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Retrieving column headers

I have the following data (data) frame from which I would like to extract and print the column headings, where there is always a non-zero element:

  M1 M2 M3 M4
2  0  1  0 1
5  1 -1  0 0
7  0  1  1 0

Required Conclusion:

2: M2 M4
5: M1 M2
7: M2 M3

So far, my R code is not working:

colnames(data)[which(data[2] !=0),]

Help will be appreciated. Thanks

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3 answers

Take a more general case of irregular results:

dat <- structure(list(M1 = c(0L, 1L, 0L), M2 = c(1L, -1L, 1L), M3 = -1:1, 
M4 = c(1L, 0L, 0L)), .Names = c("M1", "M2", "M3", "M4"), 
  class = "data.frame", row.names = c("2", "5", "7"))

> dat
  M1 M2 M3 M4
2  0  1 -1  1
5  1 -1  0  0
7  0  1  1  0

An internal approximating "loop" builds a logical set of vectors. Since R is column oriented, the second level of processing is performed on the columns. The external “loop” applied extracts the corresponding elements from the code names:

 apply( apply(dat,1, as.logical) , 2, function(ll) colnames(dat)[ll] ) 
$`2`
[1] "M2" "M3" "M4"

$`5`
[1] "M1" "M2"

$`7`
[1] "M2" "M3"

You could also extract the version of the array indicator from which (), and then process the results:

 > which(dat != 0, arr.ind=TRUE)
  row col
5   2   1
2   1   2
5   2   2
7   3   2
2   1   3
7   3   3
2   1   4
+4

:

Map(`[`, list(names(df)), split(col(df)[df != 0],
                                row(df)[df != 0]))

# [[1]]
# [1] "M2" "M4"
# 
# [[2]]
# [1] "M1" "M2"
# 
# [[3]]
# [1] "M2" "M3"

( setNames(..., rownames(df)), , df.)

+5

I add 1 to the first line to handle the irregular case

mm <- read.table(text='M1 M2 M3 M4
2  0  1  1 1
5  1 -1  0 0
7  0  1  1 0',header=TRUE)


res <- apply(mm != 0,1,function(x)colnames(mm)[x])

$`2`
[1] "M2" "M3" "M4"

$`5`
[1] "M1" "M2"

$`7`
[1] "M2" "M3"

EDIT to be consistent and return a list:

res <- apply(mm != 0,1,function(x)colnames(mm)[x])     
if (!is.null(dim(res)) split(res,seq_along(res[,1]))
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