c++ error: invalid types ‘int[int]’ for array subscript for single dimensional array

Question

I am trying to write a program to find prefix sum. It is showing error: invalid types 'int[int] for array subscript for single dimensional array code1

#include <bits/stdc++.h>
using namespace std;

long long a[(int)1e5 + 50];
int n;

void buildPrefixSum() { // O(n)
   for(int i = 1; i < n; i++) {
       a[i] += a[i - 1];
   }

}

int getSum(int i, int j) { // O(1)
   int sum = a[j];
   if(i > 0) sum -= a[i - 1];
   return sum;
}

int main()
{
    ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    int t,a,b;
    cin >> n;
    for(int i = 0; i < n; i++)
    {
        cin >> a[i];
    }
    buildPrefixSum();
    cin >> t;
    while(t--)
    {
        cin >> a >> b;
        cout << getSum(a,b) << "\n";
    }
}

But when I change the main function to this, the code runs properly.code2

int main() {
    cin >> n;
    for(int i = 0; i < n; i++) {
        cin >> a[i];
    }
    buildPrefixSum();
    int q;
    cin >> q;
    while(q-- > 0) {
        int a, b;
        cin >> a >> b;
        cout << getSum(a, b) << endl;
    }
    return 0;
}

I don't understand the difference between the two codes. Can anyone please clarify the mistake I made in the main function of the first code?

Look at where you're declaring a and what types you're declaring. — molbdnilo
– molbdnilo, Commented Jul 20, 2020 at 6:20
In the first snippet, in main(): int t,a,b; The local variable a eclipses the global variable with same name. (The global is still there but you cannot access it in the local scope.) — Scheff's Cat
– Scheff's Cat, Commented Jul 20, 2020 at 6:25

paxdiablo · Accepted Answer · 2020-07-20 07:06:39Z

1

In you first code block, your long long array a is being "shadowed" (hidden) by the a in the line:

int t,a,b;

You'll find the problem boils down to this:

int main() {
    int a;
    a[0] = 7; // error: invalid types 'int[int]' for array subscript
}

In your second code block, the declaration of the non-array a doesn't happen until after you've populated the array a - the shadowing only exists within the while loop and therefore does not raise an error.

I suspect this problem may not have occurred had you just used decent variable names :-)

edited Jul 20, 2020 at 7:06

answered Jul 20, 2020 at 6:24

paxdiablo

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