76 lines
2.3 KiB
JavaScript
76 lines
2.3 KiB
JavaScript
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// ---------------------------------------------------------------
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// Bell Triangle implementation (JavaScript)
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// ---------------------------------------------------------------
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class BellTriangle {
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/**
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* @param {number} n – number of rows to generate
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*/
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constructor(n) {
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// total number of elements in a triangular array of size n
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const length = (n * (n + 1)) / 2;
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// use a plain array, pre‑filled with zeros
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this.arr = new Array(length).fill(0);
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// initialise first element (row 1, col 0) = 1
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this.set(1, 0, 1);
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// build the triangle row by row
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for (let i = 2; i <= n; ++i) {
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// first element of a row copies the last element of the previous row
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this.set(i, 0, this.get(i - 1, i - 2));
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// the remaining entries are sums of the element above‑left
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// and the element to the left in the current row
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for (let j = 1; j < i; ++j) {
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const value = this.get(i, j - 1) + this.get(i - 1, j - 1);
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this.set(i, j, value);
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}
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}
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}
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/** Convert (row, col) into a linear index. Rows start at 1. */
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index(row, col) {
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if (row > 0 && col >= 0 && col < row) {
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// triangular number formula: sum_{k=1}^{row-1} k = row*(row-1)/2
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return (row * (row - 1)) / 2 + col;
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}
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throw new Error('Invalid row/col');
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}
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/** Return the value at (row, col). */
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get(row, col) {
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const i = this.index(row, col);
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return this.arr[i];
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}
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/** Store `value` at (row, col). */
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set(row, col, value) {
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const i = this.index(row, col);
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this.arr[i] = value;
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}
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}
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// ---------------------------------------------------------------
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// Main program – reproduces the Java `main` method
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// ---------------------------------------------------------------
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(function main() {
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const rows = 15;
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const bt = new BellTriangle(rows);
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console.log('First fifteen Bell numbers:');
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for (let i = 0; i < rows; ++i) {
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// the Bell number Bₙ is the first entry of row n (row index = n, col = 0)
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console.log(`${i + 1}: ${bt.get(i + 1, 0)}`);
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}
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// Print the whole triangle for the first 10 rows (exactly as Java did)
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for (let i = 1; i <= 10; ++i) {
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let line = `${bt.get(i, 0)}`;
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for (let j = 1; j < i; ++j) {
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line += `, ${bt.get(i, j)}`;
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}
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console.log(line);
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}
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})();
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