Week 3: Creating Functions

EMSE 4574: Intro to Programming for Analytics

John Paul Helveston

September 15, 2020

1 / 38

Quiz 2Go to #classroom channel in Slack for link
06:00
2 / 38

Thanks for the cute animals:

Olivia Z., Kyara M., Eliese O., Helena R., David R., Carolyne I., Omar A.

3 / 38

...and Alfie the alpaca:

4 / 38

Video on?

It's nice to see your faces :)

If you're okay with it, please turn on your camera - it creates a more engaging discussion environment and an opportunity for us to get to know each other better. Your privacy is important though, and I understand if you wish to keep it off. No pressure.

5 / 38

Week 3: Creating Functions1. Function syntax2. Local vs global variables3. Top-down design4. Coding style6 / 38

Week 3: Creating Functions1. Function syntax2. Local vs global variables3. Top-down design4. Coding style7 / 38

Basic function syntaxfunctionName <- function(arguments) {
    # Do stuff here
    return(something)
}

8 / 38

Basic function syntax

In English:

"functionName is a function of arguments that does..."

functionName <- function(arguments) {
    # Do stuff here
    return(something)
}

9 / 38

Basic function syntax

Example:

"squareRoot is a function of n that...returns the square root of n"

squareRoot <- function(n) {
    return(n^0.5)
}

10 / 38

Basic function syntax

Example:

"squareRoot is a function of n that...returns the square root of n"

squareRoot <- function(n) {
    return(n^0.5)
}

squareRoot(64)

## [1] 8

10 / 38

return() and cat() statements11 / 38

return() and cat() statementsisPositive <- function(n) {
    return(n > 0)
}

11 / 38

return() and cat() statementsisPositive <- function(n) {
    return(n > 0)
}

isPositive <- function(n) {
    cat(n > 0)
}

11 / 38

`return()` and `cat()` statements

isPositive <- function(n) {
    return(n > 0)
}

return() returns back a value

test <- isPositive(7)
test

TRUE

isPositive <- function(n) {
    cat(n > 0)
}

cat() prints a value as a string

test <- isPositive(7)

TRUE

test

Error: object 'test' not found

12 / 38

cat() is short for "concatenating"13 / 38

`cat()` is short for "concatenating"

print_x <- function(x) {
    cat("The value of x is", x)
}

13 / 38

`cat()` is short for "concatenating"

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

13 / 38

`cat()` is short for "concatenating"

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

print_x_squared <- function(x) {
    cat("The value of x is", x, "and the value of x^2 is", x^2)
}

13 / 38

`cat()` is short for "concatenating"

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

print_x_squared <- function(x) {
    cat("The value of x is", x, "and the value of x^2 is", x^2)
}

print_x_squared(7)

## The value of x is 7 and the value of x^2 is 49

13 / 38

cat() adds a space between values by default14 / 38

`cat()` adds a space between values by default

print_x <- function(x) {
    cat("The value of x is", x)
}

14 / 38

`cat()` adds a space between values by default

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

14 / 38

`cat()` adds a space between values by default

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

Modify separator with the sep argument:

print_x <- function(x) {
    cat("The value of x is", x, sep = ": ")
}

14 / 38

`cat()` adds a space between values by default

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

Modify separator with the sep argument:

print_x <- function(x) {
    cat("The value of x is", x, sep = ": ")
}

print_x(7)

## The value of x is: 7

14 / 38

02:00

Code tracing practice

Consider these functions:

f1 <- function(x) {
    return(x^3)
}
f2 <- function(x) {
    cat(x^3)
}
f3 <- function(x) {
    cat(x^3)
    return(x^4)
}
f4 <- function(x) {
    return(x^3)
    cat(x^4)
}

What will these lines of code produce?

