## MANAGING NETWORK DATA IN R

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# Managing network data in R
# Author: Jake Fisher

# This lab will introduce using network data with R.  As the introduction to R
# lab described, one of the main benefits to learning R is its ability to handle
# non-traditional data structures.  Key among these, for our purposes, is a
# network.

# This file will describe how to:
# 1. Create a network object from raw network data
# 2. Save and recall network (incl. vertex and edge) attributes
# 3. Calculate individual-level and global network statistics
# 4. Perform operations on a list of networks

# There are two main network packages in R.  I will focus primarily on statnet
# today.  The statnet suite of packages is mostly used for statistical models
# of networks.  The other common package is igraph, which is often used for
# computer science-style applications, like community detection and calculations
# on large graphs.  You can convert between them using the intergraph package.

# Following best practices, we will begin by setting up the workspace:
rm(list = ls())
setwd("C:/Users/fishe/Dropbox/SN&H Files/r_scripts")
library(statnet)  # lots of messages...

##### Creating a network object #####
# Statnet can create a network object from any of the common network data

# network where a vertex is a member of my immediate family, and an edge
# indicates "parent":
people <- c("mom", "dad", "sister", "me")

(adj.mat <- matrix(0, nrow = 4, ncol = 4))  # create empty matrix

# We can create a network object using the network() function

# Network objects have a number of special functions for them.  We will cover
# these in the next section.  For now, let's plot the network
plot(family)  # not terribly informative

# Note that we could have expressed the information above more compactly if we
# just listed the cell values that should be 1 (i.e., mom -> sister, mom -> me,
# etc.) and assumed everything else should be 0.  That's called an edgelist.
# We can generate a network from an edgelist, too:
(edges <- data.frame(from = c(rep("mom", 2), rep("dad", 2)),
to = rep(c("sister", "me"), 2)))
family.2 <- network(edges, matrix.type = "edgelist")

# We can show the adjacency matrix from the network object using as.matrix
as.matrix(family.2)  # order of the rows is different, but network is the same

# And finally, we could compress this even more by holding one the sender in
# one column, and the receiver in subsequent columns.  This is called an
child2 = rep("me", 2)))

# Statnet doesn't handle adjacency lists well, but they are the most common type
# of raw survey network data.  Fortunately, we can convert from an adjacency
# list to an edgelist easily, with the tidyr package
library(tidyr)
library(dplyr)  # for the pipe operator
library(magrittr)  # for the %<>% operator

gather(key = "nomination.number", value = "alter", -parent)

# Gather strings your data out into a long column.  We can select which
# variables to string out -- in this case, we wanted to keep the column for
# parent separate, and to repeat the values for each child.

# If you drop the middle column, you have an edgelist...
gather(key = "nomination.number", value = "alter", -parent) %>%
select(-nomination.number)

# ...which you can pass to network!
adj.list %>%  # note that we didn't save the output
gather(key = "nomination.number", value = "alter", -parent) %>%
select(-nomination.number) %>%
network(., matrix.type = "edgelist")

# That's great, but how do we do it with a real dataset?  Let's try with the

# So what's in here, anyway?
names(dat)

# First of all, there are several communities in these data.  Let's pick the
# first one:
cmty1 <- dat %>% filter(commcnt == 1)

# Notice that the "friend" variables contain "fnid", and no other variables do.
# We can use that to create an edgelist:
el <- cmty1 %>%
select(ego_nid, contains("fnid")) %>%
gather(key = "nomination", value = "alter", -ego_nid)

head(el)  # hmmmm.... contains 99999 and NA values.  Let's standardize those

el %<>% mutate(alter = ifelse(alter == 99999, NA, alter))

# Now you would think this would work as a network...
el %>%
select(-nomination) %>%
network(., matrix.type = "edgelist")

# but statnet also can't really deal with missing values in the edgelist.  So
# the best way to do this is to construct an empty network first, and then
# add the edges from the edgelist.  This has the advantage of including isolates
# too.

# To do that, first we get a list of the people who are involved in the network:

# Get unique list of people in the network
(ah.people <- with(el, unique(c(ego_nid, alter))))  # this would be all the people, including those out of school
# ah.people <- unique(dat\$ego_nid)  # this is only people who took the survey

el %<>% filter(!is.na(alter))  # drop missing cases

# We're not going to do that, because it includes a lot of people who didn't
# take the survey.

# Now create an empty network

# statnet adds edges by index values, not by names of variables.  So, to make
# this work, we have to create an index value of 1, 2, ..., N, where N is the
# number of people in the network
el %<>%
mutate(ego.idx = match(ego_nid, ah.people),
alter.idx = match(alter, ah.people))

# Finally, add the edges from the edgelist

# Now, as a quick gut check, let's plot it:
set.seed(919)

# (As an aside, note that you could create a list of all the networks using
# lapply...)

##### Save and recall network attributes #####
# One of the benefits of statnet is that it's possible to save attributes for
# different pieces of the network.  We can save vertex attributes, like whether
# the person is a smoker, edge attributes, like whether the ties is homophilous,
# or network attributes, like the name of the school, or the survey wave.  The
# procedure for this is simple; let's illustrate a few.

# First, network attributes are characteristics of the network as whole.  Set
# them with %n%
add.health  # note "cmty" in there

# Second, vertex attributes are characteristics of each vertex.  Set them with
# %v%
add.health %v% "sex" <- cmty1\$sex  # we can do this because vertices and data
# are sorted in the same way

# A special case of vertex attributes is vertex names, which you set with
# network.vertex.names()

# All of these methods can also be used to retrive network attributes

# Third, edge attributes are characteristics of a pair of vertices.  You can set
# them with %e%, but the indexing is more complicated, and I will not cover it
# here.

