# V7017\. A loop counter is not used for collection indexing in the inner loop\.

The analyzer has detected a possible error in two or more nested for loops, when the counter of one of the loops is not used because of a typo\.

The example:

```cpp
let sum = 0;
for (let i = 0; i < N; i++) {
  for (let j = 0; j < M; j++) {
    sum += matrix[i][i];     // <=
  }
}
```

In this case, it is intended to process all elements of a matrix and sum up them, but `i` is used instead of `j` when indexing the columns\.

The fixed code:

```cpp
let sum = 0;
for (let i = 0; i < N; i++) {
  for (let j = 0; j < M; j++) {
    sum += matrix[i][j]; 
  }
}
```