Write your answer down first, then run the code to check.

f1(2)
f2(2)
f3(2)
f4(2)

15 / 38

Week 3: Creating Functions1. Function syntax2. Local vs global variables3. Top-down design4. Coding style16 / 38

Local objectsAll objects inside function are "local" - they don't exist in the global environment17 / 38

Local objectsAll objects inside function are "local" - they don't exist in the global environmentExample:
squareOfX <- function(x) {
  y <- x^2     # y here is "local"
  return(y)
}

17 / 38

Local objects

All objects inside function are "local" - they don't exist in the global environment

Example:

squareOfX <- function(x) {
  y <- x^2     # y here is "local"
  return(y)
}

squareOfX(3)

## [1] 9

If you try to call y, you'll get an error:

Error: object 'y' not found

17 / 38

Global objectsGlobal objects exist in the main environment.18 / 38

Global objectsGlobal objects exist in the main environment.NEVER, NEVER, NEVER call global objects inside functions.18 / 38

Global objectsGlobal objects exist in the main environment.NEVER, NEVER, NEVER call global objects inside functions.print_x <- function(x) {
    cat(x)
    cat(n)  # n is global!
}
n <- 5 # Define n in the *global* environment
print_x(5)

## 55
18 / 38

Global objects

Global objects exist in the main environment.

NEVER, NEVER, NEVER call global objects inside functions.

print_x <- function(x) {
    cat(x)
    cat(n)  # n is global!
}
n <- 5 # Define n in the *global* environment
print_x(5)

## 55

n <- 6
print_x(5)

## 56

Function behavior shouldn't change with the same arguments!

18 / 38

Global objectsAll objects inside functions should be arguments to that function19 / 38

Global objectsAll objects inside functions should be arguments to that functionprint_x <- function(x, n = NULL) {
    cat(x)
    cat(n)  # n is local!
}
n <- 5 # Define n in the *global* environment
print_x(5)

## 5
19 / 38

Global objects

All objects inside functions should be arguments to that function

print_x <- function(x, n = NULL) {
    cat(x)
    cat(n)  # n is local!
}
n <- 5 # Define n in the *global* environment
print_x(5)

## 5

n <- 6
print_x(5)

## 5

Use n as argument:

print_x(5, n)

## 56

19 / 38

02:00

Code tracing practice

Consider this code:

x <- 7
y <- NULL
f1 <- function(x) {
    cat(x^3)
    cat(y, x)
}
f2 <- function(x, y = 7) {
    cat(x^3, y)
}
f3 <- function(x, y) {
    cat(x^3)
    cat(y)
}
f4 <- function(x) {
    return(x^3)
    cat(x^4)
}

What will these lines of code produce?

Write your answer down first, then run the code to check.

f1(2)
f2(2)
f3(2)
f4(2)

20 / 38

Break
05:00
21 / 38

Week 3: Creating Functions1. Function syntax2. Local vs global variables3. Top-down design4. Coding style22 / 38

"Top Down" design

23 / 38

"Top Down" design

1. Break the problem into pieces

23 / 38

"Top Down" design

1. Break the problem into pieces

2. Solve the "highest level" problem first

23 / 38

"Top Down" design

1. Break the problem into pieces

2. Solve the "highest level" problem first

3. Then solve the smaller pieces

23 / 38

Example: Given values a and b, find the value c such that the triangle formed by lines of length a, b, and c is a right triangle (in short, find hypotenuse)

24 / 38

Example: Given values a and b, find the value c such that the triangle formed by lines of length a, b, and c is a right triangle (in short, find hypotenuse)

Hypotenuse: $c = \sqrt{a^2 + b^2}$

Break the problem into two pieces:

$c = \sqrt{x}$

$x = a^2 + b^2$

25 / 38

Example: Given values a and b, find the value c such that the triangle formed by lines of length a, b, and c is a right triangle (in short, find hypotenuse)

Hypotenuse: $c = \sqrt{a^2 + b^2}$

Break the problem into two pieces:

$c = \sqrt{x}$

hypotenuse <- function(a, b) {
    return(sqrt(sumOfSquares(a, b)))
}

$x = a^b + b^2$

sumOfSquares <- function(a, b) {
    return(a^2 + b^2)
}

26 / 38

10:00

Create a function, isRightTriangle(a, b, c) that returns TRUE if the triangle formed by the lines of length a, b, and c is a right triangle and FALSE otherwise. Use the hypotenuse(a, b) function in your solution. Hint: you may not know which value (a, b, or c) is the hypotenuse.

