PHOT 110: Introduction to programming

LECTURE 05

Michaël Barbier, Spring semester (2023-2024)

Program structures so far …

Calculator: Only expressions, re-use previous result, no re-running
Scripts: variables & expressions: re-run same script, track and remove human mistakes
Branching (if, else): explicit logical expressions, execute code according to decisions
while and for Loops: Repeat code, according to condition
Functions: further re-use, abstraction of code, call whenever required, allows parameters

Functions

are separate reusable pieces of code
can be invoked or called from the script
accept 0 or more parameters
return an object(s) (possible None)

Function definition syntax

Function declaration

def area_ellipse(radius1, radius2):
  """ 
  area_ellipse calculates the area of an ellipse

  Param radius1 (float): Largest radius
  Param radius2 (float): Smalles radius
  
  Returns [float] A : area of the ellipse
  """
  pi = 3.1415
  A = radius1 * radius2 * pi

  return A

area = area_ellipse(2, 3)
print(f"The area of the ellipse is {area}")

The area of the ellipse is 18.849

Return values

Use the return keyword
the value can be an expression
Without return, function returns None
Multiple outputs possible

def add(a, b):
  """ Sum a and b """
  return a + b

sum = add(2, 5)
print(sum)

Return values

Use the return keyword

def add(a, b):
  """ Sum a and b """
  sum = a + b
  return sum

sum = add(2, 5)
print(sum)

Return values

Use the return keyword
the value can be an expression

def add(a, b):
  """ Sum a and b """
  return a + b

sum = add(2, 5)
print(sum)

Return values

Use the return keyword
the value can be an expression
Without return, function returns None

def add(a, b):
  """ Sum a and b """
  a + b

sum = add(2, 5)
print(sum)

None

Return values

Use the return keyword
the value can be an expression
Without return, function returns None
Multiple outputs possible

def add(a, b):
  """ Sum a and b and give extra outputs """
  return a + b, "a second output", a - b

out = add(2, 5)
print(out)

(7, 'a second output', -3)

Unpacking multiple outputs

import math

def polar_to_carth(radius, angle):
  """ polar_to_carth converts polar to carthesian """
  x = radius*math.cos(angle)
  y = radius*math.sin(angle)

  return x, y

x, y = polar_to_carth(3, math.pi/3)
print(f"Polar (3, pi/3) = {(x, y)}")

Polar (3, pi/3) = (1.5000000000000004, 2.598076211353316)

Multiple return statements possible

def minimum(a, b):
  """ Returns the minimum of two numbers """
  if a <= b:
    return a
  else:
    return b

print(minimum(3.4, 6.5))

3.4

Variable scope

Script variables and function parameters can have the same name

def increment(a):
  """ Increment number by 1 """
  a = a + 1
  return a

# Define the variable in the script
a = 5
print("Variable a (before) = " + str(a))
b = increment(a)
print("Variable a (after) = " + str(a))
print("Variable b = " + str(b))

Variable a (before) = 5
Variable a (after) = 5
Variable b = 6

Variable scope

Functions can use script variables from outside the function
But cannot change those variables

def increment(a):
  """ Increment number by 1 """
  a = a + x
  return a

# Define the variable in the script
a = 5
x = 4
print(increment(a))

Variable scope

Functions can use script variables from outside the function
But cannot change those variables

def increment(a):
  """ Increment number by 1 """
  a = a + x
  x = x + 3
  return a

# Define the variable in the script
a = 5
x = 4
print(increment(a))

UnboundLocalError: cannot access local variable 'x' where it is not associated with a value

Functions can call functions

def minimum(a, b):
  if a <= b:
    return a
  else:
    return b

def add(a, b, fct):
  """ Sum a, b and minimum of both """
  return a + b + fct(a, b) 

a = 10; b = 2
print(add(a, b, minimum))