Extracting a row from a data frame in R

Question

Let's say we have a matrix something like this:

> A = matrix( 
+   c(2, 4, 3, 1, 5, 7), # the data elements 
+   nrow=2,              # number of rows 
+   ncol=3,              # number of columns 
+   byrow = TRUE)        # fill matrix by rows 

> A                      # print the matrix 
     [,1] [,2] [,3] 
[1,]    2    4    3 
[2,]    1    5    7

Now, I just used this small example, but imagine if the matrix was much bigger, like 200 rows and 5 columns etc. What I want to do, is to get the minimum value from column 3, and extract that row. In other words, find and get the row where is the 3rd attribute the lowest in the entire column of that data frame.

dataToReturn <- which(A== min(A[, 3])

but this doesn't work.

In case of multiple rows meeting the minimum condition, do you want to return only the first row that meets the minimum or all rows that have the minimum value in column 3? — talat
– talat, Commented Jun 1, 2014 at 14:48

David Arenburg · Accepted Answer · 2014-06-01 14:42:42Z

2

Another way is to use which.min

A[which.min(A[, 3]), ]
##[1] 2 4 3

answered Jun 1, 2014 at 14:42

David Arenburg

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3 Comments

talat Over a year ago

This is correct. But there is of course a difference if you have multiple rows with a minimum. In such a case, which.min will only return the first row, whereas in my answer, all rows meeting the criteria, would be returned. So it really depends on the intention.

Mefhisto1 Over a year ago

Yes, I need all rows that meet the criteria, then which.min is not a viable solution ?

talat Over a year ago

No, in that case, which.min is not the right solution for you. From the ?which.min help page: "Determines the location, i.e., index of the (first) minimum or maximum of a numeric (or logical) vector."

talat · Accepted Answer · 2014-06-01 15:08:17Z

1

You can do this with a simple subsetting via [] and min:

A[A[,3] == min(A[,3]),]
[1] 2 4 3

This reads: Return those row(s) of A where the value of column 3 equals the minimum of column 3 of A.

If you have a matrix like this:

A <- matrix(c(2,4,3,1,5,7,1,3,3), nrow=3, byrow = T)
> A
     [,1] [,2] [,3]
[1,]    2    4    3
[2,]    1    5    7
[3,]    1    3    3

> A[which.min(A[, 3]), ]      #returns only the first row with minimum condition
[1] 2 4 3

> A[A[,3] == min(A[,3]),]     #returns all rows with minimum condition
     [,1] [,2] [,3]
[1,]    2    4    3
[2,]    1    3    3

edited Jun 1, 2014 at 15:08

answered Jun 1, 2014 at 14:39

talat

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