hypotenuse <- function(a, b) {
    return(sqrt(sumOfSquares(a, b)))
}

sumOfSquares <- function(a, b) {
    return(a^2 + b^2)
}

27 / 38

Week 3: Creating Functions

1. Function syntax

2. Local vs global variables

3. Top-down design

4. Coding style

28 / 38

Style matters!29 / 38

Style matters!

Which is easier to read?

V1:

sumofsquares<-function(a,b)return(a^2 + b^2)

V2:

sum_of_squares <- function(a, b) {
    return(a^2 + b^2)
}

29 / 38

Style matters!

Which is easier to read?

V1:

sumofsquares<-function(a,b)return(a^2 + b^2)

V2: <- This one is much better!

sum_of_squares <- function(a, b) {
    return(a^2 + b^2)
}

30 / 38

Use the "Advanced R" style guide:

http://adv-r.had.co.nz/Style.html

31 / 38

Use the "Advanced R" style guide:

http://adv-r.had.co.nz/Style.html

Other good style tips on this blog post

31 / 38

Style guide: Objects

32 / 38

Style guide: Objects

Use <- for assignment, not =
Put spacing around operators
(e.g. x <- 1, not x<-1)
Use meaningful variable names
This applies to file names too
(e.g. "hw1.R" vs. "untitled.R")

32 / 38

Style guide: Functions

Generally, function names should be verbs:

add()      # Good
addition() # Bad

33 / 38

Style guide: Functions

Generally, function names should be verbs:

add()      # Good
addition() # Bad

Avoid using the "." symbol:

get_hypotenuse()  # Good
get.hypotenuse()  # Bad

33 / 38

Style guide: Functions

Generally, function names should be verbs:

add()      # Good
addition() # Bad

Avoid using the "." symbol:

get_hypotenuse()  # Good
get.hypotenuse()  # Bad

Use curly braces, with indented code inside:

sum_of_squares <- function(a, b) {
    return(a^2 + b^2)
}

33 / 38

Indent by 4 spaces

Set line length to 80

34 / 38

15:00

onesDigit(x): Write a function that takes an integer and returns its ones digit.

Tests:

onesDigit(123) == 3
onesDigit(7890) == 0
onesDigit(6) == 6
onesDigit(-54) == 4

tensDigit(x): Write a function that takes an integer and returns its tens digit.

Tests:

tensDigit(456) == 5
tensDigit(23) == 2
tensDigit(1) == 0
tensDigit(-7890) == 9

35 / 38

Hint #1:

You may want to use onesDigit(x) as a helper function for tensDigit(x)

Hint #2:

The mod operator (%%) "chops" a number and returns everything to the right

123456 %% 1

## [1] 0

123456 %% 10

## [1] 6

The integer divide operator (%/%) "chops" a number and returns everything to the left

123456 %/% 1

## [1] 123456

123456 %/% 10

## [1] 12345

36 / 38

15:00

eggCartons(eggs): Write a function that reads in a non-negative number of eggs and prints the number of egg cartons required to hold that many eggs. Each egg carton holds one dozen eggs, and you cannot buy fractional egg cartons.

eggCartons(0) == 0
eggCartons(1) == 1
eggCartons(12) == 1
eggCartons(25) == 3

militaryTimeToStandardTime(n): Write a function that takes an integer between 0 and 23 (representing the hour in military time), and returns the same hour in standard time.

militaryTimeToStandardTime(0) == 12
militaryTimeToStandardTime(3) == 3
militaryTimeToStandardTime(12) == 12
militaryTimeToStandardTime(13) == 1
militaryTimeToStandardTime(23) == 11

37 / 38

HW 3

38 / 38

HW 3

Use the template

38 / 38

HW 3

Use the template
Use Polya's problem solving technique:
1. Understand the problem
2. Devise a plan
3. Carry out the plan
4. Check your work

