I am new to C , below is a code to input and display an array , a simple thing to understand how thing work..
int main() {
int N, i;
int A[N];
cin >> N;
for (i = 0; i < N; i ) {
cin >> A[i];
}
for (i = 0; i < N; i ) {
cout << A[i];
}
return 0;
}
the input is :
4
1 2 3 4
and output is :
12
i have corrected my code to:
int N, i;
cin >> N;
int A[N];
This correctly displays the array.
What exactly happened? Should N have a value before initializing A[N]? But then why didnt my initial code work? Is this something to do with Hierarchy? If so how can i get my priorities right
edit: If N is initialized as a large value, N has an input of specific number next, if so how come even if N= 9, the output remains same: 12.
CodePudding user response:
Should N have a value before initializing A[N]
Yes. It should. It should even have a value before declaring A[N]
.
How else would the runtime know how much space to allocate for A
?
CodePudding user response:
To answer your question.
int N, i;
Here, you declare N
, but because you don't assign a value to N
, N
is uninitialized and can hold any value (probably some old program's value.)
And you create an array with N
, which will lead to some unexpected behavior. (for example, a stack overflow could happen when N
is too big)
And:
The size of arrays in C has to be known at compile-time. That means when you compiler compiles the program, the array's size has to be known.
In your case, you know n
(the size) at runtime, which means only when you run the program, you know the value of n
, which is the size.
Your code run because (probably) gcc
compiler does have an extension for this to happen, but because this is not standard C , I would recommend not using it.
C has a solution for runtime array, and that is std::vector
in the <vector>
header.
You just need to change:
cin >> N;
int A[N];
to:
std::cin >> n;
std::vector<int> A(N);
note: using namespace std;
is bad, so don't use it.
CodePudding user response:
In C , the size of an array must be a compile time constant. So the following code is incorrect:
int N = 10;
int arr[N]; //incorrect
The correct way to write this would be:
const int N = 10;
int arr[N]; //correct
For the same reason the following code is incorrect in your given code snippet:
int N, i; //both N and i are **unitialized**(meaning they have **garbage value**)
cin >> N; //this is fine. This takes input from user and put it in N
int A[N]; //incorrect because N must be a constant expression
Also note that your original question used the garbage value of N to create an array. This is why it is advised that always initialize built in type in block/local scope.