Calculate correlation with cor only for numerical columns

Question

I have a dataframe and would like to calculate the correlation (with Spearman, data is categorical and ranked) but only for a subset of columns. I tried with all, but R's cor() function only accepts numerical data (x must be numeric, says the error message), even if Spearman is used.

One brute approach is to delete the non-numerical columns from the dataframe. This is not as elegant, for speed I still don't want to calculate correlations between all columns.

I hope there is a way to simply say "calculate correlations for columns x, y, z". Column references could by number or by name. I suppose the flexible way to provide them would be through a vector.

Any suggestions are appreciated.