#include <iostream>

// ALWAYS USE CONST
//     this is what happens when you don't

void experiment(int a, int b, int c, int d)
{ 
  // if a is less than b, it is very bad
  // if c is less than d, it is also very bad

  if (a < b || c < d)
    print("very bad\n");

  // if c is between 3 and 7 or between 13 and 17, it is sort of ok

  if (c >= 3 && c <= 7  ||  c >= 13 && c <= 17)
    print("sort of ok\n");

  // if b is equal to 9 it is a disaster

  if (b = 9)
    print("disaster\n");

  // if a, b, and c are all different, it isn't the end of the world

  if (a != b  &&  a != c  &&  b != c)
    print("it isn't the end of the world\n");
}

This function works as intended, except for one critical part.
No matter what inputs I give it, it always tells me it is a disaster.

If I give b the value 3, it says "disaster", and if I look, I
find that b actually changed to 9 just to make it true.

Of course the problem is that I accidentally typed b = 9 instead
of b == 9. This happens a lot, and can take a long long time to
uncover.

The correct thing to do is to start the function like this

void experiment(const int a, const int b, const int c, const int d)
{ ...

a, b, c, and d are not supposed to change during this function.
Saying const makes the compiler do all the work of making sure
they don't. It will now give me an error message as a warning
with this and many other subtle errors.