V520. The comma operator ',' in array index expression

30.05.2012

The analyzer found a potential error that may be caused by a misprint. An expression containing the ',' operator is used as an index for an array.

Here is a sample of suspicious code:

float **array_2D;
array_2D[getx() , gety()] = 0;

Most probably, it was meant to be:

array_2D[ getx() ][ gety() ] = 0;

Such errors might appear if the programmer worked earlier with a programming language where array indexes are separated by commas.

Let's look at a sample of an error found by the analyzer in one project:

float **m;
TextOutput &t = ...
...
t.printf("%10.5f, %10.5f, %10.5f,\n%10.5f, %10.5f, %10.5f,\n%10.5f, %10.5f, %10.5f)",
  m[0, 0], m[0, 1], m[0, 2],
  m[1, 0], m[1, 1], m[1, 2],
  m[2, 0], m[2, 1], m[2, 2]);

Since the printf function of the TextOutput class works with a variable number of arguments, it cannot check whether pointers will be passed to it instead of values of the float type. As a result, we will get rubbish displayed instead of matrix items' values. This is the correct code:

t.printf("%10.5f, %10.5f, %10.5f,\n%10.5f, %10.5f, %10.5f,\n%10.5f, %10.5f, %10.5f)",
  m[0][0], m[0][1], m[0][2],
  m[1][0], m[1][1], m[1][2],
  m[2][0], m[2][1], m[2][2]);
You can look at examples of errors from real projects which were detected by this diagnostic message.