38 / 38

HW 3

Use the template
Use Polya's problem solving technique:
1. Understand the problem
2. Devise a plan
3. Carry out the plan
4. Check your work
Try out the autograder (Saurav will DM you your password on Slack)

38 / 38

Week 3: Creating Functions

EMSE 4574: Intro to Programming for Analytics

John Paul Helveston

September 15, 2020

1 / 38

Quiz 2Go to #classroom channel in Slack for link
06:00
2 / 38

Thanks for the cute animals:

Olivia Z., Kyara M., Eliese O., Helena R., David R., Carolyne I., Omar A.

3 / 38

...and Alfie the alpaca:

4 / 38

Video on?

It's nice to see your faces :)

5 / 38

Week 3: Creating Functions1. Function syntax2. Local vs global variables3. Top-down design4. Coding style6 / 38

Week 3: Creating Functions1. Function syntax2. Local vs global variables3. Top-down design4. Coding style7 / 38

Basic function syntaxfunctionName <- function(arguments) {
    # Do stuff here
    return(something)
}

8 / 38

Basic function syntax

In English:

"functionName is a function of arguments that does..."

functionName <- function(arguments) {
    # Do stuff here
    return(something)
}

9 / 38

Basic function syntax

Example:

"squareRoot is a function of n that...returns the square root of n"

squareRoot <- function(n) {
    return(n^0.5)
}

10 / 38

Basic function syntax

Example:

"squareRoot is a function of n that...returns the square root of n"

squareRoot <- function(n) {
    return(n^0.5)
}

squareRoot(64)

## [1] 8

10 / 38

return() and cat() statements11 / 38

return() and cat() statementsisPositive <- function(n) {
    return(n > 0)
}

11 / 38

return() and cat() statementsisPositive <- function(n) {
    return(n > 0)
}

isPositive <- function(n) {
    cat(n > 0)
}

11 / 38

`return()` and `cat()` statements

isPositive <- function(n) {
    return(n > 0)
}

return() returns back a value

test <- isPositive(7)
test

TRUE

isPositive <- function(n) {
    cat(n > 0)
}

cat() prints a value as a string

test <- isPositive(7)

TRUE

test

Error: object 'test' not found

12 / 38

cat() is short for "concatenating"13 / 38

`cat()` is short for "concatenating"

print_x <- function(x) {
    cat("The value of x is", x)
}

13 / 38

`cat()` is short for "concatenating"

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

13 / 38

`cat()` is short for "concatenating"

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

print_x_squared <- function(x) {
    cat("The value of x is", x, "and the value of x^2 is", x^2)
}

13 / 38

`cat()` is short for "concatenating"

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

print_x_squared <- function(x) {
    cat("The value of x is", x, "and the value of x^2 is", x^2)
}

print_x_squared(7)

## The value of x is 7 and the value of x^2 is 49

13 / 38

cat() adds a space between values by default14 / 38

`cat()` adds a space between values by default

print_x <- function(x) {
    cat("The value of x is", x)
}

14 / 38

`cat()` adds a space between values by default

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

14 / 38

`cat()` adds a space between values by default

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

Modify separator with the sep argument:

print_x <- function(x) {
    cat("The value of x is", x, sep = ": ")
}

14 / 38

`cat()` adds a space between values by default

print_x <- function(x) {
    cat("The value of x is", x)
}

print_x(7)

## The value of x is 7

Modify separator with the sep argument:

print_x <- function(x) {
    cat("The value of x is", x, sep = ": ")
}

print_x(7)

## The value of x is: 7

14 / 38

02:00

Code tracing practice

Consider these functions:

f1 <- function(x) {
    return(x^3)
}
f2 <- function(x) {
    cat(x^3)
}
f3 <- function(x) {
    cat(x^3)
    return(x^4)
}
f4 <- function(x) {
    return(x^3)
    cat(x^4)
}

What will these lines of code produce?

