R_Quotation.md

R Quotation Methods

Jesse Cambon 24 November, 2019

Demonstrate the use of the quo_name() and enquo() functions to pass variable names to functions and utilize both the variables and the variable names

library(tidyverse)
library(knitr)

# Create a heatmap with two axis variables
# of the mean of a given metric variable
car_heatmap <- function(data,axis1,axis2,metric) {
  # handle quotations
  
  # The dataset can be passed easily as a function argument
  # but we need to use enquo() and the !! operator (below) 
  # for variable names
  # If you were doing this outside of function you would use quo() instead of enquo()
  axis1 <- enquo(axis1)
  axis2 <- enquo(axis2)
  metric <- enquo(metric)
  
  
  print(quo_name(metric))

  cars_agg <- data %>%
    group_by(!!axis1,!!axis2) %>%
    summarize(fill_metric = mean(!!metric),n=n())
  
   ggplot(cars_agg, aes(factor(!!axis1), factor(!!axis2))) + 
     geom_tile(aes(fill = fill_metric), colour = "grey") + 
     scale_fill_gradient(low = "white",high = "steelblue") +
     geom_text(aes(label=round(fill_metric,1))) +
     theme_minimal() +
     theme(panel.grid = element_blank()) +
     scale_x_discrete(expand=c(0,0,0,0)) +
     # quo_name() access the character name of a variable
     guides(fill=guide_legend(title=str_c('mean ',quo_name(metric)))) +
     xlab(quo_name(axis1)) + ylab(quo_name(axis2))
}

car_heatmap(mtcars,cyl,gear,mpg)

## [1] "mpg"

car_heatmap(mtcars,cyl,gear,hp)

## [1] "hp"

car_heatmap(mtcars,gear,carb,mpg)

## [1] "mpg"

To quote a character list of variables, use rlang:syms() and the !!! operator

# Find frequency counts for all variables in var list

hp_calc <- function(data,variables) {
  variables <- rlang::syms(variables)
  return(data %>% group_by(!!!variables) %>%
           summarize(n=n(),
                     mean_hp=mean(hp),
                     min_hp=min(hp),
                     max_hp=max(hp))
         )
}
 
gear_hp <- hp_calc(mtcars,c('gear')) 
vs_am_hp <- hp_calc(mtcars,c('vs','am')) 

kable(gear_hp)

gear	n	mean_hp	min_hp	max_hp
3	15	176.1333	97	245
4	12	89.5000	52	123
5	5	195.6000	91	335

kable(vs_am_hp)

vs	am	n	mean_hp	min_hp	max_hp
0	0	12	194.16667	150	245
0	1	6	180.83333	91	335
1	0	7	102.14286	62	123
1	1	7	80.57143	52	113