Intro to Data Science

Lecture 7 – Code Iteration

A Guide to Your Process

Scheduling

Learning Objectives

Practice

Supporting Information

Class Discussion

Today’s Plan

• Code Iteration with Loops
• Conditionals in Loops
• In-Class Free Work

Today’s Learning Objectives

After today’s session you will be able to:

• Describe the contexts where iteration is useful
• Apply loops to small arithmetic problems
• Create a loop with a conditional

Repeated Operations

• Often we want to repeat a given operation multiple times
  • I.e., repeat iteratively

• Could just copy/paste our code for each iteration
  • Labor intensive & inefficient & ugly (IMO)

• Copy/pasting fails if you need to do something many times
  • Code iteration is our solution to these types of problems!

Code Iteration

• Iteration process:
  1. Define the operation that you want to repeat
  2. Define the values to be passed through that operation
  3. Press “go” and sit back while the code does the work!

• Benefits:
  • Much faster than copy/pasting code
  • Code is more human-readable / navigable
  • Allows you to do something else while the code iterates itself!

• This method is called a “for loop”

Loop Syntax

• For loops repeat an operation for each value given to them

• Fundamental syntax is as follows:

# Start of loop
for ( index in vector ) {

    # Actual operation of loop
    function(index) 

} # End of loop

• More info:
  • index = placeholder for one value in vector
  • vector = set of values to pass through loop
  • curly braces ({/}) to define start/end

Loop Example

• Let’s check out an example loop to get more comfortable with these!

• Square each integer between 1 and 5

# Loop across vector of numbers
for(k in 1:5){
  
  # Square that number
  square_k <- k * k
  
  # Print that result
  print(square_k)
  
}

[1] 1
[1] 4
[1] 9
[1] 16
[1] 25

Loop Practice

• Make a vector of numbers between 5 and 25

• Write a for loop that:
  • 1. Takes the square root of each number (see ?sqrt)
  • 2. Prints that result (use print function)

Loop Practice (Answer)

• Demonstrating on only a few numbers

# Define loop
for(j in 5:8){
  
  # Take square root
  j_root <- sqrt(x = j)
  
  # Print that result
  print(x = j_root)
  
} # Close loop

[1] 2.236068
[1] 2.44949
[1] 2.645751
[1] 2.828427

More Loop Practice

• Write a second loop that:
  1. multiplies each number by 32
  2. Prints the result in the console

• Answer:

for(i in 5:25){
  i_mult <- i * 32
  print(i_mult)
}

Temperature Check

How are you Feeling?

Messaging with R

• By default, loops don’t put stuff in the Console
  • This can make figuring out what is going on difficult

• Fortunately, R contains functions for putting text into the Console
  • print is the one we’ll focus on!

• However, print only accepts one vector at a time

• paste makes multiple vectors into one vector

print + paste

• We can combine print and paste to assemble informative messages!

• For example:

# Use `paste` to combine multiple vectors
text <- paste("Hello", "my", "name", "is", "Nick")

# Then give that to `print` to return!
print(text)

[1] "Hello my name is Nick"

• This can be used in a loop to create messages that change with each iteration!

Message Example

• Let’s consider an example of a loop that uses print and paste to make messages

• For instance:

# Loop across vector of numbers
for(k in 1:5){
  
  # Square that number
  square_k <- k * k
  
  # Print a message about that result
  print(paste("The square of", k, "is", square_k))

}

[1] "The square of 1 is 1"
[1] "The square of 2 is 4"
[1] "The square of 3 is 9"
[1] "The square of 4 is 16"
[1] "The square of 5 is 25"

Write Messages

• We’ll modify your earlier practice loops to make informative messages!

• For the loop that takes the square root of the index:
  • Make the print step tell you the starting number and its square root

• Answer:

for(j in 5:25){
  j_root <- sqrt(j)
  print(paste("The square root of", j, "is", j_root))
}

More Message Practice

• For the loop that multiplies the index by 32:
  • Write a message that says each number and what it times 32 is equal to

• Answer:

for(i in 5:25){
  i_mult <- i * 32
  print(paste(i, "times 32 is equal to", i_mult))
}

Temperature Check

How are you Feeling?

Conditionals

• You can write code to do something only if some condition is met
  • Do this by using conditionals!

• Syntax is similar to logical statements for subsetting
  • Recall our earlier conversations about conditional operators:
  • ==, &, >, etc.

• Two main conditional functions: if & else

Conditional Example

# Do something if a condition is met
if(2 == 2){

  print("Math is mathing!")

  # If that condition is not met...
} else { 
  
  print("Math is--somehow--not mathing...")

}

[1] "Math is mathing!"

Conditional Syntax

• if wants:
  • Condition to check in parentheses (...)
  • What to do if the condition is met in curly braces {...}

• else wants:
  • Only curly braces {...} bits
  • Can only be used if if is used first

• The example in the previous slide is just for one if
  • Essentially ‘either this or that’

Multiple ifs

• You can add multiple conditions if desired!

# If the number is less than 0
if(x < 0){

  print("Number is negative")

# If it's more than 0
} else if(x > 0){

  print("Number is positive")

# Otherwise ...
} else { 

  print("Number is zero")

}

• See how the second condition uses else if(...){...}?

Conditionals in Loops

• You can use conditionals to make one loop handle multiple possibilities

• For example:

# Test vector
my_vec <- c(-10, -9, -8, 0, 4, 5, 6)

# Loop across it
for(value in my_vec){
  
  # Make negative numbers positive
  if(value < 0){ 
    print(value * -1)
    
    # Take the square root of positive numbers
  } else if(value > 0){
      print(sqrt(value)) }
  
} # Close loop curly brace

[1] 10
[1] 9
[1] 8
[1] 2
[1] 2.236068
[1] 2.44949

Loop Conditional Visual

Loops with Conditionals

• Write a loop that:
  • Uses g as the index
  • Uses a vector that has all the integers between 10 and 20
  • Prints each value in the vector

• Add two conditionals to that loop:
  1. If g is less than 14, print “Less than 14”
  2. Otherwise, print the value as-is

Temperature Check

How are you Feeling?

In-Class Free Work

• Draft 2 of Function Tutorials is due next week!
  • Presentations during Lecture #8
  • Requires ‘revision response’ where you discuss what changes you made due to feedback

• GitHub Presence is due next week
  • You have most of what you need for full points on that already

• Any questions about upcoming assignments / past topics?
  • I strongly recommend taking advantage of this time!

Upcoming Due Dates

Due before lecture

(By midnight)

• Homework #7
• Submit Draft 2 of Function Tutorials
  • Double check rubric to see that you’re not leaving any points on the table!
  • Remember to also submit the Revision Response

Due before lab

(By midnight)

• Homework #8
• Muddiest Point #8
• GitHub Presence