One program using three functions to compare two double-precision numbers
Your functions and their use are very curious:


A function to determine if two values are similar should be symmetric in its arguments, and return a bool.
A function determining the truth of a property should be called is@. similarValue is a bit of a puzzler, which is actually good as the functions behavior is very surprising.
A conditional expression returns a bool, which can be implicitly converted to an integer of value 0 (false) or 1 (true).
I'm not sure what I should do with valueSmaller and valueLarger, but I am sure that I don't want to see anything like them ever again.


Other points:


Seems you also don't know the conditional-operator (expr ? value-if-true : value-if-false). It can be used to write things better, but beware of getting carried away with it.
You are using using namespace std;.
That's a bad idea, read "Why is “using namespace std;” considered bad practice?".
When you output string- and char-literals consecutively, just concatenate them together. Also, if you have a length-1-string-literal, a char-literal can serve the same purpose more efficiently.
Your comments are, I'm sorry to say, useless, as they just rephrase some of the more obvious code.
When you use descriptive names, most other comments for describing what code does also become superfluous (exception: doc-comments which will be extracted for bare-bones documentation).
Comments are for giving non-obvious reasons for doing things you do, reasons for doing them the way you do them, or describing how you do something.
See this good answer by rolfl about how to properly comment.