How to get row index number in R?

Question

Suppose I have a list or data frame in R, and I would like to get the row index, how do I do that? That is, I would like to know how many rows a certain matrix consists of.

Answer 1

Perhaps this complementary example of "match" would be helpful.

Having two datasets:

first_dataset <- data.frame(name = c("John", "Luke", "Simon", "Gregory", "Mary"),
                            role = c("Audit", "HR", "Accountant", "Mechanic", "Engineer"))

second_dataset <- data.frame(name = c("Mary", "Gregory", "Luke", "Simon"))

If the name column contains only unique across collection values (across whole collection) then you can access row in other dataset by value of index returned by match

name_mapping <- match(second_dataset$name, first_dataset$name)

match returns proper row indexes of names in first_dataset from given names from second: 5 4 2 1 example here - accesing roles from first dataset by row index (by given name value)

for(i in 1:length(name_mapping)) {
    role <- as.character(first_dataset$role[name_mapping[i]])   
    second_dataset$role[i] = role
}

===

second dataset with new column:
     name       role
1    Mary   Engineer
2 Gregory   Mechanic
3    Luke Supervisor
4   Simon Accountant

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