# Log Price Transform

`D07-F08-A06` · Technical Indicators → Price Transforms · archetype `series-transform` · difficulty 2/5 · verification **verified**

Full page: https://docs.thefintechbuilder.com/technical-indicators/price-transforms/log-price-transform/
Agent skill: `npx skills add IslamBaraka90/Fintech-Algorithms-Library` — https://docs.thefintechbuilder.com/guides/agent-skill/

## Install and import

```bash
npm install fintech-algorithms
```

```ts
import { logPriceTransform } from "fintech-algorithms/technical-indicators/price-transforms/log-price-transform";
```

## Signature

```ts
logPriceTransform(input)
```

Takes the natural logarithm of each close and the first difference of those logs.

## Parameters

| Name | Type | Required | Notes |
| --- | --- | --- | --- |
| `input` | `TopicInput` | yes | `bars` needs `timestamp` and `close`, and every close must be strictly positive. This topic reads no keys from `parameters`, though `parameters` must still be an object when supplied. |

## Returns

`TopicResult`

`series` holds `log_price`, the natural log of close on every bar, and `log_return`, the difference between consecutive logs. `latest` carries the last element of each. `log_return` is null on the first bar only, which is the whole warm-up.

## Warm-up

The first `1 bar for `log_return`; none for `log_price`` positions are `null`. `ready_at` is 0 because `log_price` is available immediately.

## Errors

- When any `close` is zero or negative — throws Error
- When a `close` is not a finite number — throws Error
- When `bars` is empty — throws Error

## Complexity

Time `O(n)`, space `O(n)`.

## Worked example

Captured by running this function on the input its own test provides. Real output of real code — but not asserted against a published figure.

### Input

`input`:

```json
{
  "bars": [
    {
      "timestamp": "2024-01-02",
      "basis": "synthetic-unadjusted",
      "open": 100,
      "high": 101.45,
      "low": 98.695,
      "close": 100,
      "volume": 750000,
      "benchmark": 200
    },
    {
      "timestamp": "2024-01-03",
      "basis": "synthetic-unadjusted",
      "open": 101.49111452,
      "high": 103.38381693,
      "low": 100.05480022,
      "close": 101.78791214,
      "volume": 795117,
      "benchmark": 200.56326135
    },
    {
      "timestamp": "2024-01-04",
      "basis": "synthetic-unadjusted",
      "open": 102.45519048,
      "high": 104.6701838,
      "low": 100.91147007,
      "close": 102.9549389,
      "volume": 840234,
      "benchmark": 201.11020913
    }
  ],
  "parameters": {}
}
```

### Call

```ts
logPriceTransform(input)
```

### Returns

object with 9 fields: topic_id, title, state, ready, ready_at, series, latest, parameters, …

```json
{
  "topic_id": "D07-F08-A06",
  "title": "Log Price Transform",
  "state": "calculated",
  "ready": true,
  "ready_at": 0,
  "series": {
    "log_price": [
      4.605170185988092,
      4.622891355808754,
      4.634291406096771,
      4.6369816208940655,
      4.633089782650425,
      4.627603917225904
    ],
    "log_return": [
      null,
      0.017721169820662297,
      0.011400050288016494,
      0.002690214797294921,
      -0.0038918382436401444,
      -0.005485865424521563
    ]
  },
  "latest": {
    "log_price": 4.605078326469135,
    "log_return": -0.0146799922919163
  },
  "parameters": {},
  "diagnostics": {
    "causal": true,
    "input_count": 96
  }
}
```

## Verification and provenance

Tier: **verified** (via E).

The worked example below is the figure published in this algorithm's article, replayed and asserted by the test suite on every build. The arithmetic cannot drift without the build failing.

Both tiers guarantee the signature. Full explanation: https://docs.thefintechbuilder.com/guides/verification/

Generated from the docs.json payload shipped inside fintech-algorithms@0.13.0.
The signature and parameter list are checked against the compiled implementation at build time,
so a description that contradicts the code fails the build rather than reaching this file.

## Links

- Article (how it works, step by step): https://thefintechbuilder.com/technical-indicators/price-transforms/log-price-transform/
- Implementation source: https://github.com/IslamBaraka90/Fintech-Algorithms-Library/blob/main/src/technical-indicators/price-transforms/log-price-transform/impl.ts
- Package on npm: https://www.npmjs.com/package/fintech-algorithms
- Domain index for agents: https://docs.thefintechbuilder.com/technical-indicators/llms.txt