correlation between columns by group

Question

How do I calculate correlations between one column and all other columns in a data frame in R without using column names? I tried to use ddply and it works if I use just two col

Answer 1

As of

packageVersion("dplyr")
[1] ‘1.0.2’

The result of the code suggested in one of the answers returns a tibble

iris %>%
     group_by(Species) %>%
     do(cormat = cor(select(., -matches("Species"))))
# A tibble: 3 x 2
# Rowwise: 
  Species    cormat           
                   
1 setosa     
2 versicolor 
3 virginica  

To get the data into a rectangular shape, you can

iris_cor <- iris %>%
     group_by(Species) %>%
     do(cormat = cor(select(., -matches("Species")))) %>%
     pull(cormat) %>% melt

You will have the levels of Species codified on L1 variable.

           Var1         Var2     value L1
1  Sepal.Length Sepal.Length 1.0000000  1
2   Sepal.Width Sepal.Length 0.7425467  1
3  Petal.Length Sepal.Length 0.2671758  1
4   Petal.Width Sepal.Length 0.2780984  1
...

I am sure there's a cleaner way of doing this with unnest() and its friends, but couldn't figure out yet. Hoping this gets noticed and posts a better solution