CSE 160

Functions

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Hannah Cheung

Announcements

• HW0 grades released
• Resubmissions open! (+ tutorial)
• Regrades open! (+ tutorial)
• Due Friday (7/1) @ 11:59 PM:
• HW1 due
• Submit programming + reflection survey
• Due Wednesday (7/6) @ 11:59 PM:
• CodingBat #1 Problems due
• Resubmissions and regrades do not apply here!
• Due Friday (7/8) @ 11:59 PM:
• HW2 due
• DNA processing using loops, if statements, functions, and string manipulation

2

Math Functions

• Math uses functions
• Defining functions: f(x) = 2x + 1
• Using functions: f(2) = 5
• Python lets you use and define functions
• We have already seen some Python functions:

len, float, int, str, range

3

Python Functions

• Function packages up and names a computation
• Advantages to using functions
• Through parameters, generalizes computation to other scenarios
• Reduces repetition in programs
• Makes programs easier to understand and easier to modify and debug

4

Using (Calling) Functions

len(“hello”)

len(“”)

math.sqrt(9)

math.sqrt(7)

range(1, 5)

range(8)

math.sin(0)

str(17)

random.random()

print()

5

• Functions can take in zero or more inputs
• All of the functions here return a value
• Get to re-use code that someone else wrote

Function call examples

import math

x = 8

y = 16

z = math.sqrt(16)

u = math.sqrt(y)

v = math.sqrt(8 + 8)

w = math.sqrt(x + x)

6

What are the

• function calls or “function invocations” or “call sites”?
• arguments or parameters for each function call?

See in Python Tutor

Function call examples

greeting = “hi”

name = “Hannah”

a = len(“hello”)

b = len(greeting)

c = len(“hello” + “Hannah”)

print(“hello”)

print()

print(len(greeting + name))

7

What are the

• function calls or “function invocations” or “call sites”?
• arguments or parameters for each function call?

See in Python Tutor

Function call examples

greeting = “hi”

name = “Hannah”

a = len(“hello”)

b = len(greeting)

c = len(“hello” + “Hannah”)

print(“hello”)

print()

print(len(greeting + name))

8

What are the

• function calls or “function invocations” or “call sites”?
• arguments or parameters for each function call?

​

• math.sqrt() and len take an input and return a value.
• print produces a side effect (prints to terminal).

See in Python Tutor

Some functions are like a machine

9

• Given an input
• Produces a result or “returns” a value

2x + 1

2x + 1

2x + 1

x

x

x

2

0

100

5

f(x)

1

f(x)

201

f(x)

Defining a function

def dbl_plus(x):

return 2 * x + 1

10

keyword that defines function

2x + 1

x

f(x)

Defining a function

def dbl_plus(x):

return 2 * x + 1

11

keyword that defines function

2x + 1

x

f(x)

name of function

Defining a function

def dbl_plus(x):

return 2 * x + 1

12

keyword that defines function

2x + 1

x

f(x)

name of function

input variable name or parameter

Defining a function

def dbl_plus(x):

return 2 * x + 1

13

keyword that defines function

2x + 1

x

f(x)

name of function

input variable name or parameter

keyword that specifies result

Defining a function

def dbl_plus(x):

return 2 * x + 1

14

keyword that defines function

2x + 1

x

f(x)

name of function

input variable name or parameter

keyword that specifies result

return expression (part of return statement)

Executing a function call

def square(x):

return x * x

square(3 + 4)

15

Function definition

Function call

Executing a function call

def square(x):

return x * x

square(3 + 4)

square(3 + 4)

square(7)

16

Function definition

Function call

1. Evaluate argument(s) at call site - place where we are calling function from in program

Executing a function call

def square(x):

return x * x

square(3 + 4)

square(3 + 4)

square(7)

17

Function definition

Function call

• Evaluate argument(s) at call site - place where we are calling function from in program
• Assign argument’s value to the formal parameter name
• A new variable, not a reuse of any existing variable of the same name

​

x: 7

Executing a function call

def square(x):

return x * x

square(3 + 4)

square(3 + 4)

square(7)

18

Function definition

Function call

• Evaluate argument(s) at call site - place where we are calling function from in program
• Assign argument’s value to the formal parameter name
• A new variable, not a reuse of any existing variable of the same name
• Evaluate statements in the body of function one by one

​

x: 7

​

return 7 * 7

Executing a function call

def square(x):

return x * x

square(3 + 4)

square(3 + 4)

square(7)

19

Function definition

Function call

• Evaluate argument(s) at call site - place where we are calling function from in program
• Assign argument’s value to the formal parameter name
• A new variable, not a reuse of any existing variable of the same name
• Evaluate statements in the body of function one by one
• At return statement:
• Formal parameter variable disappears - exists only during the call
• Call expression evaluates to “returned” value

​

x: 7

​

return 7 * 7

​

square(3 + 4) = 49

Function definitions and calls

def dbl_plus(x):

return 2 * x + 1

​

def instructor_name():

return “Hannah Cheung”

​

def calc_grade(points):

grade = points * 10

return grade

20

What are the

• function names?
• function body?
• inputs or parameters?
• arguments or parameters for each function call?

