Finding the distance between two points in C++
I think @john has made some excellent comments. I agree with them, but you've already gotten them, so I'll try not to repeat the same.

Global variables

Variables defined outside any function, like these:

int ch;
double x;
double y;
double a;
double b;
double answer;


...are generally called "global", because anything, anywhere in the world program can depend on them and/or modify them.

When some parts of the code modify them and other parts depend on them, it can become difficult to figure out how the code will act under any given circumstances, how to get it to to exactly what you really want, and so on.

Usually, you want to restrict each variable to (approximately) the smallest scope necessary for it to do its job. None of these variables is used outside of the main function, so we can define them there to assure that only the code in main can use them:

int main(){
    double x;
    double y;
    double a;
    double b;
    double answer;

        cout &lt;&lt; "Enter the points for the coordinates";
        cout &lt;&lt; endl;
        cout &lt;&lt; "Point x for first coordinates: ";
        cin &gt;&gt; x;
        cout &lt;&lt; endl;
        cout &lt;&lt; endl;
        cout &lt;&lt; "Point y for first coordinate: ";
        cin &gt;&gt; y;
        cout &lt;&lt; endl;
        cout &lt;&lt; endl;
        cout &lt;&lt; "Point x for the second coordinate: ";
        cin &gt;&gt; a;
        cout &lt;&lt; endl;
        cout &lt;&lt; endl;
        cout &lt;&lt; "Point y for the second coordinate: ";
        cin &gt;&gt; b;
        cout &lt;&lt; endl;
        cout &lt;&lt; endl;
        answer = distanceBetweenTwoPoints(x, y, a, b);
        cout &lt;&lt; "The answer is " &lt;&lt; answer;
}


Avoid std::endl

Although it's used in a lot of tutorials and such, std::endl is almost never what anybody really wants. Although it won't make any real difference as you've used it here, using std::endl when (for example) you're writing lots of data to a file can easily make your program run 10x slower than it really needs to1.

Use functions

You may not have learned (much) about functions yet, but they can make your life much easier, largely by reducing repetition. For example, quite a bit of the code in main consists of a number of repetitions of the same basic sequence:

Ask user for input
Read the input


You can make your life considerably simpler by moving repeated actions like this into functions:

double get_value(char const *prompt) { 
    double value;

    std::cout &lt;&lt; "\n" &lt;&lt; prompt;
    std::cin &gt;&gt; value;
    return value;
}


Then the code in main to get values can be shortened a bit, to something like this:

cout &lt;&lt; "Enter the points for the coordinates\n";
x = get_value("Point x for first coordinate: ");
y = get_value("Point y for first coordinate: ");
a = get_value("Point x for second coordinate: ");
b = get_value("Point y for second coordinate: ");


This not only reduces the amount of typing involved, but when you make a mistake (and we all do) it improves the chance that you've only made it in one place, so when you fix it, it'll be fixed in general.

Mathematics

You should usually think twice about computing a hypotenuse at all. In a lot of cases (e.g., comparing one distance to another) you can add the squares of the X/Y coordinates, but not take the square root of the result. For a lot of common cases like "find which point is closest to the one I clicked", the square of the distance works just as well as the actual distance, but is much faster to compute (sqrt is often relatively slow).

If you do need to compute a hypotenuse, consider using std::hypot instead of re-implementing it yourself. In the worst case, this will just be a thoroughly tested and debugged version of the code you'd write on your own. In a better case, it might use a different algorithm to compute the result. For example, you might have a case where both your \$\Delta x\$ and \$\Delta y\$ are within range and the final answer would be within range, but \$\Delta x^2 + \Delta y^2\$ is out of range. In this case, the simplistic approach will overflow, but some others won't. Depending on your hardware and/or required precision, alternatives may also be faster.

Avoid using namespace std;

This using directive brings a huge number of names into scope, which can lead to all sorts of problems from colliding with names you try to define. Almost nobody really knows all the names it defines, so it's often hard to avoid them.

It's (by far) easier in the long term to bite the bullet and put up with typing things like std::cout when you need to use them.

Note that this warning applies primarily (maybe even exclusively) to the std namespace. Something like using namespace std::chrono; is often perfectly fine.

If you are going to have a using namespace std;, you usually want to restrict it to the scope of a single function:

int main() { 
     using namespace std;

}


This can still lead to the same basic problems, but at least you've restricted the scope, so when a problem arises, cleaning up the mess is less work.




1. In case you care about the reason: making an operating system call to write data to a stream usually has a fair amount of overhead, so each call takes a fair amount of time, even to write a small amount of data. To avoid this, the code in the library stores data in a buffer. It only calls the OS to write the data to the external file once in a while, when the buffer gets full.



std::endl writes a new-line to the output stream (which you want) and it flushes the stream's buffer--forces an immediate OS call to write the data in the buffer, even if it's not full (which you almost never want). For interactive input like this, it won't actually hurt much, but as a simple rule of thumb, I'd just avoid using it at all. In the (rare) cases that you really do want to flush a stream's buffer, use std::flush to do it.