Count number of rows matching a criteria

by

mydata$sCode == "CA" will return a boolean array, with a TRUE value everywhere that the condition is met. To illustrate:

> mydata = data.frame(sCode = c("CA", "CA", "AC"))
> mydata$sCode == "CA"
[1]  TRUE  TRUE FALSE

There are a couple of ways to deal with this:

  1. sum(mydata$sCode == "CA"), as suggested in the comments; because
    TRUE is interpreted as 1 and FALSE as 0, this should return the
    numer of TRUE values in your vector.

  2. length(which(mydata$sCode == "CA")); the which() function
    returns a vector of the indices where the condition is met, the
    length of which is the count of "CA".

Edit to expand upon what’s happening in #2:

> which(mydata$sCode == "CA")
[1] 1 2

which() returns a vector identify each column where the condition is met (in this case, columns 1 and 2 of the dataframe). The length() of this vector is the number of occurences.

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