Function definitions and calls

def dbl_plus(x):

return 2 * x + 1

​

def instructor_name():

return “Hannah Cheung”

​

def calc_grade(points):

grade = points * 10

return grade

​

# main program

dbp3 = dbl_plus(3)

dbp4 = dbl_plus(4)

print(dbp3 + db4)

print(instructor_name())

my_grade = calc_grade(dbp3)

21

What are the

• function definitions?
• inputs or parameters?

​

• function calls or “function invocations” or “call sites”?
• arguments or parameters for each function call?

See in Python Tutor

Function definitions and calls

def calc_grade(points):

grade = points * 10

return grade

​

def print_grade(points):

grade = points * 10

print(“Grade is:”, grade)

​

# main program

my_grade = calc_grade(10)

print_grade(10)

22

No return statement

Returns the value None

Executed for side effect

All in same file

See in Python Tutor

Functions can call functions

def fahr_to_celsius(fahr):

return (fahr - 32) / 9.0 * 5

​

def celsius_to_fahr(celsius):

result = celsius / 5.0 * 9 + 32

return result

​

def print_fahr_to_celsius(fahr):

result = fahr_to_celsius(fahr)

print(result)

​

# main program

boiling = fahr_to_celsius(212)

cold = celsius_to_fahr(-30)

print(print_fahr_to_celsius(32))

23

No return statement

Returns the value None

Executed for side effect

All in same file

See in Python Tutor

Warmup #1 - What does this print?

def celsius_to_fahr(celsius):

print(celsius / 5.0 * 9 + 32)

​

print(celsius_to_fahr(20))

24

See in Python Tutor

Warmup #1 - What does this print?

def celsius_to_fahr(celsius):

print(celsius / 5.0 * 9 + 32)

return None

​

print(celsius_to_fahr(20))

25

See in Python Tutor

Warmup #2 - What does this print?

def my_func(n):

total = 0

for i in range(n):

total = total + i

return total

​

print(my_func(4))

​

26

See in Python Tutor

Warmup #3 - What does this print?

def c_to_f(c):

print(“c_to_f”)

return c / 5.0 * 9 + 32

​

def make_message(temp):

print(“make_message”)

return “The temperature is “ + str(temp)

​

for tempc in [-40, 0, 37]:

tempf = c_to_f(tempc)

message = make_message(tempf)

print(message)

27

See in Python Tutor

Digression: Two types of output

• An expression evaluates to a value, which can be used by the containing expression/statement
• print statement writes text to the screen
• Python interpreter reads statements and expressions and executes them like a calculator
• Executing an expression prints its value
• In a program (Visual Studio Code, Python Tutor), evaluating an expression does not print it
• Printing an expression does not permit it to be used elsewhere

28

Looking up variables

• Idea: Find the nearest variable of the given name
• Check whether variable is defined in local scope
• Check whether variable is defined in intermediate scope
• not covered in CSE 160
• Check whether variable is defined in global scope
• If local and global variable have the same name, global variable is inaccessible (“shadowed” or “masked”)
• Try to avoid shadowing

29

Looking up variables

• Idea: Find the nearest variable of the given name
• Check whether variable is defined in local scope
• Check whether variable is defined in global scope

x = 22

stored = 100

def lookup():

x = 42

return stored + x

​

val = lookup()

x = 5

stored = 200

val = lookup()

30

See in Python Tutor

Looking up variables: masking

• Idea: Find the nearest variable of the given name
• Check whether variable is defined in local scope
• Check whether variable is defined in global scope

x = 22

stored = 100

def lookup():

x = 42

return stored + x

​

val = lookup()

x = 5

stored = 200

val = lookup()

x = 22

stored = 100

def lookup():

x = 42

stored = stored + x

return stored

​

val = lookup()

x = 5

stored = 200

val = lookup()

31

See in Python Tutor

Temporary variables

def store_it(arg):

stored = arg

return stored

​

stored = 0

y = store_it(22)

print(y)

print(stored)

32

See in Python Tutor

How many x variables?

def square(x):

return x * x

​

def abs(x):

if x < 0:

return -x

else:

return x

​

# main program

x = 42

sq3 = square(3)

print(x)

x = -22

result = abs(x)

print(result)

33

All in same file

See in Python Tutor

Local and global scope

myvar = 1

def outer():

myvar = 1000

​

temp = inner()

return temp

​

def inner():

return myvar

​

print(outer())

​

​

34

See in Python Tutor

Designing functions

• Breaking down a program into functions is the fundamental activity of programming
• When to use/define a function
• DRY (Don’t Repeat Yourself)
• Whenever you feel tempted to copy and paste code, don’t!

