List comprehensions in R

Package provides Python-style list comprehensions for R. List comprehension expressions use usual loops (for, while and repeat) and usual if as list producers. Syntax is very similar to Python. The difference is that returned value should be at the end of the loop body.

There are three main functions:

• to_list converts usual R loops expressions to list producers. Expression should be started with for, while or repeat. You can iterate over multiple lists if you provide several loop variables in backticks. See examples.
• to_vec is the same as to_list but return vector. See examples.
• alter return the same type as its argument but with modified elements. It is useful for altering existing data.frames or lists. See examples.

Rather unpractical example - squares of even numbers:

library(comprehenr)
to_vec(for(i in 1:10) if(i %% 2==0) i*i)

Pythagorean triples:

to_list(for (x in 1:20) for (y in x:20) for (z in y:20) if (x^2 + y^2 == z^2) c(x, y, z))

More examples:

colours = c("red", "green", "yellow", "blue")
things = c("house", "car", "tree")
to_vec(for(x in colours) for(y in things) paste(x, y))

# prime numbers
noprimes = to_vec(for (i in 2:7) for (j in seq(i*2, 99, i)) j)
primes = to_vec(for (x in 2:99) if(!x %in% noprimes) x)
primes

You can iterate over multiple lists if you provide several loop variables in backticks:

to_vec(for(`i, j` in numerate(letters)) if(i %% 2==0) paste(i, j))

set.seed(123)
rand_sequence = runif(20)
# gives only locally increasing values
to_vec(for(`i, j` in lag_list(rand_sequence)) if(j>i) j)

alter examples:

data(iris)
# scale numeric variables
res = alter(for(i in iris) if(is.numeric(i)) scale(i))
str(res)

# convert factors to characters
res = alter(for(i in iris) if(is.factor(i)) as.character(i))
str(res)

# drop factors
res = alter(for(i in iris) if(is.factor(i)) exclude())
str(res)

# 'data' argument example
# specify which columns to map with a numeric vector of positions:
res = alter(
    for(`i, value` in numerate(mtcars)) if(i %in% c(1, 4, 5)) as.character(value),
    data = mtcars
)
str(res)

# or with a vector of names:
res = alter(
    for(`name, value` in mark(mtcars)) if(name %in% c("cyl", "am")) as.character(value),
    data = mtcars
)
str(res)