Code Execution

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

total_number <- density * area
density <- c(2.8, 3.2, 1.5, 3.8)
area <- c(3, 5, 1.9, 2.7)

• Returns an error because neither density or area exists yet
• Code executes starting on the right side
• Looks up the value for density

• Can’t find the variable and so errors

• Rearrange the code so that all variables are created before they are used

density <- c(2.8, 3.2, 1.5, 3.8)
area <- c(3, 5, 1.9, 2.7)
total_number <- density * 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 density
• The second line then runs creating a vector and assigning it to the variable area
• The third line first looks up the variable density and replaces it with its value
• It then looks up the the variable area and replaces it with its value
• It then multiplies those two vectors and assigns them to the variable total_number