loop over rows of a data.frame and use them as input for a function

Question

The input values (a and b) for my foo() function come from a expand.grid() call.

I was wondering how to loop over the rows of input data.frame such that at each round, one a and b from one row of input data.frame is used in foo() as a named list?

foo <- function(a, b) { 
  b-a
  }

input <- rev(expand.grid(b=1:4,a=1:4))

#   a b
#1  1 1  # at round one, `a = 1`, `b = 1`
#2  1 2
#3  1 3
#4  1 4  # at round four, `a = 1`, `b = 4`
#.  . .
#.  . .
#.  . .

foo(a=, b=)

# Desired output:

[[1]] `a=1, b=1` 
     [1]  0

[[2]] `a=1, b=2`
     [1]   1
# .
# .
# .

You don't need to do that,you would say foo(input$a, input$b) and R will automatically do it for all rows and return a vector of the same length as a and b. — Elin
– Elin, Commented Mar 23, 2021 at 4:58

Ronak Shah · Accepted Answer · 2021-03-23 05:05:03Z

1

Assuming that the foo function is more complicated than just b-a you can use Map.

Map(foo, input$a, input$b)

answered Mar 23, 2021 at 5:05

Ronak Shah

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3 Comments

rnorouzian Over a year ago

What if I need a data.frame output instead of a list output?

Ronak Shah Over a year ago

data.frame(a = mapply(foo, input$a, input$b))

Ronak Shah Over a year ago

It depends on how the actual function foo is written.

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