error accessing array of structure pointers

Question

I am trying to build an adjacency list but get the following error

graph.c:16: error: subscripted value is neither array nor pointer

I read that this error occurs when a non array is trying to be indexed. When I am able to add an element to it directly (line 58: graph[i] = root), can I please know what is the error in assigning a member of the structure array to a NODE?

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define MAX 10
struct node{
    int data;
    struct node * link;
};

typedef struct node * NODE;
NODE graph[MAX];
void displayGraph (graph, n){
int i;
NODE cur;
for (i=0;i<n;i++){
    cur = graph[i];
    while(cur != NULL){
        printf("%d ", cur->data);
        }
    }
}

NODE insert (NODE root, NODE temp){
NODE mine;
mine = root;
    if (mine == NULL)
        return root;
    while(mine != NULL){
    mine = mine->link;
    }
mine->link = temp;
return root;
}

main ()
{
int n=0;
int i;
int val;
char * choice;
NODE temp, root;
printf("Enter the number of nodes\n");
scanf("&d", n);
for (i=0;i<n;i++){
    root = NULL;
    while(1){
        printf("Is there an adjacent node?Y:N");
        scanf("%s", choice);
        if(!strcmp(choice, "N"));
            break;
        printf("Enter the adjacent node\n");
        scanf("%d", val);
        temp = malloc(sizeof (struct node));
        temp->data = val;
        temp->link = NULL;
        root = insert(root, temp);
         }
graph[i] = root;
    }
displayGraph (graph, n);
}

On a side note there seems to be an error on line 45: scanf("&d", n);should be scanf("%d", &n); And also line 54 (?) should be scanf("%d", &val); — jpw
– jpw, Commented Oct 25, 2013 at 22:21
Your strcmp() will always result in break;. See my answer below. — ryyker
– ryyker, Commented Oct 25, 2013 at 23:27

Code Monkey · Accepted Answer · 2013-10-25 22:18:37Z

2

You haven't specified a type for the variable graph when you declared the displayGraph function.

void displayGraph (graph, n);

Seeing as graph is declared globally, you can technically omit graph as an argument to this function. You will also need to provide a type for the variable n, but if you insist on having displayGraph accept the graph array, then change:

void displayGraph (graph, n){

to

void displayGraph (NODE graph[], int n){

There are a few other problems with your code, but this should fix the error you are asking about.

answered Oct 25, 2013 at 22:18

Code Monkey

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Comments

ryyker · Accepted Answer · 2013-10-25 23:18:24Z

Besides the formatting rendering this code difficult to read, and understand, the problems in your code include:

In general, your calls to scanf() need syntax corrections:
i.e. scanf("&d", &n); should be scanf("%d", &n);
(% instead of & for first argument)

Line 12: void displayGraph (graph, n){
function definition should be:
void displayGraph (NODE * graph, int n){
I am assuming int n because int n=0; appears in the code before
and I am assuming NODE * because of declarations:
typedef struct node * NODE;
NODE graph[MAX];

cur; is a single instance of type NODE
graph[MAX]; is an array of instances of type NODE

Line 13: cur = graph[i]; is not a legal assignment.
You can to do something like this to initialize the pointer cur:

NODE graph[MAX], *cur;  
cur = &graph[0];//to initialize cur to the first position of graph

Line 48, scanf("%s", choice); choice is not initialized when created, do this:
char * choice;
then later, before you use it:
choice = malloc(NumBytes); //where NumBytes is a value of bytes large enough for the string you will scanf into it.

Line 49: the following lines result in unreachable code for line 51:

if(!strcmp(choice, "N"));
    break;              ^

Remove the ; after the if statement

Line 52: val has not been previously initialized, and here should be written &val

In the following code, I have addressed all of the mentioned issues, including formatting, however, I have not debugged your code, so I am not certain of anything beyond it will compile and build :)

#include<stdlib.h>
#include<string.h>
#define MAX 10

struct node{
    int data;
    struct node * link;
};

typedef struct node * NODE;
NODE graph[MAX];

void displayGraph (NODE * graph, int n);
NODE insert(NODE root, NODE temp);

main ()
{
    int n=0;
    int i;
    int val;
    char * choice;
    NODE temp, root;
    printf("Enter the number of nodes\n");
    scanf("%d", &n);
    for (i=0;i<n;i++)
    {
        root = NULL;
        while(1)
        {
            choice = malloc(2);//based on promted answer being only 1 byte long
            printf("Is there an adjacent node?Y:N");
            scanf("%s", choice);
            //if(!strcmp(choice, "N"))//case sensitive 
            if(!stricmp(choice, "N")) //not case sensitive
            {
                break;
            }
            printf("Enter the adjacent node\n");
            scanf("%d", &val);
            temp = malloc(sizeof (struct node));
            temp->data = val;
            temp->link = NULL;
            root = insert(root, temp);
            free(choice);
        }
        graph[i] = root;
    }
    displayGraph (graph, n);
}

void displayGraph (NODE * graph, int n)
{
    int i;
    NODE cur;
    for (i=0;i<n;i++)
    {
        cur = graph[i];
        while(cur != NULL)
        {
            printf("%d ", cur->data);
        }
    }
}

NODE insert(NODE root, NODE temp)
{
    NODE mine;
    mine = root;
    if (mine == NULL) return root;
    while(mine != NULL)
    {
        mine = mine->link;
    }
    mine->link = temp;
    return root;
}

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