In-class Exercises Week 1

Set 1, Exercise 1

Write a set of conditional(s) that satisfies the following requirements:

• If x is greater than 3 and y is less than or equal to 3 then print “Hello world!”

• Otherwise if x is greater than 3 print “!dlrow olleH”

• If x is less than or equal to 3 then print “Something else …”

• Stop execution if x is odd and y is even and report an error, don’t print any of the text strings above.

Test out your code with

1. x=1 and y=2
2. x=1 and y=3
3. x=3 and y=3

Note that because of the stop command, the .Rmd file will not knit when errors occur unless you set the chunk option error=TRUE

# write your code for problem set 1 exercise 1 here
x <- 3
y <- 3

# first need to check if x is odd and y is even
if(x%%2==1 & y%%2==0){
  stop("Error error") # x is odd and y is even so report error and terminate
}else{ # if "x is odd and y is even" is false
  if(x > 3 & y<= 3){
  "Hello world!"
} else if(x > 3){
  "!dlrow olleH"
} else{
  "Something else..."
}}

## [1] "Something else..."

Set 1, Exercise 2

Write a set of conditional(s) in R to solve equations of the form \[a x^2 + b x + c=0,\] which accounts for possible errors that might occurr in the calculation (you may use if, else if, else, stop, stopifnot)

Test out your code with

1. a=1, b=0 and c=1
2. a=1, b=0 and c=-1
3. a=9, b=4 and c=1
4. a=0, b=3 and c=3
5. a=1, b=-4 and c=4

Again, because of the stop command the .Rmd file will not knit when errors occur unless you set the chunk option error=TRUE

# write your code for problem set 1 exercise 2 here
a <- 1
b <- -4
c <- 4

root1 <- NULL
root2 <- NULL

testfunc <-(b^2-4*a*c)
if(testfunc < 0){
  stop("Can't get x with the quadratic formula")
}else{
  if(a == 0){
    stop("a is zero, so the formula is linear")
  }
  root1 <- (-b+sqrt(testfunc))/(2*a)
  root2 <- (-b-sqrt(testfunc))/(2*a)
  if(root1 != root2){
    result <- c(root1, root2)
    print(result)
  }else{ #root1 == root2
    print(root1)
  }
}

## [1] 2

Set 2

Below is a vector containing all prime numbers between 2 and 100:

primes = c( 2,  3,  5,  7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 
      43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97)

If you were given the vector

x = c(3,4,12,19,23,51,61,63,78),

write the R code necessary to print only the values of x that are not prime (without using subsetting or the %in% operator).

Your code should use nested loops to iterate through the vector of primes and x.

# write your code for problem set 2 here
# write your code for problem set 2 here
x = c(3,4,12,19,23,51,61,63,78)

notprimes = c()

for(a in x){
  found = FALSE #initialize
  for(b in primes){
    if(a == b)
      found = TRUE
  }
  if(found == FALSE) # a is not prime
    notprimes <- c(notprimes, a)
}

notprimes

## [1]  4 12 51 63 78