我正在运行产品扩散模拟研究。仿真从一个节点网络开始,并将一个初始节点数量与一个产品进行种子初始化。播种阶段之后的扩散受概率规则控制,该规则取决于采用产品的节点的邻居数量。我在R中编写了这个模型的两个版本 - 一个是矢量化的,另一个是循环的。这个想法可能更好地用代码表达。矢量化和循环版本返回不同的答案
library(igraph)
set.seed(20130810)
g <- sample_smallworld(dim = 1, size = 1000, nei = 12, p = 0.6)
n.nodes <- length(V(g))
nbr.influence <- rnorm(n = n.nodes, mean = 0.18, sd = 0.01)
# Diffusion simulation with loops
nodes.status <- rep.int(0, n.nodes)
seed <- sample(V(g), size = as.integer(0.005*n.nodes))
nodes.status[seed] <- 1
cat("Number of nodes seeded (loop version): ", sum(nodes.status), "\n")
for (node in V(g)) {
if (nodes.status[node] != 1) {
n.active.nbrs = 0
for (nbr in neighbors(g, node)) {
if (nodes.status[nbr] == 1) n.active.nbrs <- n.active.nbrs + 1
}
prob.change <- 1 - (1 - nbr.influence[node])^n.active.nbrs
if (runif(n = 1) < prob.change) nodes.status[node] = 1
}
}
cat("Number of nodes engaged after one iteration (loop version): ",
sum(nodes.status), "\n")
# Vectorized diffusion simulation
A <- get.adjacency(g)
nodes.status <- rep.int(0, n.nodes)
seed <- sample(V(g), size = as.integer(0.005*n.nodes))
nodes.status[seed] <- 1
cat("Number of nodes seeded (vectorized version): ", sum(nodes.status), "\n")
# use the adjacency matrix to count number of active neighbors for each node
n.active.neighbours <- as.vector(A %*% nodes.status)
# build the activation probability vector
prob.change <- 1 - (1 - nbr.influence)^n.active.neighbours
# see which of the nodes are ready to activate
vuln.nodes <- runif(n = n.nodes) < prob.change
# activate those nodes which are ready
nodes.status[vuln.nodes > nodes.status] <- 1
cat("Number of nodes engaged after one iteration (vectorized version): ",
sum(nodes.status), "\n")
运行该代码给出以下输出
Number of nodes seeded (loop version): 5
Number of nodes engaged after one iteration (loop version): 380
Number of nodes seeded (vectorized version): 5
Number of nodes engaged after one iteration (vectorized version): 32
两个版本的逻辑是相同的(即,扩散是由相同的概率规则),但最终的答案是广泛不同。这段代码中的错误在哪里?
非常明确的解释!我现在更了解代码! – buzaku