Zero coefficients cause panic "invalid problem: Warning"

[evolutics]

Thanks for maintaining these LP libraries.

I've noticed the following when using good_lp with Highs, hence reporting it here.

Input

Example program:

use highs::{RowProblem, Sense};

fn main() {
    let mut problem = RowProblem::new();
    // Minimize x + y subject to x ≥ 0, y ≥ 0.
    let x = problem.add_column(1., 0..);
    let y = problem.add_column(1., 0..);
    let c = 0.; // Try changing this to something nonzero.
    problem.add_row(..1, &[(x, 1.), (y, c)]); // 1 ≥ x + c y.
    let solution = problem.optimise(Sense::Minimise).solve().get_solution();
    println!("{:?}", solution.columns());
}

Actual output

When run with highs 1.2.1, it panics at the optimise call:

thread 'main' panicked at 'invalid problem: Warning', …/highs-1.2.1/src/lib.rs:200:34

Expected output

It should just print the unique solution to the problem (x = y = 0) without panicking.

highs 0.6.1 works as expected.

Workaround

The issue disappears if the coefficient c is nonzero. Thus, one can skip terms c y when generating constraints by checking if the coefficient c is zero.

In the above example, calling problem.add_row(..1, &[(x, 1.)]) instead of problem.add_row(..1, &[(x, 1.), (y, c)]) works.

[lovasoa]

I fixed it by handling highs warnings as successes, just logging a warning. But the logs are quite useless at the moment, because there is no way to know what the problem was.

But @jajhall , it would be great if there was a way to programmatically access error messages from the C api, instead of trying to use side effects directly from inside HiGHS to perform IO. Something like Highs_errmsg, on the model of sqlite3_errmsg

[jajhall]

But @jajhall , it would be great if there were a way to programmatically access error messages from the C api, instead of trying to use side effects directly from inside HiGHS to perform IO. Something like Highs_errmsg, on the model of sqlite3_errmsg

Indeed, some system of retrieving the cause of the most recent error/warning message is planned. Thanks for the pointer

[evolutics]

Thanks both for the quick reactions.