Predict the Net (Exercise)

The brief. The whole lesson is one line — net = gross + ceded, with ceded stored negative. Simple, so you already know every net, right? Predict the net for all seven claims, then press Run. Each row starts pre-filled with the naive guess that treats ceded as a positive amount to add. Every ✗ is a place the sign rule bit you — and the sign rule is the whole point.

This is Lesson 1's discipline in miniature: before you trust a cession — or a cube that sums over it — pin what the arithmetic actually does. Storing ceded as a negative amount is what lets every later lesson net by plain summing; here you prove to yourself, row by row, that adding a negative is the cession.

The rules

Gross and Ceded are the givens — the loss book with a flat facultative cession already applied. Only your prediction moves.
The Your net column is pre-filled with the naive guess gross + |ceded| — treating ceded as a positive number to add. Some rows agree; most do not.
Run, read the ✗ rows, and change each Your net to match the Net column until all seven say ✓. You have then internalised the sign rule.

No peeking at the solution until at least one row has surprised you.

Your workbench

using MeshWeaver.Layout;
using MeshWeaver.Layout.Composition;
using MeshWeaver.Layout.DataGrid;
using MeshWeaver.Utils;

// THE SIGN RULE — ceded is stored NEGATIVE, and net = gross + ceded. That is all there is.
// Your job: PREDICT the net for each claim. The `Your net` column is pre-filled with the
// naive guess that treats ceded as a POSITIVE amount to add (gross + |ceded|) — exactly the
// mistake the sign rule exists to kill. Some rows agree; most do not.
// Change each prediction to match the Net column and re-run until every row says ✓.
// Do NOT touch Gross or Ceded — those are the givens; only your prediction moves.
record Row(string Claim, double Gross, double Ceded, double Predicted, double Net, string Result);

// (Claim, Gross, Ceded [stored NEGATIVE], Predicted [<-- your guess])
(string Claim, double Gross, double Ceded, double Predicted)[] cases =
[
    ("C1",   80,    0,   80),   // nothing ceded — this one already agrees
    ("C2",  240,  -40,  280),   // 👉 your prediction — is +(-40) really +40?
    ("C3",  420, -120,  540),
    ("C4",  760, -100,  860),
    ("C5", 1300, -300, 1600),
    ("C6",   90,  -90,  180),
    ("ATT", 350,  -50,  400),
];

var rows = cases.Select(c =>
{
    var net = c.Gross + c.Ceded;                         // the sign rule — no subtraction anywhere
    var ok = Math.Abs(c.Predicted - net) < 1e-9;
    return new Row(c.Claim, c.Gross, c.Ceded, c.Predicted, net, ok ? "✓ correct" : "✗ mind the sign");
}).ToArray();

var right = rows.Count(r => r.Result.StartsWith("✓"));

Controls.Stack
    .WithView(Controls.H4($"Nets predicted — {right}/{rows.Length} correct"))
    .WithView(Controls.DataGrid(rows)
        .WithColumn(new PropertyColumnControl<string> { Property = nameof(Row.Claim).ToCamelCase() }.WithTitle("Claim"))
        .WithColumn(new PropertyColumnControl<double> { Property = nameof(Row.Gross).ToCamelCase() }.WithTitle("Gross").WithFormat("N0").WithAlign("end"))
        .WithColumn(new PropertyColumnControl<double> { Property = nameof(Row.Ceded).ToCamelCase() }.WithTitle("Ceded").WithFormat("N0").WithAlign("end"))
        .WithColumn(new PropertyColumnControl<double> { Property = nameof(Row.Predicted).ToCamelCase() }.WithTitle("Your net").WithFormat("N0").WithAlign("end"))
        .WithColumn(new PropertyColumnControl<double> { Property = nameof(Row.Net).ToCamelCase() }.WithTitle("Net = gross + ceded").WithFormat("N0").WithAlign("end"))
        .WithColumn(new PropertyColumnControl<string> { Property = nameof(Row.Result).ToCamelCase() }.WithTitle("Result"))
        .Resizable(false))
    .WithView(Controls.Markdown(
        $"**Book totals:** gross **{rows.Sum(r => r.Gross):N0}** · ceded **{rows.Sum(r => r.Ceded):N0}** · net **{rows.Sum(r => r.Net):N0}**. Plain summing over the book gives net — precisely because ceded is negative."))

When every row says ✓, you no longer think the sign rule works — you have run it seven times and watched adding a negative do the cession, on every row and in the totals.

Surprised at least once? Reveal the solution — the same seven claims with their true nets and why +(−100) is the whole cession.

← Lesson 1 exercises · Lesson 1 — From gross to net