How to permute(shuffle) dynamic/linked list in C - c

Question

I use this data structure to store a graph:
struct Vertex
{
struct Line *a;
struct Line *b;
int X;
int Y;
struct Vertex *next;
struct Edge *edges;
int edgeCount;
};
struct Edge
{
struct Vertex *dst;
struct Vertex *src;
struct Edge *next;
float weight;
bool included;
bool visited;
};
I generate a linked list of vertices(the list is my graph) in a loop:
struct Edge *edge= malloc(sizeof(struct Edge));
////here I add edge's data
if(graph->edges == NULL)
{
graph->edges = edge;
}
else
{
edge->next = graph->edges;
}
And now I need to permute resulting array. It is simple with common array of integers:
void swap(int a, int b)
{
int temp = a;
a = b;
b = temp;
}
void permutation(int* randomArray, int numIndex)
{
for (i = 0; i < numIndex; i++)
{
int randomOffset = rand() % (numIndex - i);
swap(randomArray[i], randomArray[i+randomOffset]);
}
}
But how to do the same with linked list?

Answer 1

Create an array of struct Edge * that contains all edges, built when the linked list is created, permute that array, rebuild the linked list.
The variables
struct Edge *pedges[MAXEDGES]; // or allocate dynamically (see below)
int nedges = 0;
then when list is created
struct Edge *edge= malloc(sizeof(struct Edge)); // your code
pedges[nedges++] = edge;
permutations are done the same way
void permuteEdges(struct Edge *pe, int ne) {
int i;
for (i = 0 ; i < ne ; i++) {
int randomOffset = rand() % (ne - i);
swap(pe[i], pe[i+randomOffset]);
}
}
finally, rebuild the linked list
for (i = 0 ; i < ne ; i++) {
if ( ! i) {
graph->edges = pe[i];
}
else {
pe[i-1]->next = pe[i];
}
}
ensure the last item next property points to NULL
pe[ne-1]->next = NULL;
Note that you could allocate pedges dynamically (to be freed after use). For instance if you use the array for a single shuffle, there is no need to keep that amount of memory in the heap.
Dynamic allocation
struct Edge *pedges = malloc(MAXEDGES * sizeof(*pedges));
...use it...
free(pedges);