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Some might be tempted to use float representation for calculating with currency values. That is problematic because of possible rounding errors.</p><h2 class="anchor anchorWithStickyNavbar_LWe7" id="finite-accuracy-of-representation">Finite accuracy of representation<a href="#finite-accuracy-of-representation" class="hash-link" aria-label="Direct link to Finite accuracy of representation" title="Direct link to Finite accuracy of representation"></a></h2><p>Floating points are represented like this</p><p><img loading="lazy" alt="Floating point representation" src="/assets/images/floating_point_representation-efc73a8c1b722a82b772c2cd7ee7ba99.webp" width="810" height="226" class="img_ev3q"></p><p>Not every number can be represented with a finite number of decimal places</p><p>0.01 —> 0.0000011001100110011…</p><p>Taking 17 places of the above results in 0.010000000000000001</p><p>Consider the following code snipet that shows the missing accuracy</p><div class="language-go codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-go codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#bfc7d5"><span class="token keyword" style="font-style:italic">func</span><span class="token plain"> </span><span class="token function" style="color:rgb(130, 170, 255)">main</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(199, 146, 234)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain"> </span><span class="token keyword" style="font-style:italic">var</span><span class="token plain"> n </span><span class="token builtin" style="color:rgb(130, 170, 255)">float64</span><span class="token plain"> </span><span class="token operator" style="color:rgb(137, 221, 255)">=</span><span class="token plain"> </span><span class="token number" style="color:rgb(247, 140, 108)">0</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain"> </span><span class="token keyword" style="font-style:italic">for</span><span class="token plain"> i </span><span class="token operator" style="color:rgb(137, 221, 255)">:=</span><span class="token plain"> </span><span class="token number" style="color:rgb(247, 140, 108)">0</span><span class="token punctuation" style="color:rgb(199, 146, 234)">;</span><span class="token plain"> i </span><span class="token operator" style="color:rgb(137, 221, 255)"><</span><span class="token plain"> </span><span class="token number" style="color:rgb(247, 140, 108)">1000</span><span class="token punctuation" style="color:rgb(199, 146, 234)">;</span><span class="token plain"> i</span><span class="token operator" style="color:rgb(137, 221, 255)">++</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(199, 146, 234)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain"> n </span><span class="token operator" style="color:rgb(137, 221, 255)">+=</span><span class="token plain"> </span><span class="token number" style="color:rgb(247, 140, 108)">.01</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain"> </span><span class="token punctuation" style="color:rgb(199, 146, 234)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain"> fmt</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token function" style="color:rgb(130, 170, 255)">Println</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token plain">n</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain"></span><span class="token punctuation" style="color:rgb(199, 146, 234)">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>Result: 9.999999999999831</p><h2 class="anchor anchorWithStickyNavbar_LWe7" id="money-computations">Money computations<a href="#money-computations" class="hash-link" aria-label="Direct link to Money computations" title="Direct link to Money computations"></a></h2><p>They can't be done with floating-point as it would inevitably lead to rounding errors.</p><p>Even the following packages are problematic:</p><p><a href="https://github.com/shopspring/decimal" target="_blank" rel="noopener noreferrer">github.com/shopspring/decimal</a></p><p><a href="https://github.com/Rhymond/go-money" target="_blank" rel="noopener noreferrer">github.com/Rhymond/go-money</a></p><div class="language-go codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-go codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#bfc7d5"><span class="token plain">a </span><span class="token operator" style="color:rgb(137, 221, 255)">:=</span><span class="token plain"> decimal</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token function" style="color:rgb(130, 170, 255)">NewFromInt</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token number" style="color:rgb(247, 140, 108)">2</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">b </span><span class="token operator" style="color:rgb(137, 221, 255)">:=</span><span class="token plain"> decimal</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token