# V791. The initial value of the index in the nested loop equals 'i'. Consider using 'i + 1' instead.

The analyzer has detected a faulty or suboptimal loop. A standard pattern is used where some operation is executed for every pair of elements of an array. This operation, however, is usually not required for a pair whose members are the same element, i.e. when 'i == j'.

For example:

    for (int i = 0; i < size; i++)
      for (int j = i; j < size; j++)
        ...

It is highly possible that a more correct and effective way to traverse the arrays would probably be this:

    for (int i = 0; i < size; i++)
      for (int j = i + 1; j < size; j++)
        ...

The next example is taken from a real application; the authors implemented their own algorithm to sort devices by priority but it is non-optimal:

    /* Simple bubble sort */
    for (i = 0; i < n_devices; ++i) {
      for (uint32_t j = i; j < n_devices; ++j) {
        if (devices[i]->prio > devices[j]->prio) {
          struct device_t *tmp;
          tmp = devices[i];
          devices[i] = devices[j];
          devices[j] = tmp;
        }
      }
    }

Fixed code:

    /* Simple bubble sort */
    for (i = 0; i < n_devices - 1; ++i) {
      for (uint32_t j = i + 1; j < n_devices; ++j) {
        if (devices[i]->prio > devices[j]->prio) {
          struct device_t *tmp;
          tmp = devices[i];
          devices[i] = devices[j];
          devices[j] = tmp;
        }
      }
    }

This diagnostic is classified as:

CWE-691

You can look at examples of errors detected by the V791 diagnostic.