Write your answer down first, then run the code to check.

f1(2)
f2(2)
f3(2)
f4(2)

15 / 38

Week 3: Creating Functions1. Function syntax2. Local vs global variables3. Top-down design4. Coding style16 / 38

Local objectsAll objects inside function are "local" - they don't exist in the global environment17 / 38

Local objectsAll objects inside function are "local" - they don't exist in the global environmentExample:
squareOfX <- function(x) {
  y <- x^2     # y here is "local"
  return(y)
}

17 / 38

Local objects

All objects inside function are "local" - they don't exist in the global environment

Example:

squareOfX <- function(x) {
  y <- x^2     # y here is "local"
  return(y)
}

squareOfX(3)

## [1] 9

If you try to call y, you'll get an error:

Error: object 'y' not found

17 / 38

Global objectsGlobal objects exist in the main environment.18 / 38

Global objectsGlobal objects exist in the main environment.NEVER, NEVER, NEVER call global objects inside functions.18 / 38

Global objectsGlobal objects exist in the main environment.NEVER, NEVER, NEVER call global objects inside functions.print_x <- function(x) {
    cat(x)
    cat(n)  # n is global!
}
n <- 5 # Define n in the *global* environment
print_x(5)

## 55
18 / 38

Global objects

Global objects exist in the main environment.

NEVER, NEVER, NEVER call global objects inside functions.

print_x <- function(x) {
    cat(x)
    cat(n)  # n is global!
}
n <- 5 # Define n in the *global* environment
print_x(5)

## 55

n <- 6
print_x(5)

## 56

Function behavior shouldn't change with the same arguments!

18 / 38

Global objectsAll objects inside functions should be arguments to that function19 / 38

Global objectsAll objects inside functions should be arguments to that functionprint_x <- function(x, n = NULL) {
    cat(x)
    cat(n)  # n is local!
}
n <- 5 # Define n in the *global* environment
print_x(5)

## 5
19 / 38

Global objects

All objects inside functions should be arguments to that function

print_x <- function(x, n = NULL) {
    cat(x)
    cat(n)  # n is local!
}
n <- 5 # Define n in the *global* environment
print_x(5)

## 5

n <- 6
print_x(5)

## 5

Use n as argument:

print_x(5, n)

## 56

19 / 38

02:00

Code tracing practice

Consider this code:

x <- 7
y <- NULL
f1 <- function(x) {
    cat(x^3)
    cat(y, x)
}
f2 <- function(x, y = 7) {
    cat(x^3, y)
}
f3 <- function(x, y) {
    cat(x^3)
    cat(y)
}
f4 <- function(x) {
    return(x^3)
    cat(x^4)
}

What will these lines of code produce?

Write your answer down first, then run the code to check.

f1(2)
f2(2)
f3(2)
f4(2)

20 / 38

Break
05:00
21 / 38

Week 3: Creating Functions1. Function syntax2. Local vs global variables3. Top-down design4. Coding style22 / 38

"Top Down" design

23 / 38

"Top Down" design

1. Break the problem into pieces

23 / 38

"Top Down" design

1. Break the problem into pieces

2. Solve the "highest level" problem first

23 / 38

"Top Down" design

1. Break the problem into pieces

2. Solve the "highest level" problem first

3. Then solve the smaller pieces

23 / 38

Example: Given values a and b, find the value c such that the triangle formed by lines of length a, b, and c is a right triangle (in short, find hypotenuse)

24 / 38

Example: Given values a and b, find the value c such that the triangle formed by lines of length a, b, and c is a right triangle (in short, find hypotenuse)

Hypotenuse: $c = \sqrt{a^2 + b^2}$

Break the problem into two pieces:

$c = \sqrt{x}$

$x = a^2 + b^2$

25 / 38

Example: Given values a and b, find the value c such that the triangle formed by lines of length a, b, and c is a right triangle (in short, find hypotenuse)

Hypotenuse: $c = \sqrt{a^2 + b^2}$

Break the problem into two pieces:

$c = \sqrt{x}$

hypotenuse <- function(a, b) {
    return(sqrt(sumOfSquares(a, b)))
}

$x = a^b + b^2$

sumOfSquares <- function(a, b) {
    return(a^2 + b^2)
}

26 / 38

10:00

hypotenuse <- function(a, b) {
    return(sqrt(sumOfSquares(a, b)))
}

sumOfSquares <- function(a, b) {
    return(a^2 + b^2)
}

27 / 38

Week 3: Creating Functions

1. Function syntax

2. Local vs global variables

3. Top-down design

4. Coding style

28 / 38

Style matters!29 / 38

Style matters!

