[FR] mdply equivalent

[baptiste]

It would be great if dplyr had a function similar to mapply, or better yet, mdply. It's just so useful, and clashes between dplyr and plyr mean that we can't use both.

Minimal example,

require(plyr)
p = expand.grid(mean=1:5, sd=seq(0, 1, length=10))
mdply(p, rnorm, n=5)

[hadley]

One approach:

p <- expand.grid(mean=1:5, sd=seq(0, 1, length=10))
p %>% do(data.frame(rnorm(.$mean, .$sd)))

Do you have more examples of using mdply() with data frames?

[baptiste]

I use this pattern all the time, specifically to avoid this manual mapping inside rnorm: mdply does this argument-matching for me, automatically.

Here's a tutorial example for an external tool called from the command line (with various non-vectorised parameters) https://github.com/baptiste/adda/wiki/wrapper_primer (last section)

I have lots of other examples in my wiki pages
https://github.com/baptiste/planar/wiki/bragg_stack
https://github.com/baptiste/planar/wiki/kretschmann_reflectivity (section 2)
https://github.com/baptiste/cda/wiki/dimer_linear
etc.

They're all pretty specific to light-scattering simulations, but the pattern is very general:

• define named grid of parameters
• mdply matches the names in the function call for each row, and (optionally) fills in additional parameters (...)
• the results are readily plotted with ggplot2, as already split by variables

[hadley]

This now feels more like a job for purrr.