35

Variables should represent one thing

# Legal, but confusing: don’t do this!

x = 3

…

x = “hello”

…

x = [3, 1, 4, 1, 5]

​

​

36

Each variable should contain values of only one type

Designing functions

1. Write the program as if the function already exists

# Main program

tempf = 32

print(“Temp in Fahr:”, tempf)

tempc = fahr_to_celsius(tempf)

print(“Temp in Celsius”, tempc)

​

​

37

Designing functions

• Write the program as if the function already exists
• Write a specification: describe the inputs and output, including their types

# Main program

tempf = 32

print(“Temp in Fahr:”, tempf)

tempc = fahr_to_celsius(tempf)

print(“Temp in Celsius”, tempc)

​

​

def fahr_to_celsius(fahr):

“““

Input: Number of degrees Fahrenheit

Return: Number of degrees Celsius

”””

​

​

38

Designing functions

• Write the program as if the function already exists
• Write a specification: describe the inputs and output, including their types
• Write tests: example inputs and outputs

# Main program

tempf = 32

print(“Temp in Fahr:”, tempf)

tempc = fahr_to_celsius(tempf)

print(“Temp in Celsius”, tempc)

​

​

def fahr_to_celsius(fahr):

“““

Input: Number of degrees Fahrenheit

Return: Number of degrees Celsius

”””

​

​

assert fahr_to_celsius(32) == 0

assert fahr_to_celsius(212) == 100

assert fahr_to_celsius(98.6) == 37

assert fahr_to_celsius(-40) == -40

​

​

39

Designing functions

• Write the program as if the function already exists
• Write a specification: describe the inputs and output, including their types
• Write tests: example inputs and outputs
• Write the function body (implementation)
• Write your plan in English (pseudocode), then translate to Python

# Main program

tempf = 32

print(“Temp in Fahr:”, tempf)

tempc = fahr_to_celsius(tempf)

print(“Temp in Celsius”, tempc)

​

​

def fahr_to_celsius(fahr):

“““

Input: Number of degrees Fahrenheit

Return: Number of degrees Celsius

”””

return (fahr - 32) / 9.0 * 5

​

assert fahr_to_celsius(32) == 0

assert fahr_to_celsius(212) == 100

assert fahr_to_celsius(98.6) == 37

assert fahr_to_celsius(-40) == -40

​

​

40

Documentation

Functions are an abstraction

• Abstraction = ignore some details
• Generalization = become usable in more contexts
• As long as the user knows what the function means, you don’t care how it computes that value
• You don’t care about the implementation (function body)

42

Types of documentation

1. Documentation for users/clients/callers
1. Often called the “docstring”
2. Tells what function does
3. Purpose, meaning, or abstraction of what it represents
4. Parameters, behavior, what is returned/side effects

​

def square(x):

“““

Returns the square of its argument x.

”””

# Uses “x * x” instead of “x ** 2”

return x * x

43

for users - string as first element of function body

Types of documentation

• Documentation for users/clients/callers
• Often called the “docstring”
• Tells what function does
• Purpose, meaning, or abstraction of what it represents
• Parameters, behavior, what is returned/side effects

​

• Documentation for programmers who are reading code
• Often called “comments”
• Tells how function does it
• Only necessary for tricky or interesting bits of code

def square(x):

“““

Returns the square of its argument x.

”””

# Uses “x * x” instead of “x ** 2”

return x * x

44

for users - string as first element of function body

for programmers - text after #

Multi-line strings

• Single-quote strings
• Examples: “hello” or ‘hello’
• Use in program when manipulating strings, printing

​

• Triple-quote strings
• Use this version for docstrings and file description header
• String can span multiple lines
• Can include quotation marks
• Example:

​

“““

Hello

world

”””

​

45

Comments to not write

• Comments should give information that is not apparent from the code
• Here is a counter-productive comment that clutters the code, which makes the code harder to read:

# increment value of x

x = x + 1

46

Where to write comments

• By convention, write a comment above the code that it describes
• Reasoning: Reader sees English explanation for the possibly-confusing code

​

​

​

• May appear anywhere in program, including at end of line

​

​

​

• For a comment, indentation should be consistent with surrounding code

# increment value of x

x = x + 1

x = x + 1 # increment value of x

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