How to calculate sum of values in each column based on row names in R?

Question

I can do this in python but in R I cannot find how to do conditions.

I have a matrix with colnames(mat), rownames(mat). For each column, I need to calculate sum of values if a row begins from a certain pattern. Let's say I need to sum up only the values where the row name starts from 'A'.

I tried this:

for(i in  colnames(mat)) {
  sum_A=0
  for (j in rownames(mat)) {
    sum_A<-sum(mat[ j == 'A^', i])
  }
}

A

It gives me this output:

[1] 0

I guess you need grepl. Please show a small reproducible example and expected ouptut. colSums(mat[starts_with(row.names(mat), "A"),]) — akrun
– akrun, Commented Jul 9, 2020 at 22:28

akrun · Accepted Answer · 2020-07-09 23:27:26Z

1

We can use colSums with startsWith

colSums(mat[startsWith(row.names(mat), "A"),])

answered Jul 9, 2020 at 23:27

akrun

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