CODEWITHSUNDEEP
r
antimuon
antimuon

Reputation: 262

R - Getting incorrect results for matrix mulitplication on a column from a list

I am processing list data data that I am doing matrix calculations on and I am getting some strange behavior.

When I pull the data from the list and apply the tcrossprod function I get different answers than I would expect. Below is a simplified example of the issue.

My data is of this form...

> test_list <- list(data.frame("col1" = c('a','b'), "col2" = c(229187.82,135323.01)))

> test_list
[[1]]
  col1   col2
1    a 229188
2    b 135323

Applying tcrossprod gives an incorrect result on the data...

> tcrossprod(test_list[[1]]$col2)
            [,1]        [,2]
[1,] 52527056836 31014385658
[2,] 31014385658 18312317035

This is the correct answer...

> vec <- c(229188,135323)

> vec
[1] 229188 135323

> tcrossprod(vec)
            [,1]        [,2]
[1,] 52527139344 31014407724
[2,] 31014407724 18312314329

And here is the difference between the two calculations...

> tcrossprod(vec) - tcrossprod(test_list[[1]]$col2)
        [,1]     [,2]
[1,] 82507.6 22066.26
[2,] 22066.3 -2706.46

I can't find a reason why there is a discrepancy between the two calculations...

> typeof(test_list[[1]]$col2)
[1] "double"
> typeof(vec)
[1] "double"

> class(test_list[[1]]$col2)
[1] "numeric"
> class(vec)
[1] "numeric"

Is there a way to fix this issue so the numbers calculated are correct?

Upvotes: 0

Views: 38

Answers (1)

tlysoo
tlysoo

Reputation: 171

It is because the real value in your dataframe is c(229187.82,135323.01). Then when you display they round it to: c(229188,135323)

In your example you then use the rounded numbers which are not the right ones.

> test_list <- list(data.frame("col1" = c('a','b'), "col2" = c(229187.82,135323.01)))
> a<-test_list[[1]]$col2
> vec1 <- matrix(c(229187.82,135323.01),nrow=2)
> tcrossprod(vec1)
            [,1]        [,2]
[1,] 52527056836 31014385658
[2,] 31014385658 18312317035
> vec2=matrix(a,nrow=2)
> tcrossprod(vec2)
            [,1]        [,2]
[1,] 52527056836 31014385658
[2,] 31014385658 18312317035

Upvotes: 2

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