Function set_edge_attr does not seem to work

Usage
1

The following code

library(igraph)
g1 <- make_ring(3) %>% set_edge_attr("weight", value = seq(3) )
edge_attr(g1)

gives as expected

$weight
[1] 1 2 3

However, similar code:

g2 <- make_ring(3)
set_edge_attr(g2, "weight", value = seq(3) )
edge_attr(g2)

gives
list()

I would expect the same output. Any thoughts why this isn’t the case?

2

set_edge_attr does not mutate an existing graph. It returns a new graph with modified attributes.

3

Thanks. I missed that.

In my opinion the documentation is too cryptic.
In stead of

Set edge attributes

it should be something like:

set_edge_attr creates a new graph, updating the edge attributes.

4

This is not specific to set_edge_attr but a fundamental principle of R. Like typical functional languages, R uses call-by-value, and functions cannot modify their arguments. All other functions will return modified values as well, instead of modifying variables passed to them.

5

To close this topic:

g$main <- "This is bad practice"

However:

g <- set_graph_attr(g, name="main", value="This is functional programming good practice")

6

First, I have to say that I rarely use R and can’t really say what is / isn’t considered good practice … My involvement with igraph is mainly through its C core and Mathematica interface.

However, I would not say that g$main <- "This is bad practice". I don’t think there’s anything wrong with that.

My point was that it is not possible to write an R function that changes a variable. The following is not possible:

x <- 1
f(x)
# now the value of x is 2

Only this is possible:

f <- function(a) { a + 1 }
x <- 1
x <- f(x)

This is because conceptually, R works with values, not with references. Some other languages, like Python, work with references, and you can do this:

In [1]: a=[1,2,3]

In [2]: b=a

In [3]: b[1]=100

In [4]: a
Out[4]: [1, 100, 3]

Notice that modifying b modified a as well. More accurately, a and b refer to the same data structure. In R each variable name is associated with an independent value. Modifying one variable will never affect another. Thus modifying a (local) function argument won’t change the (global) variable that was passed to the function.

7

All that said, generally speaking, improvements to the documentation are very welcome. Feel free to open pull requests with doc improvements. The R/igraph docs certainly need an update.

8

The argument is that the statement

`g$main <- "This is bad practice"`

is based on the internal structure of the graph g. In the (unlikely) case that it is decided to move $main to another position, the code will fail.

In contrast, retrieving data by functions get*attr is independent of the internal representation of g and is therefore more robust.

I admit this is a rather purist approach.