Loops

• For
• Next
• While
• Nested loops

Loops are useful for sequential, repetitive code. Loops are also useful when calling other datasets when performing functions. They are also are easier to troubleshoot and build up (e.g., can modify the 'counter').

For

Let's say you want to repeat some sequence, such as:

print(paste("I have lived in DC for", 2, "years"))
print(paste("I have lived in DC for", 3, "years"))
print(paste("I have lived in DC for", 4, "years"))

We can streamline this code – and modify it for later use – using a "for loop" (for()).

# Syntax: 
for(val in sequence){
  statement
}

for(time in 2:4){
  print(paste("I have lived in DC for", time, "years"))
}

To modify for later use:

x <- c(2:4) # x is the 'counter' - try different values for x
for(i in 1:length(x)){ 
  print(paste("I have lived in DC for", x[i], "years")) 
}

or

x <- c(2:4) # try different values for x
for(time in x){
  print(paste("I have lived in DC for", time, "years")) 
}

Exercise

# create a dataframe with species and occurrences
species <- letters[1:4]
local.1 <- rep(1, 4)
local.2 <- c(0, 0, 1, 0)
local.3 <- c(0, 0, 1, 1)
local.4 <- c(0, 1, 1, 0)
occ <- data.frame(local.1, local.2, local.3, local.4)
rownames(occ) <- species

# 1. How would you get the sum of the first row of occurrences? 
# 2. How would you make this into a loop for all rows?

Next

next() is useful if you want to omit certain data, like NAs or rainy days, to inspect/visualize data, subset data, or control flow.

# e.g., you only want results for species with more than one observation
for (i in 1:length(species)) {
  if (sum(occ[i,]) == 1){
    next  #ends reading the loop & goes to the next [i] in the sequence
  }
  print(sum(occ[i,]))
}

While

Similarly, while() is useful for evaluating specific parts of your data. As long as the condition is fulfilled, the loop continues. ALWAYS WRITE A WAY TO END THE WHILE LOOP. OR ELSE. IT. GOES. ON. FOREVER.

i = 2 # initial value
while(i < 1000) {
  print(i)
  i = i^2 #squares i each iteration
}

Exercise

Turn to a neighbor and discuss: how would you create a while loop that goes on forever? Make a while loop that repeats "hippo" indefinitely. How do you escape out of it?

Nested loops

You can nest loops inside of loops. This may be useful if you wanted to, say, cell by cell. One counter keeps track of what row you're on (j) and one counter keeps track of what column you're on (i).

# call a cell from your dataframe, e.g., occ[j, i]
occ[1,2]
0
 
#use a loop to get each cell:
for(i in 1:length(occ)){ # for each column...
  for(j in 1:length(species)){ # for each row...
    print(occ[j, i])
  }
}