89 lines
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89 lines
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<!--replace-end-7--><!--replace-end-4--><!--replace-end-1--></head><body><div class="ui fluid container universe"><!--replace-start-2--><!--replace-start-3--><!--replace-start-6--><div class="ui text container" id="zettel-container" style="position: relative"><div class="zettel-view"><article class="ui raised attached segment zettel-content"><div class="pandoc"><h1 id="title-h1">Multiple pointers</h1><p>The multiple pointers pattern can be useful for moving through an array and comparing elements against each other in a time-efficient manner and without costly multiple loops.</p><h2 id="example-moving-pairs">Example: moving pairs</h2><p>In this example we shuffle through an array creating a pair from each element encountered. Thus for an array such as <code>[4, 3, 2, 1, 3, 6, 7]</code> we would return <code>[ [ 4, 3 ], [ 3, 2 ], [ 2, 1 ], [ 1, 3 ], [ 3, 6 ], [ 6, 7 ] ]</code>.</p><pre><code class="js language-js">function movingPairs(arr) {
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const store = [];
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let left = 0;
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let right = 1;
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for (left; left < arr.length - right; left++) {
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store.push([arr[left], arr[right + left]]);
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}
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return store;
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}</code></pre><h2 id="example-identify-unique-values">Example: identify unique values</h2><p>An example application of the above pattern would be identifying the number of unique values in an array that contains duplicates. One way to do this in JavaScript would be to use a Set however this is a more generic approach that only requires a single loop:</p><pre><code class="js language-js">function uniqueValues(arr) {
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if (!arr.length) {
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return 0;
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} else {
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let count = 1;
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let left = 0;
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let right = 1;
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for (right; right < arr.length; right++) {
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if (arr[left] !== arr[right]) {
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count++;
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}
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left++;
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}
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return count;
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}
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}</code></pre><p>This works by having two pointers at either end of the array; one moving from the right and one moving from the left. Whilst the right pointer is less than the array length, the left pointer is incremented as the right moves closer to it. At each incrementation, the values of pointers (used as indexes) are compared for uniqueness, and a count is kept.</p><h2 id="example-identify-duplicates">Example: identify duplicates</h2><p>This is very similar to the above but this time we are looking for duplicates rather than unique values.</p><pre><code class="js language-js">function areThereDuplicatesPointer(...input) {
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let start = 0;
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let next = 1;
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while (next < input.length) {
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let same = input[start] === input[next];
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if (same) {
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return true;
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}
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start++;
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next++;
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}
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return false;
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}
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console.log(areThereDuplicatesPointer(1, 2, 3)); // false
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console.log(areThereDuplicatesPointer(1, 2, 2)); // true</code></pre></div></article><nav class="ui attached segment deemphasized backlinksPane" id="neuron-backlinks-pane"><h3 class="ui header">Backlinks</h3><ul class="backlinks"><li><span class="zettel-link-container cf"><span class="zettel-link"><a href="Sliding_window.html">Sliding window</a></span></span><ul class="context-list" style="zoom: 85%;"><li class="item"><div class="pandoc"><p>Whereas the <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Multiple pointers"><a href="Multiple_pointers.html">multiple pointer</a></span></span> pattern works by keeping two array indices running at once and comparing their values on each iteration, the sliding window has a running value (the ‘window’) that is updated at each iteration and which compares itself against its most recent previous value.</p></div></li></ul></li></ul></nav><nav class="ui attached segment deemphasized bottomPane" id="neuron-tags-pane"><div><span class="ui basic label zettel-tag" title="Tag">design-patterns</span></div></nav><nav class="ui bottom attached icon compact inverted menu blue" id="neuron-nav-bar"><!--replace-start-9--><!--replace-end-9--><a class="right item" href="impulse.html" title="Open Impulse"><i class="wave square icon"></i></a></nav></div></div><!--replace-end-6--><!--replace-end-3--><!--replace-end-2--><div class="ui center aligned container footer-version"><div class="ui tiny image"><a href="https://neuron.zettel.page"><img alt="logo" src="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" title="Generated by Neuron 1.9.35.3" /></a></div></div></div></body></html>
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