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

问题

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.

回答1:

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))

回答2:

The third argument of apply needs to be a function. Also, you can count logical trues with sum.

apply(data, 1, function(x)sum(x > 30))

回答3:

We can also do with Reduce and + (assuming there are no NA elements)

 Reduce(`+`, lapply(as.data.frame(data), `>`, 30))

This should be efficient as we are not converting to a matrix.

来源：https://stackoverflow.com/questions/18862114/count-number-of-columns-by-a-condition-for-each-row

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