Search code examples
rcountcol

Count number of columns by a condition (>) for each row

I am trying to work out for each row of a matrix how many columns have values greater than a specified value. I am sorry that I am asking this simple question but I wasn't able to figure it out.

I have extracted maximum temperature values from a raster stack, of multiple years of rasters, for some spatial points I am interested in. The data looks similar to:

data <- cbind('1990' = c(25, 22, 35, 42, 44), '1991' = c(23, 28, 33, 40, 45), '1992' = c(20, 20, 30, 41, 43))

    1990   1991   1992
1     25     23     20
2     22     28     20
3     35     33     30
4     42     40     41
5     44     45     43

I want to end up with the number of years that the temperature was above 30 for each location, eg.:

    yr.above   
1          0
2          0
3          2
4          3
5          3

I have tried a few things but they didn't work and were pretty illogical (e.g. trying length(data[1:length(data), which(blah blah doesn't make sense)), or apply(data, 1, length(data) > 30), I know these don't make sense but I am a bit stuck.

Solution

  • This will give you the vector you are looking for:

    rowSums(data > 30)

    It will work whether data is a matrix or a data.frame. Also, it uses vectorized functions, hence is a preferred approach over using apply which is little more than a (slow) for loop.

    If data is a data.frame, you can add the result as a column by doing:

    data$yr.above <- rowSums(data > 30)

    or if data is a matrix:

    data <- cbind(data, yr.above = rowSums(data > 30))

    You can also create a whole new data.frame:

    data.frame(yr.above = rowSums(data > 30))

    or a whole new matrix:

    cbind(yr.above = rowSums(data > 30))