C++: Pass Two Arrays into A Function But the Results are not Expected by Me

Question

I'm new C++ programmer and just started doing some practice. I came up with the following code but didn't get the result expected by myself(very likely I'm wrong but I cannot make sense of it). I really appreciate your advice on it:

Here is my code:

#include <iostream>

using namespace std;

double simulate(double *p,double *v,double u)
{

    int i = 0;
    while (u>p[i])
    {
        u-=p[i];
        i++;
    }
    cout << v[i];
}
int main()
{
    double array1[] = {0.4,0.1,0.2,0.3};
    double array2[] = {1.1,2.2,3.3,4.4};
    simulate(array1,array2,0.5);
    return 0;
}

The results give 2.2 while I expect 3.3 because: after two loops, u becomes zero so cannot execute the third loop, since we executed two loops, i becomes 2 that corresponds to index 2 in array 2, which is 3.3 instead of 2.2. Could any expert help me with this? Thanks a lot in advance!

You should pass a `size` variable make sure you don't access past the end of your arrays. — drescherjm, Nov 14 '19 at 19:49
@farbiondriven Technically, it's `0.1 > 0.1`, but yes - the loop should only be executed once, if the problems described in [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) are not observed. — Algirdas Preidžius, Nov 14 '19 at 19:52
If you want to modify the original `u` variable, you need to pass it by reference. — Thomas Matthews, Nov 14 '19 at 21:29
The `double simulate(double *p,double *v,double u)` function also needs to return a `double`. It's UB to not return anything. — drescherjm, Nov 14 '19 at 22:13

score 1 · Accepted Answer · answered Nov 14 '19 at 20:38

You're incrementing I within the loop. Think of the logic.

-The while-statement does an if-check. -You then decrement u but increment i. I now points PAST where u becomes <= 0.

This is in addition to the fact you can run off the end of your arrays, if your initial u is too big.

An addition note: please use meaningful variable names. I know this is just a practice, but it's best to get into the right habits from the beginning. Your future coworkers will appreciate it.

C++: Pass Two Arrays into A Function But the Results are not Expected by Me

1 Answers1