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V791. The initial value of the index in…
Analyzer Diagnostics
General Analysis (C++)
General Analysis (C#)
General Analysis (Java)
Diagnosis of micro-optimizations (C++)
Diagnosis of 64-bit errors (Viva64, C++)
MISRA errors
AUTOSAR errors
Additional information
Contents

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

Jun 01 2017

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:

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