Can a function return the same value inside a loop, and return different values outside of loops?

Question

It acts like this.

fun();//return 1;
for (int i=0;i++;i<100)
    fun();//return 2;
fun();//return 3;

I don't want to do it manually, like:

static int i=0;
fun(){return i};
main()
{
    i++;
    fun();//return 1;
    i++;
    for (int i=0;i++;i<100)
        fun();//return 2;
    i++;
    fun();//return 3;
}

New classes and static variables are allowed.

I am trying to design a cache replacement algorithm. Most of the time I use the LRU algorithm, but, if I use LRU algorithm inside a loop I would very likely get a cache thrashing.

https://en.wikipedia.org/wiki/Thrashing_(computer_science)

I need to know if I am inside a loop. Then I can use the LFU algorithm to avoid thrashing.

A function can return different values at different times. You'll have to come up with the logic of why. The how will be easy once you figure out why. — R Sahu, Oct 03 '15 at 05:43
No ,that won't help .The only useful answer here is the __LINE__ macro .That wouldn't be a result of any kind of logic . — iouvxz, Oct 03 '15 at 08:09

score 1 · Accepted Answer · answered Oct 03 '15 at 05:30

1

An obvious way of doing this would be using the __LINE__ macro. It will return the source code line number, which will be different throughout your function.

answered Oct 03 '15 at 05:30

Blindy

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could you please elaborate it in details with an example – surajs1n Oct 03 '15 at 06:16
Depending somewhat on what @iouvxz is trying to do, this is of course terrible, terrible practice. – Andreas Oct 03 '15 at 08:27
Excellent practice for my case . – iouvxz Oct 03 '15 at 09:55
@psyco, example of what? Simply use it and it will return you the line number. – Blindy Oct 03 '15 at 16:35
@iouvxz, just keep in mind it could return the same ID when used in different files on the same line number. Consider appending `__FILE__` to combat this if needed. – Blindy Oct 03 '15 at 16:36

score 0 · Answer 2 · edited Oct 03 '15 at 17:47

It is not possible within c++ for a function to know whether or not it is inside a loop 100% of the time. However, if you are happy to do some manual coding to tell the function that it is inside a loop then making use of C++'s default parameters you could simply implement a solution. For more information on default parameters see http://www.learncpp.com/cpp-tutorial/77-default-parameters/. Also, because Global variables are generally frowned upon I have placed them in a separate namespace in order to prevent clashes.

namespace global_variables {
    int i = 0;
}

int func(bool is_in_loop = false) {
if (is_in_loop)
    {
            //do_something;
            return global_variables::i;
    }

else 
    {
            //do_something_else;
            return global_variables::i++;
    }
}

int main()
{

    // Calling function outside of loop
    std::cout << func();

    // Calling function inside of loop
    for (int j=0;j<100;j++)
    {
            // is_in_loop will be overided to be true.
            std::cout << function(true);
    }

    return 0;
}

Can a function return the same value inside a loop, and return different values outside of loops?

2 Answers2