In FemkeThon/Sandboxy: Finding Primes

Use

primes_in(x, y) is used to find the prime numbers in a range of values.

x is the first value you want to determine of if it is a prime.

y is the last value you want to determine of if it is a prime.

All the primes between these two values are given.

The function:

```r primes_in <- function(x, y){ # Checking wheter the input is all right. if ((round(x) != x )|( round(y) != y)){ stop("This doesn't work. Numbers should be integers.") } if (y < x){ stop("Error, the first number should be lower than the second.") } if ((x < 0)|(y < 0)){ stop("Error, only positive numbers can be primes!") }

# The function itself.
# First we establish what the sequence is.
sequence <- c(x:y)
# We make an empty list to put the primes in. These are used to evaluate what other numbers are primes as well.
primes_lower_than_Y <- c()
# We don't start with 1 because everything can be divided by it.
for (n in c(2:y)){
  # Counter to count the number of numbers a number can be divided by.
  counter <- 0
  # So here, if a number can be divided by a number in primes_lower_than_Y, it isn't put in primes_lower_than_y, if it can't, it is.
  for (p in primes_lower_than_Y) {
    if ((n %% p) == 0) {
      counter <- (counter + 1)
    }
  }
  # Counter == 0 if the number can't be divided by anything.
  if (counter == 0) {
    primes_lower_than_Y <-append(primes_lower_than_Y, n)
  }
}
# Here we make sure only those numbers found in the range we put in are shown.
primes_in_range_XY <- primes_lower_than_Y[primes_lower_than_Y %in% sequence]
# Just to make sure that, in some cases, 1 isn't shown as though it were a prime.
if (y < 2){
  primes_in_range_XY <- c()
}
return(primes_in_range_XY)

}

```

Examples

Some examples of how the code works.

```r primes_in(0, 1)

primes_in(1, 7)

primes_in(40, 120)

# Wrong input (negative values, non-integers, a y value higher than the x value) Will give error messages.

```

FemkeThon/Sandboxy documentation built on May 6, 2019, 4:36 p.m.