Question

I am pushing stuff into a priority Queue and I’ve made a comparison method for elements that go in there , when I checked the order , I realized I wanted elements to go in the other way around , so I went and changed all the boolean returns from my comparison method to be opposite of what they originally were and now it gives me a million assertion failures. If I ignore them , the program still works as expected , but why the assert fails ? Why should the program care ?

 bool operator()(charElement &operand1,charElement &operand2)
{
    // If the frequency of op1 < op2 then return true
    if(operand1.i_frequency < operand2.i_frequency) return true; //change to false

    // If the frequency of op1 > op2 then return false
    if(operand1.i_frequency > operand2.i_frequency)return false; //change to true

    // If the frequency of op1 == op2 then return true (that is priority is indicated to be less even though frequencies are equal)
    if(operand1.i_frequency == operand2.i_frequency)return true; //change to false

    // By default , return true (that is priority is indicated to be less)
    return true; //change to false
}

By the way, why was my query downvoted ? I try to ask sane questions and I believe this is one of them.

Specifics : I changed the operator definition to what @sehe suggested. Now if I add a not operator to the return value, it gives me assertion failures (invalid operator<) inside the include file , line number 2457. I checked the file , it seems that the error is in an inline method called Push_heap_0 , but I am stumped as to what that code does

Answer 1

Can’t you simply reverse the order of the parameters? That is, replace:

bool operator()(charElement &operand1,charElement &operand2)

with:

bool operator()(charElement &operand2,charElement &operand1)

Then you can leave the body of the function as it is.

On a side note, comparisons are read-only operations, and we should always respect const correctness:

bool operator()(const charElement& operand2, const charElement& operand1)

Also, you are not allowed to return true if the elements are equal:

if(operand1.i_frequency == operand2.i_frequency)return true;   // HUGE PROBLEM!

You must always return false in that case, no matter if you want “normal” or “backwards” order.

The opposite of < is not >, but >=. This seems to be hard for programmers to grasp.