum of 12 and 45 is ') show_sum(12, 45) def show_sum(num1, num2): result = num1 + num2 print(result) main()
main
num1
show_sum
num2 12 45
result 57
none
The sum of 12 and 45 is 57
CS 1110

8. Local Variables
number = 0 def main(): number = int(input('Enter a number: ')) show_number() def show_number(): print('The number you entered is ', number) main()
Enter a number: 7
The number you entered is 0
CS 1110

9. Global Variables
number = 0 def main(): global number number = int(input('Enter a number: ')) show_number() def show_number(): print('The number you entered is ', number) main()
Enter a number: 7
The number you entered is 7
CS 1110

10. Calling Functions with Named/Optional Parameters
def my_function(name="Mary", school="UVa"): print(name + " goes to " + school) # call a function without param passing my_function() # call a function with some params (ordering) my_function("Tom") # call a function with a named param my_function(school="GMU") # call a function with param passing (match number of params and ordering) my_function("Ann", "VT") # call a function with named params in any order my_function(school="GMU", name="Ann")
Default values
Mary goes to Uva
Tom goes to Uva
Mary goes to GMU
Ann goes to VT
Ann goes to GMU
CS 1110