Control Structures

• Allow you to encode decisions about running code:

  • run code if a certain condition is met: if, if-else
  • run code multiple times: loops. for(), while(), repeat {}. Use break, next to stop or skip.

if this condition is true, then carry out this task

Use:

if(cond) expr

• cond: a logical vector of length one.

• expr: code to be evaluated if cond == TRUE.

Example:

x <- 1
if(x < 10){print('x is small')}

[1] "x is small"

if this condition is true, carry out task A, otherwise carry out task B

Use:

    if (cond) { expr } else { expr }

Example:

x <- 25

if(x < 10){
  print('x is small')
} else {
    print('x is big')
  } 

[1] "x is big"

Loops allow you to run chunks of code multiple times.

The loop will run until a certain condition has been reached.

Use for(), while(), repeat() to construct loops.

next and break are used to modify the loop.

The for loop progresses along a vector (seq), each element in turn (var).

Use:

for(var in seq) { expr }

Example:

    x <- c(1,2,3,4,5)

    for(i in 1:5) {
    print(x[i])
    }

[1] 1
[1] 2
[1] 3
[1] 4
[1] 5

The while loop runs while a condition is met.

Use:

while(cond) { expr }

Example:

    x <- 1

    while(x < 4) {
      x <- x + 1
      print(x)
    }

[1] 2
[1] 3
[1] 4

The repeat loop runs over a piece of code infinitely.

We need to use the break command within it to stop the loop.

Use:

repeat { expr }

Example:

    x <- 1

    repeat {
      print(x)
      x <- x + 1
      if(x > 5) break
    }

[1] 1
[1] 2
[1] 3
[1] 4
[1] 5

The control next jumps to the next cycle in the loop, without completing the current one.

Example

    x <- 1:4
    for (i in x) {
    if (i == 2) {
    next
    }
    print(i)
    }

[1] 1
[1] 3
[1] 4

• Allow you to encode decisions about running code:

  • run code if a certain condition is met: if, if-else
  • run code multiple times: loops. for(), while(), repeat {}. Use break, next to stop or skip.

• SWIRL lesson covers for loops