我试图用Python 3实现Prim的算法,它计算它产生的MST的总重量。我正在做一些与众不同的事情,使用"数组"跟踪未访问的节点。
这是我的代码:
def Prim(Graph):
# row 1 is "still in R"
# row 2 is the connector vertex
# row 3 is the cost
total = 0
A = []
n = len(Graph)
A = [[None for x in range(0, n)] for y in range(1, 4)]
#Debugging purposes
#print(A)
for x in range(1, n):
A[0][x] = 'Y'
A[1][x] = 0
A[2][x] = 0
for neighbour in Graph[1]:
A[1][neighbour-1] = 1
A[2][neighbour-1] = Graph[1][neighbour]
#Debugging purposes
#print("Neighbour: ", neighbour, "Weight: ", Graph[1][neighbour])
current = 1
T = [current]
MST_edges = {}
count = 0
while len(T) < n:
x = search_min(current, A)
T.append(x)
MST_edges[x] = A[1][x]
A[0][x] = 'N'
total += A[2][x]
#print(Graph)
#print(A)
for neighbour in Graph[x]:
#print(neighbour)
#print(A[2][neighbour-1])
if A[0][neighbour-1] != 'N':
if Graph[x][neighbour] < A[2][neighbour-1]:
A[1][neighbour-1] = x
A[2][neighbour-1] = Graph[x][neighbour]
count += 1
current = T[count]
return total
def search_min(current, A):
minimum_cost = 100
minimum_vertex = 1
for x in range(1,len(A[0])):
if A[1][x] != None and A[0][x] != 'N' and A[2][x] < minimum_cost:
minimum_cost = A[2][x]
minimum_vertex = x
#Debugging
## print("x", x)
## print("cost",minimum_cost)
## print("vertex",x)
return minimum_vertex
它有时给我像20这样可笑的低重量(这几乎是不可能的,因为所有边缘的最小重量都是10)。问题可能出在while循环中:
while len(T) < n:
x = search_min(current, A)
T.append(x)
MST_edges[x] = A[1][x]
A[0][x] = 'N'
total += A[2][x]
#print(Graph)
#print(A)
for neighbour in Graph[x]:
#print(neighbour)
#print(A[2][neighbour-1])
if A[0][neighbour-1] != 'N':
if A[2][neighbour-1] != None and Graph[x][neighbour] < A[2][neighbour-1]:
A[1][neighbour-1] = x
A[2][neighbour-1] = Graph[x][neighbour]
count += 1
current = T[count]
但我不知道哪个部分。变得很晚,我的头疼,任何可以提供帮助的人都会很棒。
编辑以下是它生成的MST示例。由于某种原因,有一些顶点有0个加权边。
graph = construct_graph(20) Prim(图){3:0,5:0,8:0,16:0,6:5,9:3,7:8,11:5,15:11,12:11,2:8,18 :2,19:2,1:19,10:19,14:10,17:5,13:16,4:1}
(仔细查看我的代码,你可以看到,对于值x:y,x是顶点的值,而y是连接边的权重。由于某种原因,顶点加权为0)
答案 0 :(得分:0)
经过建议,我改变了这行代码:
A[2][x] = 0
对此:
A[2][x] = math.inf
这样阵列不会意外地看到'woot,edge with 0 weight',因为那意味着它没有连接。因此,所有这些都应该归结为非法价值。