# T109

> type is not indexable

**Severity** Error

`[...]` was applied to a value that has no index operation. Subscription fields with history (`chart.close[3]`), arrays, rolling buffers, maps and strings can be indexed; plain values such as a trade's price, a `context.*` constant or a local number cannot.

If you need earlier values of something that has no history, push it into a `rolling<T>` buffer in `state` and index that.

## Failing example

```flowscope
script "Trade acceleration"
data tape = subscribe(data.trades, minNotional: 50_000.0)
on tape.trade(tr) {
  if tr.size > tr.size[1] * 3.0 {
    log.info(str.format("size jump at {0}", tr.price))
  }
}
```

## Fixed example

```flowscope
script "Trade acceleration"
data tape = subscribe(data.trades, minNotional: 50_000.0)
state sizes = rolling<float>(2)
on tape.trade(tr) {
  let prev = sizes.last()
  if prev != null && tr.size > prev * 3.0 {
    log.info(str.format("size jump at {0}", tr.price))
  }
  sizes.push(tr.size)
}
```

## Related

- [T420](/docs/scripting/reference/diagnostics/t420)
- [C006](/docs/scripting/reference/diagnostics/c006)