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Week 2: Preclass homework for Monday

For the duration of the semester, you should complete these homework assignments before class. In class, we will generally build on the pre-class work or you will share your work with a small group.

1. Skill of the week: version control using Git.

Once you have received an email from me (sometime Friday), take control of your repo (see git02_your_repo.md). In git03_basics.md learn the basics of how Git works, how to make a repository, add files to it, track changes, and backtrack to a previous state.

2. Watch the video lecture on "Selection structures"

3. Programming problems on selection structures

This is a problem-set assignment. You should submit only one file of R code. The filename should be coding_assignment1.R (or .Rmd if you prefer) but you can add any prefixes to the filename that work with your file naming scheme. Push your file to your GitHub repository. To push, you first commit to your local repository, then do

git push

I suggest you follow the class coding style ebio5460_r_style_guide.md but if you have a strong style preference from long experience I won't mind.

You must not use any R packages for these problems!

Single selection structure
if ( condition ) {
    expression
}
Question 1

What goes wrong with the following code and why? (Provide your answer as a comment). Fix the code.

student_grade <- 75
if ( student_grade < 60 )
    print("Failed")
    print("You need to take the course again")
Question 2

Why doesn't the following code work as intended? (Provide your answer as a comment). Fix the code while retaining the single selection structures. You can find the data file in the data folder in the class-materials repository.

Hint: sketch the flowchart for two stacked if single selection structures.

grades <- read.csv("grades.csv")
class_grades <- grades$class1
if ( mean(class_grades) < 60 ) {
    adj <- 60 - mean(class_grades)         #calculate adjustment
    class_grades <- class_grades + adj     #adjust the original grades
    message <- "Mean for raw grades was a fail: grades adjusted"
}
if ( mean(class_grades) >= 60 ) {
    message <- "Average grade was a pass: no adjustment made"
}
print(message)
print(class_grades)    #Should now be adjusted if necessary
Double selection structure
if ( condition1 ) {
    expression1
} else {
    expression2
}
Question 3

Fix the code in (2) using a double selection structure. Which option works best, stacked single selection structures or a double selection structure, and why? (Provide your answer as a comment).

Question 4

Where should I put braces and indents? Modify the following code to produce the patterns shown in tables (a) and (b) depending on the input. You may not make any changes other than indentation or inserting braces. Your answer should be clear, readable code with good style.

pattern <- NULL
if ( y == 2 )
if ( x == 1 )
pattern <- c(pattern,"@")
else
pattern <- c(pattern,"#")
pattern <- c(pattern,"$")
pattern <- c(pattern,"&")
print(pattern,quote=FALSE)

You will also want to add assignment statements for y and x before this code so you can vary the input values of y and x, e.g.

y <- 3
x <- 1

Your answer will require separate code for each of table (a) and table (b).

Table (a) Case 1 Case 2 Case 3
With input: y = 3, x = 1 y = 2, x = 3 y = 2, x = 1
Pattern output: & # $ & @ $ &

A single algorithm should produce all of the patterns in table (a) by changing only the input values of y and x.

Table (b) Case 1 Case 2 Case 3
With input: y = 3, x = 1 y = 2, x = 3 y = 2, x = 1
Pattern output: # $ & & @ &

A single algorithm should produce all of the patterns in table (b) by changing only the input values of y and x.

Question 5

Write an algorithm (as R code) that determines whether a 5 digit integer reads the same forwards as backwards (i.e. a palindrome). Your algorithm should take the integer as input (no other inputs are allowed). The algorithm should print out "The number is a palindrome" or "The number is not a palindrome". Use only basic operators to do the necessary calculations (see ?Arithmetic and ?Syntax for a list of allowed operators). You may not use any R functions, except for print(). Resist the temptation to Google for an answer (there are many algorithms for palindromes). If you don't think about it, you won't get as much benefit, and you will end up with someone else's algorithm.

Hint: use the division (/) and modulus (%%) operators to separate the number into individual digits (and store them as objects) first. The modulus operator returns the remainder after dividing by a number. For example

6245 %% 1000
[1] 245

Alternatively, you might find the modulus division (%/%) operator to be useful.

6245 %/% 1000
[1] 6

The modulus operators are very useful in computer programming.