creating a vector of values of elements rather than of 1 element in R

Question

I have a data frame of the following:

   > head(casted)
    ID nobs  sulfate   nitrate
  1  1  117 3.880701 0.5481368
  2  2 1041 4.460811 0.9474492
  3  3  243 4.327613 0.6585144
  4  4  474 4.214956 0.8701622
  5  5  402 4.210072 1.0939005
  6  6  228 4.102132 0.5206404

I would like to add a column "cor" with that uses the cor() on the sulfate and nitrate column per ID, but when I used the following code, I just get 1 value populating the entire column:

  casted$cor <- cor(casted$sulfate, casted$nitrate)
  > head(casted)
  ID nobs  sulfate   nitrate        cor
1  1  117 3.880701 0.5481368 0.00940941
2  2 1041 4.460811 0.9474492 0.00940941
3  3  243 4.327613 0.6585144 0.00940941
4  4  474 4.214956 0.8701622 0.00940941
5  5  402 4.210072 1.0939005 0.00940941
6  6  228 4.102132 0.5206404 0.00940941

I know I'm doing something wrong, but I can still can't figure it out.

Thanks! Meera

Answer 1

first you should know what a correlation is. Correlation is a statistical method to find the relationship between two samples. So to calculate the correlation, you need to have two series instead of two numbers. For example, we cannot tell the correlation between 1 and 2 cause we don't have enough information here, or we can say we cannot create a covariance matrix based on two numbers. What you did is right, the cor column is the correlation between sulfate column and nitrate column. The calculation uses every number in two columns, so it will give only one result.