For this code challenge, you need to write a function that takes in an argument n and returns all the prime numbers that are less than or equal to n.

Rules:

To win



Hint coming at the 5 minute mark…















Hint

The modulo operator takes one number and reports its remainder when it is divided by another number. This can be computed in R using the %% operator. For example,

5 %% 3
## [1] 2



Solution below…















# a function to check whether a number is prime or not
is_prime <- function(x) {
  possibly_prime <- TRUE
  
  # make i go from 2 to x-1
  for (i in 2:(x-1)) {
    # check if x mod i == 0 
    # if this condition holds, then x is not prime
    if (x %% i == 0) {
      possibly_prime <- FALSE
    }
  }
  
  # handle x=2 separately
  if (x==2) possibly_prime <- TRUE
  
  # if we make it all the way to the end, 
  # and possibly_prime hasn't been set to FALSE,
  # then x is a prime number; otherwise, 
  # is not a prime number
  possibly_prime
}

# function to compute all prime numbers up to n
prime <- function(n) {
  # list of possible prime numbers
  candidates <- seq(2,n)
  
  # apply the is_prime function to all of them
  candidate_prime <- sapply(candidates, is_prime)
  
  # return whichever ones were prime
  candidates[candidate_prime]
}


# check that it works
prime(511)
##  [1]   2   3   5   7  11  13  17  19  23  29  31  37  41  43  47  53  59  61  67
## [20]  71  73  79  83  89  97 101 103 107 109 113 127 131 137 139 149 151 157 163
## [39] 167 173 179 181 191 193 197 199 211 223 227 229 233 239 241 251 257 263 269
## [58] 271 277 281 283 293 307 311 313 317 331 337 347 349 353 359 367 373 379 383
## [77] 389 397 401 409 419 421 431 433 439 443 449 457 461 463 467 479 487 491 499
## [96] 503 509
length(prime(511))
## [1] 97
prime(423)
##  [1]   2   3   5   7  11  13  17  19  23  29  31  37  41  43  47  53  59  61  67
## [20]  71  73  79  83  89  97 101 103 107 109 113 127 131 137 139 149 151 157 163
## [39] 167 173 179 181 191 193 197 199 211 223 227 229 233 239 241 251 257 263 269
## [58] 271 277 281 283 293 307 311 313 317 331 337 347 349 353 359 367 373 379 383
## [77] 389 397 401 409 419 421
length(prime(423))
## [1] 82
prime(761)
##   [1]   2   3   5   7  11  13  17  19  23  29  31  37  41  43  47  53  59  61
##  [19]  67  71  73  79  83  89  97 101 103 107 109 113 127 131 137 139 149 151
##  [37] 157 163 167 173 179 181 191 193 197 199 211 223 227 229 233 239 241 251
##  [55] 257 263 269 271 277 281 283 293 307 311 313 317 331 337 347 349 353 359
##  [73] 367 373 379 383 389 397 401 409 419 421 431 433 439 443 449 457 461 463
##  [91] 467 479 487 491 499 503 509 521 523 541 547 557 563 569 571 577 587 593
## [109] 599 601 607 613 617 619 631 641 643 647 653 659 661 673 677 683 691 701
## [127] 709 719 727 733 739 743 751 757 761
length(prime(761))
## [1] 135
prime(888)
##   [1]   2   3   5   7  11  13  17  19  23  29  31  37  41  43  47  53  59  61
##  [19]  67  71  73  79  83  89  97 101 103 107 109 113 127 131 137 139 149 151
##  [37] 157 163 167 173 179 181 191 193 197 199 211 223 227 229 233 239 241 251
##  [55] 257 263 269 271 277 281 283 293 307 311 313 317 331 337 347 349 353 359
##  [73] 367 373 379 383 389 397 401 409 419 421 431 433 439 443 449 457 461 463
##  [91] 467 479 487 491 499 503 509 521 523 541 547 557 563 569 571 577 587 593
## [109] 599 601 607 613 617 619 631 641 643 647 653 659 661 673 677 683 691 701
## [127] 709 719 727 733 739 743 751 757 761 769 773 787 797 809 811 821 823 827
## [145] 829 839 853 857 859 863 877 881 883 887
length(prime(888))
## [1] 154