[SOLVED] What is the purpose of "!" in this code from "Point in Polygon Test"?

What is the purpose of "!" in this code from "Point in Polygon Test"?

Point in Polygon Test Code

int pnpoly(int nvert, float *vertx, float *verty, float testx, float testy)
{
  int i, j, c = 0;
  for (i = 0, j = nvert-1; i < nvert; j = i++) {
    if ( ((verty[i]>testy) != (verty[j]>testy)) &&
     (testx < (vertx[j]-vertx[i]) * (testy-verty[i]) / (verty[j]-verty[i]) + vertx[i]) )
       c = !c;
  }
  return c;
}

What is the purpose of ! in c = !c; ?

Solution

Any non-zero number or non-null pointer is considered to have a logical value of "true", whereas a number with value 0 or a null pointer is considered to have a logical value of "false".

The ! operator is the logical NOT operator (as opposed to a bit-wise NOT operator). It inverts the logical value of its operand, producing the integer value 0 if the operand has a logical value of "true", and the value 1 if the operand has a logical value of "false".