R igraph: Finding shortest path in igraph, adding weight to it and search for alternative

Question

i play around with https://rdrr.io/rforge/osmar/src/demo/navigator.R (Navigator Demo). I would like to find several paths insted of just one.

It seems i can not use the function all_simple_paths since it will never terminate. Would it be possible after i find a shortest path with

route <- get.all.shortest.paths(gr_muc, from = as.character(hway_start_node), to = as.character(hway_end_node))[[1]]

increase weight of the whole route and search again with the function get.all.shortest.paths to find n ext alternative? Is this the right approach or is there an alternative?

Thank you in advance!

    library(tidyverse)
library(osmdata)
library(osmar) # (geosphere is inclued in osmar)
library(sf)
library(ggmap)
library(prettymapr)
library(leaflet)
library(igraph)
library(stplanr)
library(rgeos)

### Download and extract data: #######################################

download.file("http://osmar.r-forge.r-project.org/muenchen.osm.gz",
              "muenchen.osm.gz")

system("gzip -d muenchen.osm.gz")



### Import subset based on bounding box: #############################

src <- osmsource_osmosis(file = "muenchen.osm",
                         osmosis = "osmosis")

muc_bbox <- center_bbox(11.575278, 48.137222, 60000, 60000)

muc <- get_osm(muc_bbox, src)

hways_muc<-muc
gr_muc <- as_igraph(hways_muc)

 hway_start_node <- local({
  id <- find(muc, node(tags(v == "Sendlinger Tor")))[1]
  find_nearest_node(muc, id, way(tags(k == "highway")))
})

hway_start <- subset(muc, node(hway_start_node))

hway_end_node <- local({
  id <- find(muc, node(attrs(lon > 11.59 & lat > 48.150)))[1]
  find_nearest_node(muc, id, way(tags(k == "highway")))
})
hway_end <- subset(muc, node(hway_end_node))



route <- get.all.shortest.paths(gr_muc,
                                    from = as.character(hway_start_node),
                                    to = as.character(hway_end_node),
                                    mode = "all")[[1]]

does the navigator.R example work for you? I'm getting an error when calling get_osm. — Waldi, Jun 12 '20 at 21:22
i think there is a bug. Try to use this example : https://stackoverflow.com/questions/62246073/shortest-path-from-osmar-object-to-igraph-in-r-trying-to-replicate-an-osmar-doc — Andreas, Jun 12 '20 at 21:28
It would help if you could supply the complete working example up to get.all.shortest.path. I would like to test if the solution I imagine answers your needs. — Waldi, Jun 13 '20 at 06:47
@waldi i added some code to better explain the problem. Please adjust the size of the bbox if it does not work. — Andreas, Jun 13 '20 at 08:42

Waldi · Accepted Answer · 2020-06-13T22:22:57.377

Increasing the weight of the path works exactly as expected :

library(osmar)
library(igraph)

### 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)
}

# Number of new ways to look for 
nways <- 10
# Weight factor applied to already found way
weightfactor <- 2

for (numway in 1:nways) {
  r <- route(hway_start_node,hway_end_node)
  color <- colorRampPalette(c("springgreen","royalblue"))(nways)[numway]
  plot.route(r,color)
  # Modify current route weight
  E(gr)[r]$weight <- E(gr)[r]$weight * weightfactor
}

Thank you very much! you are awesome. I think it should be the right way to do it. Strangely after i run your code the pot i get looks differently and uncomplete. https://imgur.com/a/rSSxaGo — Andreas, Jun 13 '20 at 21:17
It was a nice bounty, enjoyed it! Regarding display, perhaps remove some layers or open a zoomed window. — Waldi, Jun 13 '20 at 21:41
Found the problem: in plot.route function it should be hways instead hways_muc — Andreas, Jun 13 '20 at 22:19
would it be possible to add weight to just a part of the route? — Andreas, Jun 13 '20 at 22:23
for sure.E(gr)[r] is the list of all the edges/segments of the route. As explained in [Navigator.R](https://rdrr.io/rforge/osmar/src/demo/navigator.R), you can get the detail of each edge (name of the street), and you could affect a different weight to each segment according to the streets you want to follow or avoid. — Waldi, Jun 13 '20 at 22:34

R igraph: Finding shortest path in igraph, adding weight to it and search for alternative

1 Answers1