When I change the variable that is in an array, why does it not update the array?-CodePudding

Essentially my question is that, in C and C , if I update a variable that I have previously placed in an array, why does it not update the array?

Ex:

int var = 0;  
int list[1];  
list[0] = var;  
var = 1;

printf("%i", list[0]);

I don't understand why it is printing 0 instead of 1.

CodePudding user response：

list[0] = var; copies the value in var into list[0] - var and list[0] are two separate ints.

If you want changes to one variable to be seen through another varible, you need to use pointers or references. Example:

int main() {
    int var = 0;  
    int* list[1];   // an array of pointers to `int`
    list[0] = &var; // list[0] now points at `var` 
    var = 1;
    printf("%i\n", *list[0]); // dereferencing list[0] to get the value in `var`
}

Output:

CodePudding user response：

The variable var is a separate object relative to the array list

int var = 0;  
int list[1];

They occupy different extents of memory.

In this assignment

list[0] = var;

the copy of the value of the variable var is assigned to another object list[0]. var and list[0] have no any relation between each other.

You could declare an array of pointers like for example

#include <stdio.h>

int main( void )
{
    int var = 0;  
    int * list[1];

    list[0] = &var;

    var = 1;

    printf( "*list[0] = %d\n", *list[0] );  
}

The program output is

*list[0] = 1

In this case the pointer list[0] points to the variable var. So using the pointer you can output the current value of the variable var.

Pay attention to that even in C you may not declare an array of references. Similarly to the shown above approach with pointers you could declare an array of objects pf the type std::reference_wrapper as for example

#include <iostream>
#include <functional>

int main()
{
    int var = 0;  
    std::reference_wrapper<int> list[1] = { var };

    var = 1;

    std::cout << "list[0] = " << list[0].get() << '\n';  
}

The program output is

list[0] = 1