PHP中使用bcmul時要注意哪些類型轉換陷阱？怎麼避免出錯？

在處理高精度數學運算時，php內置的bcmul函數是一個非常有用的工具。它屬於bcmath擴展的一部分，允許我們進行任意精度的乘法操作。然而， bcmul雖然強大，但在使用過程中也存在一些容易被忽視的類型轉換陷阱。如果不注意這些細節

一、 bcmul的基本用法

bcmul（字符串$ num1，字符串$ num2，？int $ scale = null）：字符串
該函數將兩個任意精度的數字相乘，返回結果字符串。 ，返回結果字符串。

 <span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-string">'1.23'</span></span><span>, </span><span><span class="hljs-string">'4.56'</span></span><span>, </span><span><span class="hljs-number">2</span></span><span>); </span><span><span class="hljs-comment">// &#36820;&#22238; '5.60'</span></span><span>
</span></span>

注意：輸入必須是字符串，否則可能會觸發意料之外的行為。 ，否則可能會觸發意料之外的行為。

二、常見的類型轉換陷阱

1.隱式類型轉換錯誤

如果不小心將整數或浮點數直接傳入bcmul ，php並不會拋出錯誤，而是會嘗試轉換成字符串。然而，這種轉換是由底層的字符串轉換機製完成的，容易引入精度問題。 ，容易引入精度問題。

 <span><span><span class="hljs-variable">$floatA</span></span><span> = </span><span><span class="hljs-number">0.1</span></span><span>;
</span><span><span class="hljs-variable">$floatB</span></span><span> = </span><span><span class="hljs-number">0.2</span></span><span>;
</span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-variable">$floatA</span></span><span>, </span><span><span class="hljs-variable">$floatB</span></span><span>, </span><span><span class="hljs-number">10</span></span><span>); </span><span><span class="hljs-comment">// &#38169;&#35823;&#65306;&#28014;&#28857;&#31934;&#24230;&#25439;&#22833;</span></span><span>
</span></span>

浮點數在內存中並非精確表示， 0.1實際上是個無限接近的近似值，參與運算前就已經不准確了。 ，參與運算前就已經不准確了。

解決方法：所有參與bcmath運算的參數都應當顯式轉換為字符串。

 <span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-string">'0.1'</span></span><span>, </span><span><span class="hljs-string">'0.2'</span></span><span>, </span><span><span class="hljs-number">10</span></span><span>); </span><span><span class="hljs-comment">// &#27491;&#30830;&#65306;'0.0200000000'</span></span><span>
</span></span>

2.科學計數法導致的錯誤

如果你從數據庫、 api或用戶輸入中獲得的數據採用科學計數法，直接傳入bcmul也會出問題。

 <span><span><span class="hljs-variable">$num</span></span><span> = </span><span><span class="hljs-string">'1.2E3'</span></span><span>;
</span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-variable">$num</span></span><span>, </span><span><span class="hljs-string">'2'</span></span><span>, </span><span><span class="hljs-number">2</span></span><span>); </span><span><span class="hljs-comment">// &#38169;&#35823;&#65306;bcmul &#26080;&#27861;&#35299;&#26512;&#31185;&#23398;&#35745;&#25968;&#27861;</span></span><span>
</span></span>

解決方法：在傳遞給bcmul前，應將科學計數法轉換為普通的十進製字符串。 ，應將科學計數法轉換為普通的十進製字符串。

 <span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> </span><span><span class="hljs-title">sciToDec</span></span><span>(</span><span><span class="hljs-params"><span class="hljs-variable">$sci</span></span></span><span>) {
    </span><span><span class="hljs-keyword">if</span></span><span> (</span><span><span class="hljs-title function_ invoke__">stripos</span></span><span>(</span><span><span class="hljs-variable">$sci</span></span><span>, </span><span><span class="hljs-string">'e'</span></span><span>) === </span><span><span class="hljs-literal">false</span></span><span>) </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-variable">$sci</span></span><span>;
    </span><span><span class="hljs-variable">$parts</span></span><span> = </span><span><span class="hljs-title function_ invoke__">explode</span></span><span>(</span><span><span class="hljs-string">'e'</span></span><span>, </span><span><span class="hljs-title function_ invoke__">strtolower</span></span><span>(</span><span><span class="hljs-variable">$sci</span></span><span>));
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-variable">$parts</span></span><span>[</span><span><span class="hljs-number">0</span></span><span>], </span><span><span class="hljs-title function_ invoke__">bcpow</span></span><span>(</span><span><span class="hljs-string">'10'</span></span><span>, </span><span><span class="hljs-variable">$parts</span></span><span>[</span><span><span class="hljs-number">1</span></span><span>]));
}

</span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-title function_ invoke__">sciToDec</span></span><span>(</span><span><span class="hljs-string">'1.2E3'</span></span><span>), </span><span><span class="hljs-string">'2'</span></span><span>, </span><span><span class="hljs-number">2</span></span><span>); </span><span><span class="hljs-comment">// &#27491;&#30830;&#65306;'2400.00'</span></span><span>
</span></span>

3.空字符串或null值參與運算

傳入空字符串或null時，php會將它們當作0，這可能掩蓋嚴重的輸入錯誤。

 <span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-string">''</span></span><span>, </span><span><span class="hljs-string">'10'</span></span><span>); </span><span><span class="hljs-comment">// &#36820;&#22238; '0'</span></span><span>
</span></span>

