1. CSE 231 Lab 4

2. Functions Global VS local variables
Topics to cover
Functions
Global VS local variables

3. Think a math function! F(x) = x^2 What is F(2) ? F(2) = 4
Functions
Think a math function! F(x) = x^2 What is F(2) ? F(2) = 4
x = 2

4. Functions - Think a math function!
F(x) = x^2 Functions have a Domain and a Range What is a Domain? All possible input of a function. What is a Range? All possible output of a function.

5. Functions – Back to Python
What are functions?
Why do we use functions?
Examples of functions?

6. What are functions? Allow you to write code once and reuse it later
Make code more readable, easier to debug
Some examples of functions we’ve used:
print(), input()
Computer Science is full of buzzword, When you think functions, you think Abstraction.

7. Functions – Back to Python
Functions in Python (and everywhere else in the world) have the option of:
Accepting input. (Domain)
Giving output. (Range)

8. Functions – What do they look like?
Functions have a definition, parameters, and some have return value(s)

9. What is the output? Examples Example 1: def print_hi(): print(“Hi!”)
def print_hi(name):
print(“Hi”, name, “!”)
print_hi(‘Bob’)
Hi Bob!
Example 3:
def square(num):
return num**2
Nothing
Example 4:
def print_hi(name):
print(“Hi”, name, “!”)
print_hi()
Error

10. 2.5 15 Examples def average(number1, number2):
return average
avg = average(2,3)
print(avg)
2.5
num1 = 10
num2 = 20
avg = average(num1,num2)
print(avg) 15

11. Functions – Multiple returns
def get_double_and_triple(num):
return num*2 num*3
double_two, triple_two = get_double_and_triple(2)
print(“2 * 2 =”, double_two, ”| 2 * 3 =”, triple_two”)
>>> SyntaxError: invalid syntax Why?

12. Functions – Multiple returns
def get_double_and_triple(num): return num*2, num*3 double_two, triple_two = get_double_and_triple(2) print(“2 * 2 =”, double_two, ”| 2 * 3 =”, triple_two”) >>> 2 * 2 = 4 | 2 * 3 = 6
IMPORTANT

13. Global VS Local Variables

14. Local
def local_test():
local_variable = “Hi, I’m Local!”
print(local_variable)
What’s the issue here?

15. Local
def local_test():
local_variable = “Hi, I’m Local!”
print(local_variable)
local_variable only accessible inside local_test()!

16. Global x = 10 y = 4 def print_globals(y): y = 3 print(x) #10
print(y) #3
print_globals(2)
print(y) #4

17. Global x = 10 y = 20 def mess_with_globals(): x += 10 y += 10
print(x) #What is the output?
print(y) #What is the output?

18. Global x = 10 y = 20 def mess_with_globals(): x += 10 y += 10
print(x) #10
print(y) #20
X and y are global because they were defined outside of a function
Nothing happen because you did not call the function

19. Global x = 10 y = 20 def mess_with_globals(): x += 10 y += 10
print(x)
print(y)
X and y are global because they were defined outside of a function
UnboundLocalError: local variable ‘x' referenced before assignment

20. Global x = 10 y = 20 def mess_with_globals(): global x global y
print(x) #20
print(y) #30
x and y are global because they were defined outside of a function
All good. But this is not a good practice. If you want to change a variable, pass the variable to the function and return the result

21. All good Global x = 10 y = 20 def mess_with_globals(x,y): x += 10
return x,y
x,y = mess_with_globals(x,y)
print(x) #20
print(y) #30
All good

22. Local + some global  local_variable = “I’m a liar” def local_test():
local_variable = “Hi, I’m Local!”
print(local_variable)
#I’m a liar
local_variable is actually a global variable.

23. Some things to remember!
Function variables are independent of your main variables
Do NOT use globally defined variables in functions
Do NOT declare functions inside loops
Coding standards, no. 2
If a function needs to access/modify a variable, pass the variable to the function and return the result