Programming with data
Writing functions
Ben Baumer
SDS 192
Oct 19, 2020
(http://beanumber.github.io/sds192/lectures/mdsr_scale_01-functions.html)
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Functions
Do you ever find yourself with .Rmd files that look like this?
my_df1 %>% ... # Do some stuff to my_df1 ...my_df2 %>% ... # Do the same stuff to my_df2 ...my_df3 %>% ... # And again to my_df3 ...2 / 11
For example
What if I want to draw the same kind of plot several times?
my_df1 %>% ggplot(aes(x = var1, y = var2, color = var3)) + geom_point() + geom_line()my_df2 %>% ggplot(aes(x = varA, y = varB, color = varC)) + geom_point() + geom_line()my_df3 %>% ggplot(aes(x = var1A, y = var2B, color = var3C)) + geom_point() + geom_line()3 / 11
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User-defined functions
name_of_function <- function(data, var = "value") { ... ... <valid R code> ... ... return(x)}inputs:
- the arguments:
data,var datais requiredvaris optional -- has a default value of"value"
- the arguments:
output:
- the return value
- by default output of last line in function
- here, explicitly the object
x
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Scoping: global variables OK
my_cars <- function(mod, n = 6) { mpg %>% filter(model == mod) %>% head(n)}my_cars("civic")## # A tibble: 6 x 11## manufacturer model displ year cyl trans drv cty hwy fl class ## <chr> <chr> <dbl> <int> <int> <chr> <chr> <int> <int> <chr> <chr> ## 1 honda civic 1.6 1999 4 manual(… f 28 33 r subcomp…## 2 honda civic 1.6 1999 4 auto(l4) f 24 32 r subcomp…## 3 honda civic 1.6 1999 4 manual(… f 25 32 r subcomp…## 4 honda civic 1.6 1999 4 manual(… f 23 29 p subcomp…## 5 honda civic 1.6 1999 4 auto(l4) f 24 32 r subcomp…## 6 honda civic 1.8 2008 4 manual(… f 26 34 r subcomp…6 / 11
Default values
my_cars <- function(mod = "civic", n = 6) { mpg %>% filter(model == mod) %>% head(n)}my_cars()## # A tibble: 6 x 11## manufacturer model displ year cyl trans drv cty hwy fl class ## <chr> <chr> <dbl> <int> <int> <chr> <chr> <int> <int> <chr> <chr> ## 1 honda civic 1.6 1999 4 manual(… f 28 33 r subcomp…## 2 honda civic 1.6 1999 4 auto(l4) f 24 32 r subcomp…## 3 honda civic 1.6 1999 4 manual(… f 25 32 r subcomp…## 4 honda civic 1.6 1999 4 manual(… f 23 29 p subcomp…## 5 honda civic 1.6 1999 4 auto(l4) f 24 32 r subcomp…## 6 honda civic 1.8 2008 4 manual(… f 26 34 r subcomp…7 / 11
Default values overridden
my_cars("jetta", n = 3)## # A tibble: 3 x 11## manufacturer model displ year cyl trans drv cty hwy fl class ## <chr> <chr> <dbl> <int> <int> <chr> <chr> <int> <int> <chr> <chr> ## 1 volkswagen jetta 1.9 1999 4 manual(m5) f 33 44 d compa…## 2 volkswagen jetta 2 1999 4 manual(m5) f 21 29 r compa…## 3 volkswagen jetta 2 1999 4 auto(l4) f 19 26 r compa…8 / 11
Naming arguments optional
my_cars("camry", 2)## # A tibble: 2 x 11## manufacturer model displ year cyl trans drv cty hwy fl class ## <chr> <chr> <dbl> <int> <int> <chr> <chr> <int> <int> <chr> <chr> ## 1 toyota camry 2.2 1999 4 manual(m5) f 21 29 r midsi…## 2 toyota camry 2.2 1999 4 auto(l4) f 21 27 r midsi…my_cars(mod = "corolla", 2)## # A tibble: 2 x 11## manufacturer model displ year cyl trans drv cty hwy fl class ## <chr> <chr> <dbl> <int> <int> <chr> <chr> <int> <int> <chr> <chr> ## 1 toyota corolla 1.8 1999 4 auto(l3) f 24 30 r compa…## 2 toyota corolla 1.8 1999 4 auto(l4) f 24 33 r compa…9 / 11
Order of arguments matter if unnamed
my_cars(2, "corolla")## # A tibble: 0 x 11## # … with 11 variables: manufacturer <chr>, model <chr>, displ <dbl>,## # year <int>, cyl <int>, trans <chr>, drv <chr>, cty <int>, hwy <int>,## # fl <chr>, class <chr>my_cars(2, mod = "corolla")## # A tibble: 2 x 11## manufacturer model displ year cyl trans drv cty hwy fl class ## <chr> <chr> <dbl> <int> <int> <chr> <chr> <int> <int> <chr> <chr> ## 1 toyota corolla 1.8 1999 4 auto(l3) f 24 30 r compa…## 2 toyota corolla 1.8 1999 4 auto(l4) f 24 33 r compa…my_cars(n = 2, "corolla")## # A tibble: 2 x 11## manufacturer model displ year cyl trans drv cty hwy fl class ## <chr> <chr> <dbl> <int> <int> <chr> <chr> <int> <int> <chr> <chr> ## 1 toyota corolla 1.8 1999 4 auto(l3) f 24 30 r compa…## 2 toyota corolla 1.8 1999 4 auto(l4) f 24 33 r compa…10 / 11
Writing functions checklist
Pay attention to:
names of arguments
default argument values
local vs. global objects
return values
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