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Writing a for-loop in R

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There may be no R topic that is more controversial than the humble for-loop. And, to top it off, good help is hard to find. I was astounded by the lack of useful posts when I googled “for loops in R” (the top return linked to a page that did not exist). In fact, even searching for help within R is not easy and not even that helpful when successful (?for won’t get you anywhere. ?'for' will get you the help page but it is by no means exhaustive.) So, at the request of Sam, a faithful reader of the Paleocave blog, I’m going to throw my hat into the ring and brace myself for the potential onslaught of internet troll wrath.

How to loop in R

Use the for loop if you want to do the same task a specific number of times.
It looks like this.

for (counter in vector) {commands}

I’m going to set up a loop to square every element of my dataset, foo, which contains the odd integers from 1 to 100 (keep in mind that vectorizing would be faster for my trivial example – see below).


foo = seq(1, 100, by=2)

foo.squared = NULL

for (i in 1:50 ) {
foo.squared[i] = foo[i]^2
}

If the creation of a new vector is the goal, first you have to set up a vector to store things in prior to running the loop. This is the foo.squared = NULL part. This was a hard lesson for me to learn. R doesn’t like being told to operate on a vector that doesn’t exist yet. So, we set up an empty vector to add stuff to later (note that this isn’t the most speed efficient way to do this, but it’s fairly fool-proof). Next, the real for-loop begins. This code says we’ll loop 50 times(1:50). The counter we set up is ‘i’ (but you can put whatever variable name you want there). For our new vector foo.squared, the ith element will equal the number of loops that we are on (for the first loop, i=1; second loop, i=2).
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