我正在从 osmar 对象构建的 igraph 中搜索一组边，并希望更改这些边的权重。由于我的图表很大，因此这项任务需要很长时间。由于我在循环中运行此函数，因此运行时会变得更大。
有没有办法优化这个？
这是代码：
library(osmar)
library(igraph)
library(tidyr)
library(dplyr)
### Get data ----
src <- osmsource_api(url = "https://api.openstreetmap.org/api/0.6/")
muc_bbox <- center_bbox(11.575278, 48.137222, 1000, 1000)
muc <- get_osm(muc_bbox, src)
### Reduce to highways: ----
hways <- subset(muc, way_ids = find(muc, way(tags(k == "highway"))))
hways <- find(hways, way(tags(k == "name")))
hways <- find_down(muc, way(hways))
hways <- subset(muc, ids = hways)
#### Plot data ----
## Plot complete data and highways on top:
plot(muc)
plot_ways(muc, col = "lightgrey")
plot_ways(hways, col = "coral", add = TRUE)
### Define route start and end nodes: ----
id<-find(muc, node(tags(v %agrep% "Sendlinger Tor")))[1]
hway_start_node <-find_nearest_node(muc, id, way(tags(k == "highway")))
hway_start <- subset(muc, node(hway_start_node))
id <- find(muc, node(attrs(lon > 11.58 & lat > 48.15)))[1]
hway_end_node <- find_nearest_node(muc, id, way(tags(k == "highway")))
hway_end <- subset(muc, node(hway_end_node))
## Add the route start and and nodes to the plot:
plot_nodes(hway_start, add = TRUE, col = "red", pch = 19, cex = 2)
plot_nodes(hway_end, add = TRUE, col = "red", pch = 19, cex = 2)
### Create street graph ----
gr <- as.undirected(as_igraph(hways))
### Compute shortest route: ----
# Calculate route
route <- function(start_node,end_node) {
get.shortest.paths(gr,
from = as.character(start_node),
to = as.character(end_node),
mode = "all")[[1]][[1]]}
# Plot route
plot.route <- function(r,color) {
r.nodes.names <- as.numeric(V(gr)[r]$name)
r.ways <- subset(hways, ids = osmar::find_up(hways, node(r.nodes.names)))
plot_ways(r.ways, add = TRUE, col = color, lwd = 2)
}
nways <- 1
numway <- 1
r <- route(hway_start_node,hway_end_node)
# Plot route
color <- colorRampPalette(c("springgreen","royalblue"))(nways)[numway]
plot.route(r,color)
## Route details ----
# Construct a new osmar object containing only elements
# related to the nodes defining the route:
route_nodes <- as.numeric(V(gr)[r]$name)
route_ids <- find_up(hways, node(route_nodes))
osmar.route <- subset(hways, ids = route_ids)
osmar.nodes.ids <- osmar.route$nodes$attrs$id
# Extract the nodes’ coordinates,
osmar.nodes.coords <- osmar.route$nodes$attrs[, c("lon", "lat")]
osmar.nodes <- cbind(data.frame(ids = osmar.nodes.ids),
data.frame(ids_igraph = as.numeric(V(gr)[r]) ),
osmar.nodes.coords)
## Find edges ids containing points of interest ----
wished.coords <- data.frame(wlon = c(11.57631),
wlat = c(48.14016))
# Calculate all distances
distances <- crossing(osmar.nodes,wished.coords) %>%
mutate(dist = geosphere::distHaversine(cbind(lon,lat),cbind(wlon,wlat)))
# Select nodes below maximum distance :
mindist <- 50 #m
wished.nodes <- distances %>% filter(dist < mindist)
# Select edges incident to these nodes :
selected.edges <- unlist(incident_edges(gr,V(gr)[wished.nodes$ids_igraph]))
This is where the slowdown occurs: Weight of selected edges, change it by multiplying it with 10
E(gr)[selected.edges]$weight<-E(gr)[selected.edges]$weight*10
这就是减速发生的地方：所选边缘的权重，通过将其乘以 10 来更改它
E(gr)[selected.edges]$weight<-E(gr)[selected.edges]$weight*10
也许我可以使用哈希图？
更新
哈希图
单位：秒
Hashmap:
expr min lq mean median uq max neval
Hashmap 3.248543 3.289474 3.472038 3.324417 3.734050 4.188924 100
Without 3.267549 3.333012 3.557179 3.367015 3.776429 5.643784 100
Sadly it does not seemt to bring a lot of improvement.
library(hashmap)
#https://github.com/nathan-russell/hashmap
H <- hashmap(E(gr)[selected.edges],E(gr)[selected.edges]$weight)
sapply(H$find(E(grr)[selected.edges]), function(x) x * 10)
更新： 根据 igraph doc，igraph 是线程安全的，所以我可以使用并行。
我目前正在尝试这个：
no_cores <- detectCores(logical = FALSE)
data <- split(selected.edges,factor(sort(rank(selected.edges)%%no_cores)))
c_result <- mclapply(1:no_cores, function(x) {
E(gr)[unlist(data[[x]])]$weight * 1000 / mean_value }, mc.cores = no_cores)
E(gr)[unlist(data)]$weight<-unlist(c_result)
我想知道为什么我必须在并行循环之外执行“编写步骤”。当我试图在循环中将权重写回 igraph 时，它不起作用，即权重没有得到更新。
先感谢您！BR