Differentiate between a number (of type double) with decimal places and one without - c++

Question

I am trying to implement a simple decimation algorithm in c++. I have two arrays, say p & q, where the subscripts are related to each other by the following relation:

p[k] = q[0.5*k]. This means that the following sequence should hold valid:

p[0] = q[0]  
p[1] = 0  
p[2] = q[1]  
p[3] = 0  
p[4] = q[2]  

and so on...

Please note that p[k] takes on a value only and only when the result of (0.5*k) contains no decimal places (or has 0 in decimal) and does not use any rounding off etc.

My question is: Is there a way to distinguish between an integer (a number with no decimal places or only 0 in decimal, say 2.0) and a number with decimal places in C++, provided both are cast to double?

eg.) 2.0 is an integer cast to double. 2.1 is a number with decimal places.
eg. 2) * 0.9*2 should put 0 into array p while 0.9*10 should put q[9] into array p.*

If I use the statement, (int) (0.5*k), then I end up with an integer in every case, irrespective of the value of k.

Edit: The 0.5 in the above case is only illustrative. It could be any number, say 2, 2.5, 0.9, 0.95 etc.)

Any help is most welcome,
Thanks,
Sriram.

Answer 1

Assuming k is of an integer type, you could use if (k % 2 == 0) ... to check if kis divisible by two:

if (k % 2 == 0)
  p[k] = q[k / 2];
else
  p[k] = 0;

This can also be expressed using the ternary operator:

p[k] = (k % 2 == 0) ? q[k / 2] : 0;

Answer 2

Presuming that the coef can be anything else,

p[floor(coef*k)] = (fabs(coef*k-floor(coef*k))<1E-6)?q[k]:0;