In R, What Does a Negative Index Do

In R, what does a negative index do?

This is covered in section 2.7 of the manual: http://cran.r-project.org/doc/manuals/R-intro.html#Index-vectors

It is a negative index into the cnt2 object specifying all rows and all columns except the first column.

What does negative index mean in a data frame?

Minus means remove the element with that index, just like "no minus" extract that element with the index

R: Why doesn't the negative index work to create the complement set?

You can just shuffle the indices (sampling without replacement) and then get the first few for testing and the others for training.

indices <- sample(seq(20))
test <- indices[1:10]
train <- indices[11:20]

train
#>  [1] 10  8 12  1  7 20 13 18  4 11
test
#>  [1] 19  3 15  2  6  9 16 14 17  5

^{Created on 2021-09-09 by the reprex package (v2.0.0)}

Interpretation of negative index when subsetting a data.frame

In the context [, -7] it means drop the 7th column from the data frame longley (or take all columns but the 7th from longley).

This is R 101 and you'd do well to read some introductory material. For example, this is covered very early on in the An Introduction to R manual that comes with R or is accessible from the R website. Or you could read ?Extract.

Here is an example

> head(longley)
     GNP.deflator     GNP Unemployed Armed.Forces Population Year Employed
1947         83.0 234.289      235.6        159.0    107.608 1947   60.323
1948         88.5 259.426      232.5        145.6    108.632 1948   61.122
1949         88.2 258.054      368.2        161.6    109.773 1949   60.171
1950         89.5 284.599      335.1        165.0    110.929 1950   61.187
1951         96.2 328.975      209.9        309.9    112.075 1951   63.221
1952         98.1 346.999      193.2        359.4    113.270 1952   63.639
> names(longley)
[1] "GNP.deflator" "GNP"          "Unemployed"   "Armed.Forces" "Population"  
[6] "Year"         "Employed"    
> names(longley)[7]
[1] "Employed"
> head(longley[, -7])
     GNP.deflator     GNP Unemployed Armed.Forces Population Year
1947         83.0 234.289      235.6        159.0    107.608 1947
1948         88.5 259.426      232.5        145.6    108.632 1948
1949         88.2 258.054      368.2        161.6    109.773 1949
1950         89.5 284.599      335.1        165.0    110.929 1950
1951         96.2 328.975      209.9        309.9    112.075 1951
1952         98.1 346.999      193.2        359.4    113.270 1952

Select one data point per row using indexing vector with negative values

Maybe this is another naive approach. We loop over every row in the matrix and remove index specified in b.

t(sapply(seq_len(nrow(a)), function(x) a[x, -b[x]]))

#     [,1] [,2]
#[1,]    4    7
#[2,]    2    5
#[3,]    6    9

Or using mapply with split

t(mapply(`[`, split(a, seq_len(nrow(a))), -b))

Is for loop indexing possible with negative values

R in principle is designed to avoid loops where ever possible. In addition to that there are some syntax errors in your code...

If you want to loop along a vector you need to use i in seq_along(x) not i in x. A solution to your problem would be indexing the vector directly

    for (i in seq_along(x)){ 
      nums[i] <- sum(x < x[i])
    }

Edit: The reason you only looped over the positive values is that i in x starts indexing by -10 wich works for comparing x against i but not to asignment your start asigning at nums[-10] which is not really suitable...

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