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<!DOCTYPE html>
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<title>iOS 数据精度及大数的处理 | 锐意翱翔</title>
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<a class="nav-item" href="https://think.cc/"><h1 class="title is-4">锐意翱翔</h1></a>
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<h2 class="subtitle is-6">May 3, 2018</h2>
<h1 class="title">iOS 数据精度及大数的处理</h1>
<div class="content">
<p>在 iOS 开发中，我们很容易遇到使用 CGFloat 来标示浮点数，但这样的表示会造成精度失真。这时我们可以使用<code>NSDecimalNumber</code>来处理这个问题。</p>
<p><code>NSDecimalNumber</code>是<code>NSNumber</code>的子类，可以处理大数运算及数据的精度问题。</p>
<h3 id="大数相乘可能导致的问题">大数相乘可能导致的问题</h3>
<p>我们先上一段代码：</p>
<pre><code>NSString *priceStr = @"";
NSDecimalNumber *number = [NSDecimalNumber decimalNumberWithString:priceStr];
NSDecimalNumber *countNum = [NSDecimalNumber decimalNumberWithString:stringWithNSInteger(NSIntegerMax)];
number = [number decimalNumberByMultiplyingBy:countNum];
</code></pre>
<p>在这段代码中，number的值为：NaN，即：not a number ，非数值；
而countNum 是一个最大的整数，
最后，将NaN和最大的整数相乘，导致了overflow的crash。</p>
<p>解决方案：</p>
<pre><code>//定义数值处理的行为
NSDecimalNumberHandler *roundUp = [NSDecimalNumberHandler
decimalNumberHandlerWithRoundingMode:NSRoundBankers
scale:2
raiseOnExactness:NO
raiseOnOverflow:NO
raiseOnUnderflow:NO
raiseOnDivideByZero:NO];
NSString *priceStr = @"";
NSDecimalNumber *number = [NSDecimalNumber decimalNumberWithString:priceStr];
NSDecimalNumber *countNum = [NSDecimalNumber decimalNumberWithString:stringWithNSInteger(NSIntegerMax)];
//使用数据处理行为的约定来进行运算，防止crash
number = [number decimalNumberByMultiplyingBy:countNum withBehavior:roundUp];
</code></pre>
<p>上面这个例子不会crash了，但是最终number的值为NaN，需要后续的业务逻辑进行判断处理；</p>
<p>NSDecimalNumberHandler 用到的参数，其中：</p>
<h4 id="nsroundbankers">NSRoundBankers</h4>
<p>枚举，截断的方式；完整的定义如下：</p>
<pre><code>// Rounding policies :
// Original
// value 1.2 1.21 1.25 1.35 1.27
// Plain 1.2 1.2 1.3 1.4 1.3
// Down 1.2 1.2 1.2 1.3 1.2
// Up 1.2 1.3 1.3 1.4 1.3
// Bankers 1.2 1.2 1.2 1.4 1.3
typedef NS_ENUM(NSUInteger, NSRoundingMode) {
NSRoundPlain, // Round up on a tie
NSRoundDown, // Always down == truncate
NSRoundUp, // Always up
NSRoundBankers // on a tie round so last digit is even
};
</code></pre>
<p><strong>scale</strong></p>
<p>小数点后面的位数（精度）</p>
<p><strong>raiseOnExactness</strong></p>
<blockquote>
<p>The exception raised if there is an exactness error.</p>
</blockquote>
<p><strong>raiseOnOverflow</strong></p>
<p>是否抛出溢出错误，如果为YES，则APP会捕获溢出错误，这会导致APPcrash；</p>
<blockquote>
<p>The exception raised on overflow.</p>
</blockquote>
<p><strong>raiseOnUnderflow</strong></p>
<blockquote>
<p>The exception raised on underflow.</p>
</blockquote>
<p><strong>raiseOnDivideByZero</strong></p>
<blockquote>
<p>The exception raised on divide by zero.</p>
</blockquote>
<h4 id="补充知识">补充知识：</h4>
<ul>
<li>判断一个数值是否为NaN可以使用系统方法：isnan(x)；注：x为数值类型，不是NSDecimalNumber，更不是NSNumber;</li>
<li>如果要判断一个NSDecimalNumber 是否为 NAN ，则使用下面的方法：</li>
</ul>
<pre><code>if([number isEqualToNumber:NSDecimalNumber.notANumber]){
NSLog(@"number is nan");
}else{
NSLog(@"number:%@",number);
}
</code></pre>
<p>参考链接：<a href="https://www.jianshu.com/p/c8feb12ecba5">iOS 两个数相乘导致 NSDecimalNumber overflow exception 错误的分析及解决</a></p>
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