如何将结构数组作为 C 中的指针传递给函数？

Question

I have done the following program, to input a graph and color it in at most 6 colors (given degree at most 5 etc.)

Most of the code is self explanatory, with comments to explain what each line does.

I have included my code below and the errors that are raised, but I'm especially curious whats causing the warning saying the parameter I've input isn't of expected type. This has become sort of a recurring error for me, and I'd love some feedback!

Any suggestions on the code itself would be appreciated too, but my main issue at the moment is related to the passing of parameters

struct NODE{
    int node_no;
    char color;
};

char Colors[6]={'a','b','c','d','e','f'};

// l->left index, right index is fixed at n-1, as we colors from  0 to n-1
void ColorTree(struct NODE *Tree[], int l, int n, int Edges [][2], int e)
{
    if(n==1)    // if only one node, color node with lowest color
    {
        Tree[0]->color='a';
    }
    
    int i,j; char used_colors[6], c='N';
    
    for(i=0;i<=e;i++)   // colors 
    {
        if(Edges[i][0] == Tree[l]->node_no)
        {
            c=Tree[Edges[i][1]]->color;
            continue;
        }
        if(Edges[i][1] == Tree[l]->node_no)
        {
            c=Tree[Edges[i][0]]->color;
        }

        for(j=0;used_colors[j]!='\0';j++);  // finds empty position of used_colors
        if(c!='N')
            used_colors[j]=c;   // if the color is not N, add to already used colors
    }

    for(i=0;Colors[i]!='\0';i++)
        for(j=0;used_colors[j]!='\0';j++)
        {   
            if(Colors[i]==used_colors[j])   // the color is already used
                continue;
            else if(used_colors[j+1]=='\0')     // the color is not used
                break;
        }

    Tree[l]->color=Colors[i];   // the unused color being assigned to Tree[l]

    if(l==(n-1))    // all nodes have been colored
        return;

    ColorTree(&Tree,l+1,(n-1),Edges, e);
}

int main()
{     
    int n,e,i;

    scanf("%d", &n);
    struct NODE Tree[n];    // n-> no. of vertices 

    for(i=0;i<n;i++)     // initialising the tree, with node 0,1,2...n-1 and  color = 'N'
    {
        Tree[i].node_no=i;
        Tree[i].color='N';    // 'N'->no color
    }

    scanf("%d", &e);    // e-> no.of edges

    int Edges[e][2];
    
    for(i=0;i<e;i++)    // entering the edges
    {
        scanf("%d", &Edges[i][0]);
        scanf("%d", &Edges[i][1]);
    }

    ColorTree(&Tree,0,(n-1),Edges,e);

    printf("%c", Tree[0].color);    // printing the colors in the graph
    for(i=1;i<n;i++)
        printf("\n%c", Tree[i].color);
    
    return 0;
}

The errors i'm getting are:

<file_name>.c:51:15: warning: passing argument 1 of ‘ColorTree’ from incompatible pointer type [-Wincompatible-pointer-types]
   51 |     ColorTree(&Tree,l+1,(n-1),Edges, e);
      |               ^~~~~
      |               |
      |               struct NODE ***
<file_name>.c:11:29: note: expected ‘struct NODE **’ but argument is of type ‘struct NODE ***’
   11 | void ColorTree(struct NODE *Tree[], int l, int n, int Edges [][2], int e)
      |                ~~~~~~~~~~~~~^~~~~~
<file_name>.c: In function ‘main’:
<file_name>.c:77:15: warning: passing argument 1 of ‘ColorTree’ from incompatible pointer type [-Wincompatible-pointer-types]
   77 |     ColorTree(&Tree,0,(n-1),Edges,e);
      |               ^~~~~
      |               |
      |               struct NODE (*)[(sizetype)(n)]
<file_name>.c:11:29: note: expected ‘struct NODE **’ but argument is of type ‘struct NODE (*)[(sizetype)(n)]’
   11 | void ColorTree(struct NODE *Tree[], int l, int n, int Edges [][2], int e)
      |   

My program eventually terminates through Segmentation fault after the inputs. What am I doing wrong in passing the array of structures as reference?

Answer 1

Your current implementation expects a NODE pointer array (eg an array of NODE* ). In reality, your passing it &Tree from main() , which is not NODE pointer array, its the address of a hard array. Worse (if there is such a thing), in ColorTree you're pouring more salt on the wound by recursing with &Tree , in that context, &Tree is actually NODE ***

None of this needs to be done this way. Simply passing Tree in both calling scenarios and changing the formal argument to NODE *Tree (or NODE Tree[] ), and fixing all the code therein will work. An example is below (that utilizes dynamic allocations because my C compiler is dumber than bag of hammers and doesn't understand VLAs):

#include <stdio.h>
#include <stdlib.h>

struct NODE {
    int node_no;
    char color;
};

char Colors[6] = { 'a','b','c','d','e','f' };

// l->left index, right index is fixed at n-1, as we colors from  0 to n-1
void ColorTree(struct NODE Tree[], int l, int n, int Edges[][2], int e)
{
    if (n == 1)    // if only one node, color node with lowest color
    {
        Tree[0].color = 'a';
    }

    int i, j; char used_colors[6], c = 'N';

    for (i = 0; i < e; i++)   // colors 
    {
        if (Edges[i][0] == Tree[l].node_no)
        {
            c = Tree[Edges[i][1]].color;
            continue;
        }
        if (Edges[i][1] == Tree[l].node_no)
        {
            c = Tree[Edges[i][0]].color;
        }

        for (j = 0; used_colors[j] != '\0'; j++);  // finds empty position of used_colors
        if (c != 'N')
            used_colors[j] = c;   // if the color is not N, add to already used colors
    }

    for (i = 0; Colors[i] != '\0'; i++)
        for (j = 0; used_colors[j] != '\0'; j++)
        {
            if (Colors[i] == used_colors[j])   // the color is already used
                continue;
            else if (used_colors[j + 1] == '\0')     // the color is not used
                break;
        }

    Tree[l].color = Colors[i];   // the unused color being assigned to Tree[l]

    if (l == (n - 1))    // all nodes have been colored
        return;

    ColorTree(Tree, l + 1, (n - 1), Edges, e);
}

int main()
{
    int n, e, i;

    scanf("%d", &n);

    struct NODE *Tree = malloc(n * sizeof *Tree);

    for (i = 0; i<n; i++)     // initialising the tree, with node 0,1,2...n-1 and  color = 'N'
    {
        Tree[i].node_no = i;
        Tree[i].color = 'N';    // 'N'->no color
    }

    scanf("%d", &e);    // e-> no.of edges

    int(*Edges)[2] = malloc(e * sizeof *Edges);

    for (i = 0; i<e; i++)    // entering the edges
    {
        scanf("%d", &Edges[i][0]);
        scanf("%d", &Edges[i][1]);
    }

    ColorTree(Tree, 0, (n - 1), Edges, e);

    printf("%c", Tree[0].color);    // printing the colors in the graph
    for (i = 1; i<n; i++)
        printf("\n%c", Tree[i].color);

    return 0;
}

Whether this solves every issue you code faces beyond the stated question isn't guaranteed (for all I know it could be chock full of logic errors), but it will fix the problem(s) mentioned. I did, however, take liberty to fix the blatant overreach of the for-loop in ColorTree.