315 lines
8.7 KiB
Plaintext
315 lines
8.7 KiB
Plaintext
---
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title: Typed Arrays and ML
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---
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<head>
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<script src="https://docs.sheetjs.com/tfjs/tf.min.js"></script>
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</head>
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Machine learning libraries in JS typically use "Typed Arrays". Typed Arrays are
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not JS Arrays! With some data wrangling, translating between SheetJS worksheets
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and typed arrays is straightforward.
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This demo covers conversions between worksheets and Typed Arrays for use with
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[TensorFlow.js](https://js.tensorflow.org/js/) and other ML libraries.
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:::note
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The live code blocks in this demo load the standalone TensorFlow.js build from
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version `3.18.0`. It is shipped in the NodeJS package at `/dist/tf.min.js`.
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[Official installation notes](https://www.tensorflow.org/js/tutorials/setup)
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:::
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## CSV Data Interchange
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`tf.data.csv` generates a Dataset from CSV data. The function expects a URL.
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Fortunately blob URLs are supported, making data import straightforward:
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```js
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function worksheet_to_csv_url(worksheet) {
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/* generate CSV */
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const csv = XLSX.utils.sheet_to_csv(worksheet);
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/* CSV -> Uint8Array -> Blob */
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const u8 = new TextEncoder().encode(csv);
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const blob = new Blob([u8], { type: "text/csv" });
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/* generate a blob URL */
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return URL.createObjectURL(blob);
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}
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```
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[This demo mirrors `TFjs` docs](https://js.tensorflow.org/api/latest/#data.csv),
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fetching [an XLSX export of the example dataset](https://sheetjs.com/data/bht.xlsx).
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<details><summary><b>TF CSV Demo using XLSX files</b> (click to show)</summary>
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```jsx live
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function SheetJSToTFJSCSV() {
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const [output, setOutput] = React.useState("");
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const doit = React.useCallback(async () => {
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/* fetch file */
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const f = await fetch("https://sheetjs.com/data/bht.xlsx");
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const ab = await f.arrayBuffer();
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/* parse file and get first worksheet */
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const wb = XLSX.read(ab);
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const ws = wb.Sheets[wb.SheetNames[0]];
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/* generate CSV */
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const csv = XLSX.utils.sheet_to_csv(ws);
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/* generate blob URL */
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const u8 = new TextEncoder().encode(csv);
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const blob = new Blob([u8], {type: "text/csv"});
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const url = URL.createObjectURL(blob);
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/* feed to tfjs */
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const dataset = tf.data.csv(url, {columnConfigs:{"medv":{isLabel:true}}});
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/* this part mirrors the tf.data.csv docs */
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const flat = dataset.map(({xs,ys}) => ({xs: Object.values(xs), ys: Object.values(ys)})).batch(10);
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const model = tf.sequential();
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model.add(tf.layers.dense({inputShape: [(await dataset.columnNames()).length - 1], units: 1}));
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model.compile({ optimizer: tf.train.sgd(0.000001), loss: 'meanSquaredError' });
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let base = output;
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await model.fitDataset(flat, { epochs: 10, callbacks: { onEpochEnd: async (epoch, logs) => {
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setOutput(base += "\n" + epoch + ":" + logs.loss);
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}}});
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model.summary();
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});
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return ( <pre><b><a href="https://js.tensorflow.org/api/latest/#data.csv">Original CSV demo</a></b><br/><br/>
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<button onClick={doit}>Click to run</button>
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{output}
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</pre> );
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}
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```
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</details>
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In the other direction, `XLSX.read` will readily parse CSV exports.
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## JS Array Interchange
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[The official Linear Regression tutorial](https://www.tensorflow.org/js/tutorials/training/linear_regression)
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loads data from a JSON file:
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```json
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[
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{
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"Name": "chevrolet chevelle malibu",
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"Miles_per_Gallon": 18,
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"Cylinders": 8,
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"Displacement": 307,
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"Horsepower": 130,
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"Weight_in_lbs": 3504,
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"Acceleration": 12,
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"Year": "1970-01-01",
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"Origin": "USA"
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},
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{
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"Name": "buick skylark 320",
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"Miles_per_Gallon": 15,
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"Cylinders": 8,
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"Displacement": 350,
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"Horsepower": 165,
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"Weight_in_lbs": 3693,
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"Acceleration": 11.5,
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"Year": "1970-01-01",
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"Origin": "USA"
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},
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// ...
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]
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```
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In real use cases, data is stored in [spreadsheets](https://sheetjs.com/data/cd.xls)
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Following the tutorial, the data fetching method is easily adapted. Differences
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from the official example are highlighted below:
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```js
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/**
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* Get the car data reduced to just the variables we are interested
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* and cleaned of missing data.