# These are useful for plotting and advanced statistical methods.

##### Calculate individual-level and global network statistics #####
# statnet has functions for virtually every network statistic.  They come in two
# flavors, individual-level network statistics, and global network statistics.

# Individual level statistics are calculated for each person.  These include
# things like centrality.

# Typically, these are most useful in a dataset.  We can construct that with the
# data.frame command:
(ind.net.stats <- data.frame(
indegree = degree(add.health, cmode = "indegree"),
outdegree = degree(add.health, cmode = "outdegree")
))

# Then you would merge these on to a regular dataset, and run a regression.

# Global network statistics are characteristics of the network as a whole.
# statnet has a number of commands for these as well:

# Often you would calculate these on a single network, and include them as
# text in a plot, or somewhere in the text of a paper.  As we increasingly
# work with several networks, however, you might have a list of network objects,
# which you could summarize using the network level statistics for each.  The
# next section will explain how to do that.

##### Performing operations with lists of network data #####
# The faux Add Health data contains many communities.  (This is increasingly
# common with network data.)  The easiest way to perform a calculation on each
# of these networks is to create a list of network objects.

# To save each of the networks in the faux Add Health data, we will perform the
# operations above for each of the networks.  The easiest way to do this is to
# construct a list, and loop through it with lapply.

# We begin by splitting up the data into a list of data.frames, one for each
# community.
cmties <- split(dat, dat[, c("commcnt", "sdummy")], drop = T)

# note that this is now a list of data.frames, which are split up by unique
# combinations of commcnt and sdummy
sapply(cmties, class)

# Now, for each one of these, we want to run the same set of commands that we
# ran earlier.  This is an ideal case for writing your own function!
createNetwork <- function(indat) {
# Creates a network object from an adjacency list for the faux Add Health data
#
# Args:
#   indat: a data.frame with columns for ego_nid, and at least one column with
#          a name containing "fnid"
#
# Returns:
#   A statnet network object

# Save edgelist without missings
el <- indat %>%
select(ego_nid, contains("fnid")) %>%
gather(key = "nomination", value = "alter", -ego_nid) %>%
mutate(alter = ifelse(alter == 99999, NA, alter))

# Get list of unique people in edgelist
# people <- unique(with(el, c(ego_nid, alter)))  # everyone involved
people <- unique(el\$ego_nid)  # only people who completed survey

# Create an empty network
out <- network.initialize(n = length(people))

# Save the index numbers to the edgelist
el %<>%
filter(!is.na(alter)) %>%
mutate(ego.idx = match(ego_nid, people), alter.idx = match(alter, people))

# Add those edges to the edgelist

# Save the names of the people as vertex names
network.vertex.names(out) <- people

# Return the network object
return(out)
}

# Now we can loop through all of data sets and create a network for each one...
nets <- lapply(cmties, createNetwork)

# And then we can create a data.frame of descriptive network statistics for each
# of those:
net.stats <- data.frame(
net.id = names(nets),

# Network-level statistics
reciprocity = sapply(nets, grecip, USE.NAMES = FALSE),
transitivity = sapply(nets, gtrans, USE.NAMES = FALSE),
density = sapply(nets, gden, USE.NAMES = FALSE),
size = sapply(nets, network.size, USE.NAMES = FALSE),

# Aggregate individual-level statistics
n.isolates = sapply(nets, isolates, USE.NAMES = FALSE) %>%
sapply(., length, USE.NAMES = FALSE),
mean.degree = sapply(nets, degree, cmode = "outdegree", USE.NAMES = FALSE) %>%
sapply(., mean, USE.NAMES = FALSE)
) %>%
# Function from tidyr to separate values of the form community.school into
# separate columns
separate(col = net.id, into = c("commcnt", "sdummy"))

# Or, equivalently, write a function to output a data.frame
calculateNetStats <- function(net) {
# Calculates a variety of global and aggregate individual network statistics
#
# Args:
#   net: a statnet network object
#
# Returns:
#   a data.frame with columns for reciprocity, transitivity, density, size,
#   number of isolates, and mean degree
return(data.frame(reciprocity = grecip(net), transitivity = gtrans(net),
density = gden(net), size = network.size(net),
n.isolates = length(isolates(nets)),
mean.degree = mean(degree(net, cmode = "outdegree"))))
}

# This crashed my computer...
# net.stats.equiv <- lapply(nets, calculateNetStats) %>%
#   bind_rows %>%
#   mutate(net.id = names(nets)) %>%
#   separate(col = net.id, into = c("commcnt", "sdummy"))

# Finally, we could also save individual level statistics from each of the
# networks, and merge them on to the main data.
calculateIndNetStats <- function(net) {
# Calculates isolation and degree from a network
#
# Args:
#   net: a statnet network object
#
# Returns:
#   a data.frame with columns for ego_nid (the vertex names), isolate (T/F),
#   indegree, and outdegree
return(data.frame(ego_nid = network.vertex.names(net),
isolate = is.isolate(net, ego = seq(network.size(net))),
indegree = degree(net, cmode = "indegree"),
outdegree = degree(net, cmode = "outdegree")))

}

ind.net.stats <- lapply(nets, calculateIndNetStats) %>%
bind_rows(., .id = "sch") %>%  # save a column indicating which list it's from
separate(sch, into = c("commcnt", "sdummy")) %>%
# Convert them to integers, so that they can be merged
mutate_each(funs(as.integer(as.character(.))), commcnt, sdummy)

# This can then be merged onto the original data by ego_nid, commcnt, and sdummy
# e.g.,
dat %<>%
left_join(., ind.net.stats)

##### Practice! #####
# Now let's practice doing this ourselves!  Tom Valente collected similar data
# http://www-hsc.usc.edu/~tvalente/enp/enp_individual.xls