Strange error of vector when recieving inputs

Question

Hi , what does this error show ? " request for member "push_back" in 'adj' , which is non-class type 'std::vector [100005] "
I declared a vector (out of main function) and made a for loop to get some inputs :
void Defines(){
#define pb push_back
}
int n,m;
const int N =1e5 + 5;
vector<int> adj[N];
...
int main(){
Defines();
cin>>n>>m;
for(int i=1; i<=m; i++){
int a,b; cin>>a>>b;
adj.pb(a);
adj.pb(b);
}
... }

Answer 1

Ali_combination
Just to further explain how macros work, they are a pre-processor directive, they are evaluated just prior to compilation of your program. Typically, they are placed at the top of the file outside of any function. The file is parsed for macros prior to being compiled and then the macros are used to do a literal replacement within the program and then compilation occurs.

So, with;
#define hello "Hello, World!"
...
...
cout << hello;

The hello text in the file will literally be changed to the value "Hello, World!" so that just before it is compiled it will read.
cout << "Hello, World!";

Answer 2

Rohit
What's the point of the Defines() function in your code? Putting a macro definition inside a function does nothing. The macro will be defined irrespective of whether you call the Defines() function or not.

Answer 3

XXX i tried to keep the code as similar to the Ali_combination's example code.

Yeah I agree with you, there's no point to put macros inside function.

Answer 4

Ali_combination What @XXX pointed out is true, no need to call Defines() inside the main function because macros will be defined irrespectively like he said.

Just comment line 20 in my code above and run, you will see it still works so you got the point

Example:
#include <iostream>
void LOL(){
#define hello "Hello world"
}
using namespace std;

int main() {
// notice we didn't call the function LOL here
cout << hello;
}

O/P: Hello world

Answer 5

Rohit oh really sorry, I didn't see that it was the OP's code

Answer 6

Hello Ali_combination ,

adj[N] with square brackets "[ ]" is for defining the size of an array and not vector in c++, for vector we use parenthesis "( )" like adj(N)

edit:
// check this code to see it
/*
2 3
6 5
7 5
4 6
*/ // INPUT
#include <iostream>
#include <vector>
using namespace std;

void Defines(){
#define pb push_back
}

int n,m;
const int N =1e5 + 5;
vector<int> adj(N);

int main(){
Defines(); // no need to call, it'll still works
cin>>n>>m;
for(int i=1; i<=m; i++){
int a,b; cin>>a>>b;
adj.pb(a);
adj.pb(b);
}
}

Answer 7

Rohit I got the point fellas..thank you =)

Answer 8

ChaoticDawg thanks for your nice explanation:))

Answer 9

The problem is that you're accessing the vector outside of its bounds. The vector is empty, but v[i] tries to access elements of the vector that don't exist. Therefore the behavior of the program is undefined.

I suspect that you may have intended to use the vector's constructor that takes a count of elements as an argument.

This May Help,
Peter

Answer 10

Rohit Hello gentleman , thank you so much .

Answer 11

XXX hmm..i just wanted my code to be cleaner . So I made a void function and declared a macro definition . When I call it in my main function , I actually use #defines(isnt it the point?)

Answer 12

Peter thank you Peter

Answer 13

Adding to the category of "another way to get this error" that I don't see already list:
If you have a class method, and you forget to include the object as your first argument (typically by calling the method with obj. before the method), you can get this error.
Also, you will not be able to validate the correct number of arguments with nargin('functionName') either, because it will report -1 for your class methods.

Regards,
Rachel Gomez

Answer 14

https://www.sololearn.com/discuss/3209319/?ref=app

Answer 15

Good Time (Travel) excuse me? What is this ? What do you mean by that ?

Answer 16

The error message you're seeing, "request for member 'push_back' in 'adj', which is non-class type 'std::vector [100005]'", indicates that the variable 'adj' is not being recognized as a vector.

The issue seems to stem from how you're using the 'pb' macro (defined in the 'Defines' function) to push elements into the vector 'adj'. The 'pb' macro is intended to be used with vectors, but in your code, it's being used with 'adj', which is an array of vectors. This is likely causing the error.

To fix this, you should push elements into the correct vector within the 'adj' array. Here's the corrected code:
#include <iostream>
#include <vector>
using namespace std;

void Defines() {
#define pb push_back
}

int n,m;
const int N = 1e5 + 5;
vector<int> adj[N];

int main() {
Defines();
cin >> n >> m;
for(int i = 1; i <= m; i++) {
int a, b;
cin >> a >> b;
adj[a].pb(b); // Push 'b' into the vector at index 'a' in the 'adj' array
adj[b].pb(a); // Push 'a' into the vector at index 'b' in the 'adj' array
}
// Additional code...
return 0;
}
In this corrected code, 'adj[a]' and 'adj[b]' represent the vectors at indices 'a' and 'b' respectively in the 'adj' array. By using 'pb' with 'adj[a]' and 'adj[b]', you're correctly pushing elements into the respective vectors.
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