# 下劃線 ### 下劃線的特殊性 1. 在import中使用 2. ``` <pre class="calibre15">``` 在代碼中使用 ``` ``` 3. ``` <pre class="calibre13">``` <span class="token2">package</span> main <span class="token2">import</span> <span class="token1">(</span> <span class="token4">"os"</span> <span class="token1">)</span> func <span class="token3">main</span><span class="token1">(</span><span class="token1">)</span> <span class="token1">{</span> buf <span class="token1">:</span><span class="token">=</span> <span class="token3">make</span><span class="token1">(</span><span class="token1">[</span><span class="token1">]</span>byte<span class="token1">,</span> <span class="token5">1024</span><span class="token1">)</span> f<span class="token1">,</span> _ <span class="token1">:</span><span class="token">=</span> os<span class="token1">.</span><span class="token3">Open</span><span class="token1">(</span><span class="token4">"/Users/***/Desktop/text.txt"</span><span class="token1">)</span> defer f<span class="token1">.</span><span class="token3">Close</span><span class="token1">(</span><span class="token1">)</span> <span class="token2">for</span> <span class="token1">{</span> n<span class="token1">,</span> _ <span class="token1">:</span><span class="token">=</span> f<span class="token1">.</span><span class="token3">Read</span><span class="token1">(</span>buf<span class="token1">)</span> <span class="token2">if</span> n <span class="token">==</span> <span class="token5">0</span> <span class="token1">{</span> <span class="token2">break</span> <span class="token1">}</span> os<span class="token1">.</span>Stdout<span class="token1">.</span><span class="token3">Write</span><span class="token1">(</span>buf<span class="token1">[</span><span class="token1">:</span>n<span class="token1">]</span><span class="token1">)</span> <span class="token1">}</span> <span class="token1">}</span> ``` ``` ``` <pre class="calibre13">``` 下劃線意思是忽略這個變量<span class="token1">.</span> 比如os<span class="token1">.</span>Open，返回值為<span class="token">*</span>os<span class="token1">.</span>File，error 普通寫法是f<span class="token1">,</span>err <span class="token1">:</span><span class="token">=</span> os<span class="token1">.</span><span class="token3">Open</span><span class="token1">(</span><span class="token4">"xxxxxxx"</span><span class="token1">)</span> 如果此時不需要知道返回的錯誤值 就可以用f<span class="token1">,</span> _ <span class="token1">:</span><span class="token">=</span> os<span class="token1">.</span><span class="token3">Open</span><span class="token1">(</span><span class="token4">"xxxxxx"</span><span class="token1">)</span> 如此則忽略了error變量 ``` ``` ``` <pre class="calibre13">``` 占位符，意思是那個位置本應賦給某個值，但是咱們不需要這個值。 所以就把該值賦給下劃線，意思是丟掉不要。 這樣編譯器可以更好的優化，任何類型的單個值都可以丟給下劃線。 這種情況是占位用的，方法返回兩個結果，而你只想要一個結果。 那另一個就用 <span class="token4">"_"</span> 占位，而如果用變量的話，不使用，編譯器是會報錯的。 ``` ``` ``` <pre class="calibre13">``` <span class="token2">import</span> <span class="token4">"database/sql"</span> <span class="token2">import</span> _ <span class="token4">"github.com/go-sql-driver/mysql"</span> ``` ``` 第二個import就是不直接使用mysql包，只是執行一下這個包的init函數，把mysql的驅動注冊到sql包里，然后程序里就可以使用sql包來訪問mysql數據庫了。