Basic code execution

  • When we run a chunk of code it runs from top to bottom in order
  • It runs the first line, then the second line, and so on
  • This means that order is important
  • Write code to calculate a total number from density and area
density <- number / area
number <- c(9, 16, 3, 10)
area <- c(3, 5, 1.9, 2.7)
  • Returns an error because neither number or area exists yet

  • Within a line code executes everything before assignment the output to a variable
  • So it starts to the right of the assignment operator
  • Looks up the value for number
  • Can’t find the variable and so errors

  • Rearrange the code so that all variables are created before they are used
number <- c(9, 16, 3, 10)
area <- c(3, 5, 1.9, 2.7)
density <- number / area
  • This executes in the following sequence
  • The right hand side of the first line creates a vector
  • It is then stored or “assigned” to the variable number
  • The second line then runs creating a vector and assigning it to the variable area
  • The third line first looks up the variable number and replaces it with its value
  • It then looks up the the variable area and replaces it with its value
  • It then divides those two vectors and assigns the resulting vector to the variable density

Code executes inside to outside

  • Code that is enclosed by brackets or parentheses executes before the surrounding code
density[density > 3]
  • So this line is the same as
density_filter <- density > 3