First row occurrence of each value : <p>I have two varaibles a and amount sorted by a</p>

<pre><code>a      amount

112    12000 
112    15000 
113    14000
114    18000
114    17000 
115    19000 
115    17000
</code></pre>

<p>I want the  first row occurrence of each value in a variable</p>

<pre><code>output 

 a    amount
112  12000
113  14000
114  18000
115  19000 
</code></pre>