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*/
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async function getData() {
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// highlight-start
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/* fetch file */
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const carsDataResponse = await fetch('https://sheetjs.com/data/cd.xls');
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/* get file data (ArrayBuffer) */
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const carsDataAB = await carsDataResponse.arrayBuffer();
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/* parse */
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const carsDataWB = XLSX.read(carsDataAB);
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/* get first worksheet */
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const carsDataWS = carsDataWB.Sheets[carsDataWB.SheetNames[0]];
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/* generate array of JS objects */
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const carsData = XLSX.utils.sheet_to_json(carsDataWS);
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// highlight-end
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const cleaned = carsData.map(car => ({
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mpg: car.Miles_per_Gallon,
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horsepower: car.Horsepower,
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}))
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.filter(car => (car.mpg != null && car.horsepower != null));
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return cleaned;
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}
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```
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## Low-Level Operations
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:::caution
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While it is more efficient to use low-level operations, JS or CSV interchange
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is strongly recommended when possible.
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:::
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### Data Transposition
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A typical dataset in a spreadsheet will start with one header row and represent
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each data record in its own row. For example, the Iris dataset might look like
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`XLSX.utils.sheet_to_json` will translate this into an array of row objects:
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```js
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var aoo = [
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{"sepal length": 5.1, "sepal width": 3.5, ...},
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{"sepal length": 4.9, "sepal width": 3, ...},
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...
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];
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```
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TF.js and other libraries tend to operate on individual columns, equivalent to:
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```js
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var sepal_lengths = [5.1, 4.9, ...];
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var sepal_widths = [3.5, 3, ...];
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```
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When a `tensor2d` can be exported, it will look different from the spreadsheet:
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```js
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var data_set_2d = [
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[5.1, 4.9, ...],
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[3.5, 3, ...],
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...
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]
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```
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This is the transpose of how people use spreadsheets!
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#### Typed Arrays and Columns
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A single typed array can be converted to a pure JS array with `Array.from`:
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```js
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var column = Array.from(dataset_typedarray);
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```
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Similarly, `Float32Array.from` generates a typed array from a normal array:
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```js
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var dataset = Float32Array.from(column);
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```
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### Exporting Datasets to a Worksheet
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`XLSX.utils.aoa_to_sheet` can generate a worksheet from an array of arrays.
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ML libraries typically provide APIs to pull an array of arrays, but it will
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be transposed. To export multiple data sets, manually "transpose" the data:
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```js
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/* assuming data is an array of typed arrays */
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var aoa = [];
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for(var i = 0; i < data.length; ++i) {
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for(var j = 0; j < data[i].length; ++j) {
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if(!aoa[j]) aoa[j] = [];
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aoa[j][i] = data[i][j];
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}
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}
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/* aoa can be directly converted to a worksheet object */
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var ws = XLSX.utils.aoa_to_sheet(aoa);
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```
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### Importing Data from a Spreadsheet
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`sheet_to_json` with the option `header:1` will generate a row-major array of
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arrays that can be transposed. However, it is more efficient to walk the sheet
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manually:
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```js
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/* find worksheet range */
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var range = XLSX.utils.decode_range(ws['!ref']);
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var out = []
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/* walk the columns */
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for(var C = range.s.c; C <= range.e.c; ++C) {
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/* create the typed array */
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var ta = new Float32Array(range.e.r - range.s.r + 1);
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/* walk the rows */
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for(var R = range.s.r; R <= range.e.r; ++R) {
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/* find the cell, skip it if the cell isn't numeric or boolean */
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var cell = ws[XLSX.utils.encode_cell({r:R, c:C})];
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if(!cell || cell.t != 'n' && cell.t != 'b') continue;
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/* assign to the typed array */
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ta[R - range.s.r] = cell.v;
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}
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out.push(ta);
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}
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```
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If the data set has a header row, the loop can be adjusted to skip those rows.
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### TF.js Tensors
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A single `Array#map` can pull individual named fields from the result, which
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can be used to construct TensorFlow.js tensor objects:
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```js
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const aoo = XLSX.utils.sheet_to_json(worksheet);
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const lengths = aoo.map(row => row["sepal length"]);
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const tensor = tf.tensor1d(lengths);
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```
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`tf.Tensor` objects can be directly transposed using `transpose`:
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```js
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var aoo = XLSX.utils.sheet_to_json(worksheet);
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// "x" and "y" are the fields we want to pull from the data
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var data = aoo.map(row => ([row["x"], row["y"]]));
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// create a tensor representing two column datasets
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var tensor = tf.tensor2d(data).transpose();
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// individual columns can be accessed
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var col1 = tensor.slice([0,0], [1,tensor.shape[1]]).flatten();
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var col2 = tensor.slice([1,0], [1,tensor.shape[1]]).flatten();
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```
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For exporting, `stack` can be used to collapse the columns into a linear array:
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```js
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/* pull data into a Float32Array */
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var result = tf.stack([col1, col2]).transpose();
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var shape = tensor.shape;
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var f32 = tensor.dataSync();
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/* construct an array of arrays of the data in spreadsheet order */
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var aoa = [];
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for(var j = 0; j < shape[0]; ++j) {
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aoa[j] = [];
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for(var i = 0; i < shape[1]; ++i) aoa[j][i] = f32[j * shape[1] + i];
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}
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/* add headers to the top */
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aoa.unshift(["x", "y"]);
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/* generate worksheet */
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var worksheet = XLSX.utils.aoa_to_sheet(aoa);
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```
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