# Why does not this Erdos-Renyi C code work?

## Homework Statement

I need to write an Erdos-Renyi random graph, by using the adjacency matrix (or alternatively list) and calculate the fitness of the graph.
Definition: G(n, p) is a random graph with n vertices where each possible edge has probability p of existing.

## Homework Equations

The number of edges in a G(n,p) graph is a random variable with a binomial expected value.

## The Attempt at a Solution

I wrote the following C code:
Mod note: Edited by mentor to change code tag to code=c tag, and deleted extraneous spaces between lines.
C:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define VOL 30
#define CONNECTIVITY 1

int main(){

int i , j , neighbour[VOL], connected[VOL];
int matrix[VOL][VOL];

for ( i = 0; i < VOL; i ++ ){
for (  j = 0; j < VOL; j ++ ){
matrix[i][j] = 0;
matrix[j][i] = 0;
}
}

srand((unsigned)time(NULL));

int num_edges = 0;

for(i = 0; i < VOL; i++){
connected[i] = 0;
for(j=VOL-1;j>=i;j--){
double prob=(rand()/(double)(RAND_MAX));
if(prob < CONNECTIVITY){
j = prob*VOL;
if(i != j ){
matrix[i][j] = 1;
matrix[j][i] = 1;
neighbour[i] = j;
neighbour[j] = i;
num_edges++;
printf("matrix[%d][%d]\n", i,j);
}
else{
j = prob*VOL;
matrix[i][j] = 1;
matrix[j][i] = 1;
neighbour[i] = j;
neighbour[j] = i;
connected[i] = 1;
}
}
}
return 0;
}
}
However there is something wrong. Any advice or help would be greatly appreciated. Thank you.

Last edited:

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Mark44
Mentor

## Homework Statement

I need to write an Erdos-Renyi random graph, by using the adjacency matrix (or alternatively list) and calculate the fitness of the graph.
Definition: G(n, p) is a random graph with n vertices where each possible edge has probability p of existing.

## Homework Equations

The number of edges in a G(n,p) graph is a random variable with a binomial expected value.

## The Attempt at a Solution

I wrote the following C code:

C:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define VOL 30
#define CONNECTIVITY 1

int main(){

int i , j , neighbour[VOL], connected[VOL];
int matrix[VOL][VOL];

for ( i = 0; i < VOL; i ++ ){
for (  j = 0; j < VOL; j ++ ){
matrix[i][j] = 0;
matrix[j][i] = 0;
}
}

srand((unsigned)time(NULL));

int num_edges = 0;

for(i = 0; i < VOL; i++){
connected[i] = 0;
for(j=VOL-1;j>=i;j--){
double prob=(rand()/(double)(RAND_MAX));
if(prob < CONNECTIVITY){
j = prob*VOL;
if(i != j ){
matrix[i][j] = 1;
matrix[j][i] = 1;
neighbour[i] = j;
neighbour[j] = i;
num_edges++;
printf("matrix[%d][%d]\n", i,j);
}
else{
j = prob*VOL;
matrix[i][j] = 1;
matrix[j][i] = 1;
neighbour[i] = j;
neighbour[j] = i;
connected[i] = 1;
}
}
}
return 0;
}
}
However there is something wrong. Any advice or help would be greatly appreciated. Thank you.
What exactly is wrong? Does your program compile? If it compiles, do the results it produces differ from what you think it should be produced? If so, tell us what it should produce and what it actually produces.

The normal course of action is to use a debugger to single-step through the program, noting the values of variables, and comparing the expected values of them with their actual values as produced by your code.

Last edited:
It doesn't compile the adjacency matrix: the indices of the matrix are the edges between the nodes, but I didn't want to do that. I wanted something 'cleaner' and elegant, like an adjacency list or matrix in order to calculate the fitness (...but I don't know how to do that).

Code:
    for(i = 0; i < VOL; i++){
connected[i] = 0;
for(j=VOL-1;j>=i;j--){
double prob=(rand()/(double)(RAND_MAX));
if(prob < CONNECTIVITY){
j = prob*VOL;
This assignment to j can't be right. You are already uning j for the loop, and I have also no idea why this statement is here at all.

I assume that CONNECTIVITY is 1 for debugging and that you will use some value <1 later.

I don't know what neighbour is supposed to mean. A node can have more than one neighbour. The information will get overwritten.

Thank you, Willem. I didn't understand what you mean about the CONNECTIVITY.
Yes, you have supposed right about neighbour: a node can have more than one neighbour. The probability of edges may change.

