R - Add a new column to a dataframe using matching values of another dataframe

Question

I am trying to fill in table1 with matching val2 values of table2

table1$New_val2 = table2[table2$pid==table1$pid,]$val2

But I get the warning

longer object length is not a multiple of shorter object length

which is fair enough because the table lengths are not the same.

Please kindly direct me on the correct way to do this.

Answer 1

You can just apply the following command:

merge(table1, table2[, c("pid", "col2")], by="pid")

Add in the all.x=TRUE argument to keep all of the pids in table1 that don't have matches in table2.

You were on the right track. Here's a way of using the match:

table1$val2 <- table2$val2[match(table1$pid, table2$pid)]

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