# Issue 3: [BUG] PMT/FV/PV/NPER silently ignore the 5th "type" argument (annuity-due)

[Sh-TB]

Issue 3: [BUG] PMT/FV/PV/NPER silently ignore the 5th "type" argument (annuity-due)

Classification

Bug

Severity

Medium — wrong result only when 5th arg is used, but silent

File

Core/Formula/FormulaEvaluator.Functions.cs:1118-1149

Root Cause

EvalPmt, EvalFv, EvalPv, and EvalNper only read args[0..3]. The 5th Excel argument type (0 = payment at end of period [default], 1 = payment at beginning) is never read or applied:

private static FormulaResult? EvalPmt(List<object> args)
{
    if (args.Count < 3) return null;
    double rate = args[0] is FormulaResult r ? r.AsNumber() : 0,
           nper = args[1] is FormulaResult r2 ? r2.AsNumber() : 0,
           pv = args[2] is FormulaResult r3 ? r3.AsNumber() : 0;
    var fv = args.Count > 3 && args[3] is FormulaResult r4 ? r4.AsNumber() : 0;
    // ← args[4] (type) is NEVER read
    if (rate == 0) return FR(-(pv + fv) / nper);
    return FR(-(rate * (pv * Math.Pow(1 + rate, nper) + fv) / (Math.Pow(1 + rate, nper) - 1)));
    // ← no (1+rate) multiplier for type=1
}

The same pattern applies to EvalFv (line 1126), EvalPv (line 1134), and EvalNper (line 1142) — all read up to args[3] and none read args[4].

Repro (PoC)

=PMT(0.05/12, 12, 1000, 0, 0)   → ordinary annuity
=PMT(0.05/12, 12, 1000, 0, 1)   → annuity-due (payment at beginning)

Expected vs Actual

	type=0	type=1
Expected (Excel)	-85.607465	-85.251971 (different!)
Actual (this code)	-85.607482	-85.607482 (identical)

Runtime Verification

Compiled and executed with .NET SDK 8.0.423. Both type=0 and type=1 return -85.607482, confirming the type argument is silently ignored.

Why maintainer accepts this

The functions are registered in the dispatch table and accept 4 arguments — they are "implemented" but incomplete. If type=1 were unsupported, the function should return #N/A or #VALUE!, not silently return the wrong (ordinary-annuity) result. Excel documentation: PMT function.

Fix

Read args[4] (default 0) and apply the standard annuity-due adjustment (multiply by (1 + rate) when type == 1) per the standard financial formula.

Classification

Bug

Severity

Medium — wrong result only when 5th arg is used, but silent

File

Core/Formula/FormulaEvaluator.Functions.cs:1118-1149

Root Cause

EvalPmt, EvalFv, EvalPv, and EvalNper only read args[0..3]. The 5th Excel argument type (0 = payment at end of period [default], 1 = payment at beginning) is never read or applied:

private static FormulaResult? EvalPmt(List<object> args)
{
    if (args.Count < 3) return null;
    double rate = args[0] is FormulaResult r ? r.AsNumber() : 0,
           nper = args[1] is FormulaResult r2 ? r2.AsNumber() : 0,
           pv = args[2] is FormulaResult r3 ? r3.AsNumber() : 0;
    var fv = args.Count > 3 && args[3] is FormulaResult r4 ? r4.AsNumber() : 0;
    // ← args[4] (type) is NEVER read
    if (rate == 0) return FR(-(pv + fv) / nper);
    return FR(-(rate * (pv * Math.Pow(1 + rate, nper) + fv) / (Math.Pow(1 + rate, nper) - 1)));
    // ← no (1+rate) multiplier for type=1
}

The same pattern applies to EvalFv (line 1126), EvalPv (line 1134), and EvalNper (line 1142) — all read up to args[3] and none read args[4].

Repro (PoC)

=PMT(0.05/12, 12, 1000, 0, 0)   → ordinary annuity
=PMT(0.05/12, 12, 1000, 0, 1)   → annuity-due (payment at beginning)

Expected vs Actual

	type=0	type=1
Expected (Excel)	-85.607465	-85.251971 (different!)
Actual (this code)	-85.607482	-85.607482 (identical)

Runtime Verification

Compiled and executed with .NET SDK 8.0.423. Both type=0 and type=1 return -85.607482, confirming the type argument is silently ignored.

Why maintainer accepts this

The functions are registered in the dispatch table and accept 4 arguments — they are "implemented" but incomplete. If type=1 were unsupported, the function should return #N/A or #VALUE!, not silently return the wrong (ordinary-annuity) result. Excel documentation: PMT function.

Fix

Read args[4] (default 0) and apply the standard annuity-due adjustment (multiply by (1 + rate) when type == 1) per the standard financial formula.

[mvanhorn]

Implemented in #232 - PMT/FV/PV/NPER now read the 5th type argument and apply the annuity-due (beginning-of-period) adjustment; type=0/omitted results are unchanged, and post-fix values match Excel and LibreOffice.

[goworm]

Verified fixed: PMT/FV/PV/NPER all honor the 5th type argument with the annuity-due factor (1 + rate*type). =PMT(0.05/12,12,1000,0,0) → -85.6074817884675, =PMT(0.05/12,12,1000,0,1) → -85.2522640217103 (exactly type-0 ÷ (1+rate), the closed-form relation; note the expected value quoted in the report is itself slightly off). Closing.