“Invalid operands to binary expressions” Error

Question

I keep getting this error:

Invalid operands to binary expressions ('struct node'and 'int')

on two lines that I've marked below with ** in the function "reachR". Whats wrong and how can I fix it? The code is for a data structure assignment.

#include <stdio.h>
#include <stdlib.h>
#define Vertex int
#define maxV 10000

typedef struct digraph *Digraph;
typedef struct node *teste;
static int lbl[maxV];

typedef struct {
    Vertex v, w;
} Arc;

struct digraph {
    int V;
    int A;
    teste *adj;
};

struct node {
    Vertex w;
    teste next;
};


teste NEWnode( Vertex w, teste next) {
    teste a = malloc( sizeof (struct node));
    a->w = w;
    a->next = next;
    return a;
}

Arc ARC( Vertex v, Vertex w) {
    Arc a;
    a.v = v, a.w = w;
    return a;
}

void reachR( Digraph G, Vertex v) {
    Vertex w;
    lbl[v] = 1;
    for (w = 0; w < G->V; w++)
        **if (G->adj[v][w] == 1 && lbl[w] == 0)**
            reachR( G, w);
}

int DIGRAPHreach( Digraph G, Vertex s, Vertex t) {
    Vertex v;
    for (v = 0; v < G->V; v++)
        lbl[v] = 0;
    reachR( G, s);
    if (lbl[t] == 0) return 0;
    else return 1;
}

int digraphcycle( Digraph G) {
    Vertex v; teste a; int output;
    for (v = 0; v < G->V; v++)
        for (a = G->adj[v]; a != NULL; a = a->next) {
            output = DIGRAPHreach( G, a->w, v);
            if (output == 1) return 1;
        }
    return 0;
}

Digraph DIGRAPHinit( int V) {
    Vertex v;
    Digraph G = malloc( sizeof *G);
    G->V = V;
    G->A = 0;
    G->adj = malloc( V * sizeof (teste));
    for (v = 0; v < V; v++)
        G->adj[v] = NULL;
    return G;
}

void DIGRAPHinsertA( Digraph G, Vertex v, Vertex w) {
    teste a;
    for (a = G->adj[v]; a != NULL; a = a->next)
        if (a->w == w) return;
    G->adj[v] = NEWnode( w, G->adj[v]);
    G->A++;
}

int main (){
    Digraph G = DIGRAPHinit(4);
    DIGRAPHinsertA(G, 1, 2);
    DIGRAPHinsertA(G, 1, 3);
    DIGRAPHinsertA(G, 2, 3);
    DIGRAPHinsertA(G, 3, 4);
    if (digraphcycle(G)==1){ 
        printf("SIM!"); 
    } 
}

I tried to replace with the one you suggested and the compiler said to use G->adj[v]->w Is it the same thing? — Marcos Guimaraes, Mar 02 '16 at 19:31
I haven't noticed that adj is array of pointers, you should use `->` — Iłya Bursov, Mar 02 '16 at 19:31
Thanks for the correction. I tried to run the code but it gave me another error message on that line. It says "Thread 1: EXC_BAD_ACCESS (code=1, address=0x2)" and it doesn't run. Do you have any idea of what does that mean? — Marcos Guimaraes, Mar 02 '16 at 19:38
it means that you're accessing unallocated memory, use debugger — Iłya Bursov, Mar 02 '16 at 19:38
I don't know, `Digraph G = malloc( sizeof *G);` looks suspicions though — Iłya Bursov, Mar 02 '16 at 19:48
Thank you very much, I'll take a look at this. Really thanks! — Marcos Guimaraes, Mar 02 '16 at 19:53

score 0 · Accepted Answer · answered Mar 02 '16 at 19:28

You are trying to compare a struct with an int. Obviously cannot be done. The equals operator (==) is a binary one, and it requires two compare-able objects on both sides.
You can access the value of the Vertex (an int) like mentioned in the comments by performing
G->adj[v]->w

“Invalid operands to binary expressions” Error

1 Answers1