For loops

For Loops

Loops are a fundamental structure for repetition in programming. for loops perform the same action for each item in a list of things. The basic syntax is:

for (item in list_of_items) {
  do_something(item)
}

We can create a vector on the fly to loop a particular number of times:

for (i in 1:5){
  print(i)
}

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

Or use a pre-existing vector or list.

volumes = c(1.6, 3, 8)
for (volume in volumes){
  mass <- 2.65 * volume ^ 0.9
  print(mass)
}

[1] 4.045329
[1] 7.12287
[1] 17.21975

We also might want to loop over indices so we can access multiple vectors.

as <- c(2.65, 1.28, 3.29)
bs <- c(0.9, 1.1, 1.2)
volumes = c(1.6, 3, 8)
masses <- vector(mode="numeric", length=length(volumes))
for (i in 1:length(volumes)){
   mass <- as[i] * volumes[i] ^ bs[i]
   masses[i] <- mass
}
masses

[1]  4.045329  4.285913 39.893660

We can use functions inside loops. For example, let’s take a function that returns an estimated mass if the volume > 5 and NA if it’s not.

est_mass <- function(volume, a, b){
  if (volume > 5) {
    mass <- a * volume ^ b
  } else {
    mass <- NA
  }
  return(mass)
}

We can then call the function to populate a vector item by item.

masses <- vector(mode="numeric", length=length(volumes))
for (i in 1:length(volumes)){
   mass <- est_mass(volumes[i], as[i], bs[i])
   masses[i] <- mass
}
masses

[1]       NA       NA 39.89366

To note, this is the for loop equivalent of an mapply statement.

masses_apply <- mapply(est_mass, volumes, as, bs)
masses_apply

[1]       NA       NA 39.89366

In R we often want to use `apply` statements as opposed to explicitly writing loops.

The materials in this lesson have been adapted from work created by the (HBC)](http://bioinformatics.sph.harvard.edu/) and Data Carpentry (http://datacarpentry.org/), as well as materials created by Laurent Gatto, Charlotte Soneson, Jenny Drnevich, Robert Castelo, and Kevin Rue-Albert. These are open access materials distributed under the terms of the Creative Commons Attribution license (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

For Loops

In R we often want to use apply statements as opposed to explicitly writing loops.

In R we often want to use `apply` statements as opposed to explicitly writing loops.