Interpretation of negative index when subsetting a data.frame

Question

I am very new to R and at times get stuck with the codes. I came across one of this code as below. What does -7 mean in the code below?

round(cor(longley[,-7]),3)

I understand: round for rounding, longley as data.frame, 3: digits for rounding, but not the -7.

Solution 1

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

Solution 2

The command longley[,-7] means: All columns from longley except the 7th. This is called negative indexing.

Have a look at ?Extract for further information.