If I consider the following dfs function, I don't find the only nearest neighbour of my code:

Code:
int dfs(int i)
{
int j;
neighbours[i]=1;

printf("%d->",i+1);

for(j=0;j<VOL;j++)
{
if(matrix[j][i]==1&&neighbours[j]==0)
dfs(j);
}
return j;
}

Last edited:
Thank you, Willem. I didn't understand what you mean about the CONNECTIVITY.
It would appear that CONNECTIVITY is supposed to be the probability that any pair of nodes is connected (the constant p from G(n,p)).

Code:
for(i = 0; i < VOL; i++){
connected[i] = 0;
for(j=VOL-1;j>=i;j--){
double prob=(rand()/(double)(RAND_MAX));
if(prob < CONNECTIVITY) {
martix[i][j] = 1;
matrix[j][i] = 1;
You enumerated all the possible edges (i,j) here, and you use a random number, to find out wether they should be connected.
You seem to be about done here.
Unfortunately I still have no idea wat neigbours or dfs(i) is supposed to represent, so I have no idea wether this is correct.

Unfortunately I still have no idea wat neigbours or dfs(i) is supposed to represent, so I have no idea wether this is correct.
dfs(i) is the depth first search function.
I would like to find the neighbourhood of a node(my source). However it doesn't work, because it prints all the numbers and not only the neighbours of a selected vertex.

dfs(i) is the depth first search function.
I would like to find the neighbourhood of a node(my source). However it doesn't work, because it prints all the numbers and not only the neighbours of a selected vertex.
Your description of dfs() is still very vague. What does the parameter mean, and what does the return value mean?

If you want the neigbourhood from the node i, you need to test all potential edges from node i to all other nodes to see if they exist.
I can't see why you would want to use a recursive function for this. You can just use a simple loop.

Do you want to find ALL the other nodes connected to node, including indirect connections? dfs() seems to do that. How is the neighbours[] array initialized and reset?

Your description of dfs() is still very vague. What does the parameter mean, and what does the return value mean?
This function returns the printf, where there is all information about the connection of all nodes with a selected one (source). However I should find only the nearest neighbours and possibile consider a directed graph.

If you want the neigbourhood from the node i, you need to test all potential edges from node i to all other nodes to see if they exist.
I can't see why you would want to use a recursive function for this. You can just use a simple loop.
Sorry, Willem: what should it be into the loop? Because I didn't understand how to check the potential edges from a node i to all others ones.

[/QUOTE]

Do you want to find ALL the other nodes connected to node, including indirect connections? dfs() seems to do that. How is the neighbours[] array initialized and reset?
Yes, I do. I would like to understand how I can consider the direct and indirect connections respectively.
neighbours[] array in the dfs() function is all there. It was initialised equals to 0.

Mark44
Mentor
Your description of dfs() is still very vague. What does the parameter mean, and what does the return value mean?
This function returns the printf, where there is all information about the connection of all nodes with a selected one (source). However I should find only the nearest neighbours and possibile consider a directed graph.
"Returns the printf" makes no sense.
Here's the definition of dfs() that you showed in post #5.
C:
int dfs(int i)
{
int j;
neighbours[i]=1;

printf("%d->",i+1);

for(j=0;j<VOL;j++)
{
if(matrix[j][i] == 1 && neighbours[j] == 0)
dfs(j);
}
return j;
}
Some questions:
1) How does this code tie into main() as you showed in post #1? There is no mention of dfs() in that code.
2) The code just above references the neighbours and matrix arrays. main() has local arrays named neighbour (different from neighbours) and matrix. Since neighbours and matrix aren't passed as parameters to dfs(), there must be some global arrays named neighbours and matrix somewhere that we haven't seen. It's very bad practice for a function to modify global variables as your dfs() function appears to be doing.
3) Your code for dfs() should have comments, one of which would give a bried explanation of what it does, and another that explains the meaning of the parameter i. In fact, there is not a single comment in any of the code you have shown here, which makes it very difficult for people reading your code to understand what you're trying to do. Further, comments can help the person writing the code keep focused on what the code is supposed to be doing.
If you want the neigbourhood from the node i, you need to test all potential edges from node i to all other nodes to see if they exist.
I can't see why you would want to use a recursive function for this. You can just use a simple loop.
I don't see the purpose of the recursive call, either.

To get a better understanding of what should happen, I recommend that you start with a small graph of, say, 6 nodes, and draw paths between some of them. By hand, fill out your arrays. This might help you understand what your code needs to be doing.

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