function" style="color:rgb(130, 170, 255)">NewFromFloat</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token number" style="color:rgb(247, 140, 108)">300.99</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">c </span><span class="token operator" style="color:rgb(137, 221, 255)">:=</span><span class="token plain"> a</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token function" style="color:rgb(130, 170, 255)">Mul</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token plain">b</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">d </span><span class="token operator" style="color:rgb(137, 221, 255)">:=</span><span class="token plain"> c</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token function" style="color:rgb(130, 170, 255)">Div</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token plain">decimal</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token function" style="color:rgb(130, 170, 255)">NewFromInt</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token number" style="color:rgb(247, 140, 108)">3</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><h2 class="anchor anchorWithStickyNavbar_LWe7" id="solution">Solution<a href="#solution" class="hash-link" aria-label="Direct link to Solution" title="Direct link to Solution"></a></h2><p>Use Int by representing money in cents:</p><ul><li>10.99 -> 1099 (cents)</li><li>10.9900 -> 109900 (4 digit tax)</li></ul><h2 class="anchor anchorWithStickyNavbar_LWe7" id="conclusion">Conclusion<a href="#conclusion" class="hash-link" aria-label="Direct link to Conclusion" title="Direct link to Conclusion"></a></h2><p><strong>Division is a problem!</strong></p><p>1/3 - > 0.33333333…
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Correct way: 0.33, 0.33, 0.34</p><p>When doing money calculations one should avoid division as it inevitably leads to loss of accuracy.
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When dividing make sure to round to cent and deal with diffs.</p><p>Division by 10^k is ok as long as we are inside of the range of the data type.</p></div><footer class="row docusaurus-mt-lg blogPostFooterDetailsFull_mRVl"><div class="col"><b>Tags:</b><ul class="tags_jXut padding--none margin-left--sm"><li class="tag_QGVx"><a class="tag_zVej tagRegular_sFm0" href="/blog/tags/golang">golang</a></li><li class="tag_QGVx"><a class="tag_zVej tagRegular_sFm0" href="/blog/tags/currency">currency</a></li><li class="tag_QGVx"><a class="tag_zVej tagRegular_sFm0" href="/blog/tags/decimal-accuracy">decimal accuracy</a></li></ul></div><div class="col margin-top--sm"><a href="https://github.com/foomo/foomo-docs/tree/main/foomo/blog/2023-03-06-accuracy-of-decimal-computations/index.mdx" target="_blank" rel="noreferrer noopener" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_Z9Sw" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div></footer></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Blog post page navigation"><a class="pagination-nav__link pagination-nav__link--prev" href="/blog/go-race-conditions-testing-and-coverage"><div class="pagination-nav__sublabel">Newer Post</div><div class="pagination-nav__label">Go race conditions testing and coverage</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/blog/why-bundle-size-is-important"><div class="pagination-nav__sublabel">Older Post</div><div class="pagination-nav__label">Why bundle size is important?</div></a></nav></main><div class="col col--2"><div class="tableOfContents_bqdL thin-scrollbar"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#intro" class="table-of-contents__link toc-highlight">Intro</a></li><li><a href="#finite-accuracy-of-representation" class="table-of-contents__link toc-highlight">Finite accuracy of representation</a></li><li><a href="#money-computations" class="table-of-contents__link toc-highlight">Money computations</a></li><li><a href="#solution" class="table-of-contents__link toc-highlight">Solution</a></li><li><a href="#conclusion" class="table-of-contents__link toc-highlight">Conclusion</a></li></ul></div></div></div></div></div><footer class="footer"><div class="container container-fluid"><div class="row footer__links"><div class="col footer__col"><div class="footer__title">github</div><ul class="footer__items clean-list"><li class="footer__item"><a href="https://github.com/foomo" target="_blank" rel="noopener noreferrer" class="footer__link-item">https://github.com/foomo</a></li></ul></div><div class="col footer__col"><div class="footer__title">legal</div><ul class="footer__items clean-list"><li class="footer__item"><a class="footer__link-item" href="/etc/imprint">Imprint</a></li></ul></div></div><div class="footer__bottom text--center"><div class="footer__copyright">© 2023 bestbytes</div></div></div></footer></div>
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