# MQL5 traps

## Index direction

In MQL5 a series array is often read newest first, so index 0 is the current bar. A port that keeps `close[1]` for "the previous bar" reads the wrong bar.

```python
import forge as fg

FORGE_API = 1


def last_two_closes(close):
    # Oldest first: [-1] is the latest closed bar, [-2] the one before it.
    return close[-2], close[-1]
```

> **Trap.** An MQL5 index of 1 is a Forge index of -2. There is no reversed series mode.

## Point, Digits and pips

`_Point` is the smallest price step and `_Digits` its number of decimals. A pip is 10 points on symbols with 5 or 3 digits and 1 point elsewhere. Forge prices are raw prices such as 1.08543, never points or pips. A pip or point amount appears only where a price amount is typed, and is resolved through the broker facts.

> **Trap.** Converting a stop from pips by multiplying with a fixed 0.0001 is wrong on JPY pairs and on every symbol with a different step. Let Forge resolve the amount from the broker facts.

## EMPTY_VALUE

An indicator buffer holds `EMPTY_VALUE` for "no value", which is a very large number. Forge uses NaN, and a comparison with NaN is false.

```python
import forge as fg
from forge import ta

FORGE_API = 1


def ready(value):
    # A missing value is NaN, never a sentinel number.
    return ~ta.isna(value)
```

> **Trap.** A test such as `value != EMPTY_VALUE` has no Forge equivalent. Test for NaN with `ta.isna`, or leave the bar empty.

## Hedging and netting

An MQL5 account is either hedging, which allows several positions on one symbol, or netting, which keeps one. Forge reads the account's margin mode from the broker facts. A strategy that opens an opposite order on a netting account reduces or reverses the position, and an order the account does not allow is rejected with a named reason.

> **Trap.** An expert advisor that hedges by opening both sides works on a hedging account and does not on a netting one. Check the account's mode before porting the logic.

## Ticks and bars

An expert advisor reacts in `OnTick`, on every price update. A Forge strategy decides on closed bars and fills at the next real price. A port that depended on the tick it was called on needs another design, or a bar type built from ticks.

## Magic numbers

A magic number tags the orders of one expert advisor. Forge orders carry a `tag`, and a strategy lists its own positions by tag.

## prev_calculated

`OnCalculate` receives `prev_calculated`, the number of bars already done. Forge passes `start`, the first bar to fill, to `calculate`. Fill every output from `start` on and do not keep your own count.

> **Trap.** An indicator that loops from 0 on every call works on a chart and is slow in a sweep. Fill from `start`.