Which is easier to read?

V1:

sumofsquares<-function(a,b)return(a^2 + b^2)

V2:

sum_of_squares <- function(a, b) {
    return(a^2 + b^2)
}

29 / 38

Style matters!

Which is easier to read?

V1:

sumofsquares<-function(a,b)return(a^2 + b^2)

V2: <- This one is much better!

sum_of_squares <- function(a, b) {
    return(a^2 + b^2)
}

30 / 38

Use the "Advanced R" style guide:

http://adv-r.had.co.nz/Style.html

31 / 38

Use the "Advanced R" style guide:

http://adv-r.had.co.nz/Style.html

Other good style tips on this blog post

31 / 38

Style guide: Objects

32 / 38

Style guide: Objects

Use <- for assignment, not =
Put spacing around operators
(e.g. x <- 1, not x<-1)
Use meaningful variable names
This applies to file names too
(e.g. "hw1.R" vs. "untitled.R")

32 / 38

Style guide: Functions

Generally, function names should be verbs:

add()      # Good
addition() # Bad

33 / 38

Style guide: Functions

Generally, function names should be verbs:

add()      # Good
addition() # Bad

Avoid using the "." symbol:

get_hypotenuse()  # Good
get.hypotenuse()  # Bad

33 / 38

Style guide: Functions

Generally, function names should be verbs:

add()      # Good
addition() # Bad

Avoid using the "." symbol:

get_hypotenuse()  # Good
get.hypotenuse()  # Bad

Use curly braces, with indented code inside:

sum_of_squares <- function(a, b) {
    return(a^2 + b^2)
}

33 / 38

Indent by 4 spaces

Set line length to 80

34 / 38

15:00

onesDigit(x): Write a function that takes an integer and returns its ones digit.

Tests:

onesDigit(123) == 3
onesDigit(7890) == 0
onesDigit(6) == 6
onesDigit(-54) == 4

tensDigit(x): Write a function that takes an integer and returns its tens digit.

Tests:

tensDigit(456) == 5
tensDigit(23) == 2
tensDigit(1) == 0
tensDigit(-7890) == 9

35 / 38

Hint #1:

You may want to use onesDigit(x) as a helper function for tensDigit(x)

Hint #2:

The mod operator (%%) "chops" a number and returns everything to the right

123456 %% 1

## [1] 0

123456 %% 10

## [1] 6

The integer divide operator (%/%) "chops" a number and returns everything to the left

123456 %/% 1

## [1] 123456

123456 %/% 10

## [1] 12345

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15:00

eggCartons(0) == 0
eggCartons(1) == 1
eggCartons(12) == 1
eggCartons(25) == 3

militaryTimeToStandardTime(n): Write a function that takes an integer between 0 and 23 (representing the hour in military time), and returns the same hour in standard time.

militaryTimeToStandardTime(0) == 12
militaryTimeToStandardTime(3) == 3
militaryTimeToStandardTime(12) == 12
militaryTimeToStandardTime(13) == 1
militaryTimeToStandardTime(23) == 11

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HW 3

38 / 38

HW 3

Use the template

38 / 38

HW 3

Use the template
Use Polya's problem solving technique:
1. Understand the problem
2. Devise a plan
3. Carry out the plan
4. Check your work

38 / 38

HW 3

Use the template
Use Polya's problem solving technique:
1. Understand the problem
2. Devise a plan
3. Carry out the plan
4. Check your work
Try out the autograder (Saurav will DM you your password on Slack)

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