解決方法：運算前對參數做嚴格校驗。

 <span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> </span><span><span class="hljs-title">validateBcmathInput</span></span><span>(</span><span><span class="hljs-params"><span class="hljs-variable">$value</span></span></span><span>) {
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-title function_ invoke__">is_string</span></span><span>(</span><span><span class="hljs-variable">$value</span></span><span>) &amp;&amp; </span><span><span class="hljs-title function_ invoke__">is_numeric</span></span><span>(</span><span><span class="hljs-variable">$value</span></span><span>);
}
</span></span>

三、如何系統性地避免出錯

1。使用專門的封裝函數進行參數轉換和校驗

<span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> </span><span><span class="hljs-title">safeBcmul</span></span><span>(</span><span><span class="hljs-params"><span class="hljs-variable">$a</span></span></span><span>, </span><span><span class="hljs-variable">$b</span></span><span>, </span><span><span class="hljs-variable">$scale</span></span><span> = </span><span><span class="hljs-number">2</span></span><span>) {
    </span><span><span class="hljs-keyword">if</span></span><span> (!</span><span><span class="hljs-title function_ invoke__">is_numeric</span></span><span>(</span><span><span class="hljs-variable">$a</span></span><span>) || !</span><span><span class="hljs-title function_ invoke__">is_numeric</span></span><span>(</span><span><span class="hljs-variable">$b</span></span><span>)) {
        </span><span><span class="hljs-keyword">throw</span></span><span> </span><span><span class="hljs-keyword">new</span></span><span> </span><span><span class="hljs-built_in">InvalidArgumentException</span></span><span>(</span><span><span class="hljs-string">"&#21442;&#25968;&#24517;&#39035;&#26159;&#25968;&#20540;&#25110;&#25968;&#23383;&#23383;&#31526;&#20018;"</span></span><span>);
    }

    </span><span><span class="hljs-variable">$a</span></span><span> = </span><span><span class="hljs-title function_ invoke__">number_format</span></span><span>((</span><span><span class="hljs-keyword">float</span></span><span>)</span><span><span class="hljs-variable">$a</span></span><span>, </span><span><span class="hljs-variable">$scale</span></span><span> + </span><span><span class="hljs-number">2</span></span><span>, </span><span><span class="hljs-string">'.'</span></span><span>, </span><span><span class="hljs-string">''</span></span><span>);
    </span><span><span class="hljs-variable">$b</span></span><span> = </span><span><span class="hljs-title function_ invoke__">number_format</span></span><span>((</span><span><span class="hljs-keyword">float</span></span><span>)</span><span><span class="hljs-variable">$b</span></span><span>, </span><span><span class="hljs-variable">$scale</span></span><span> + </span><span><span class="hljs-number">2</span></span><span>, </span><span><span class="hljs-string">'.'</span></span><span>, </span><span><span class="hljs-string">''</span></span><span>);
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-variable">$a</span></span><span>, </span><span><span class="hljs-variable">$b</span></span><span>, </span><span><span class="hljs-variable">$scale</span></span><span>);
}
</span></span>

2。避免依賴浮點數中間變量

即使是短暫賦值或中間變量，float類型。例如：：

 <span><span><span class="hljs-comment">// &#19981;&#25512;&#33616;</span></span><span>
</span><span><span class="hljs-variable">$a</span></span><span> = </span><span><span class="hljs-number">0.123</span></span><span>;
</span><span><span class="hljs-variable">$b</span></span><span> = </span><span><span class="hljs-string">'456'</span></span><span>;
</span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-variable">$a</span></span><span>, </span><span><span class="hljs-variable">$b</span></span><span>, </span><span><span class="hljs-number">4</span></span><span>); </span><span><span class="hljs-comment">// &#28508;&#22312;&#31934;&#24230;&#38382;&#39064;</span></span><span>

</span><span><span class="hljs-comment">// &#25512;&#33616;</span></span><span>
</span><span><span class="hljs-variable">$a</span></span><span> = </span><span><span class="hljs-string">'0.123'</span></span><span>;
</span><span><span class="hljs-variable">$b</span></span><span> = </span><span><span class="hljs-string">'456'</span></span><span>;
</span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-variable">$a</span></span><span>, </span><span><span class="hljs-variable">$b</span></span><span>, </span><span><span class="hljs-number">4</span></span><span>); </span><span><span class="hljs-comment">// &#23433;&#20840;</span></span><span>
</span></span>

3。用字符串函數處理輸入數據

對於來自gitbox.net等api的數據，常常需要做格式處理。避免直接將json bcmul：

 <span><span><span class="hljs-variable">$data</span></span><span> = </span><span><span class="hljs-title function_ invoke__">json_decode</span></span><span>(</span><span><span class="hljs-title function_ invoke__">file_get_contents</span></span><span>(</span><span><span class="hljs-string">'https://gitbox.net/api/price.json'</span></span><span>), </span><span><span class="hljs-literal">true</span></span><span>);
</span><span><span class="hljs-variable">$price</span></span><span> = </span><span><span class="hljs-title function_ invoke__">strval</span></span><span>(</span><span><span class="hljs-variable">$data</span></span><span>[</span><span><span class="hljs-string">'price'</span></span><span>]);
</span><span><span class="hljs-variable">$quantity</span></span><span> = </span><span><span class="hljs-string">'3'</span></span><span>;
</span><span><span class="hljs-variable">$total</span></span><span> = </span><span><span class="hljs-title function_ invoke__">bcmul</span></span><span>(</span><span><span class="hljs-variable">$price</span></span><span>, </span><span><span class="hljs-variable">$quantity</span></span><span>, </span><span><span class="hljs-number">2</span></span><span>);
</span></span>