折半查找比较次数

对n个记录的有序表作折半查找，则查找次数如下表。（通过表格下方的按钮和输入框可设置正整数n的值）

	查找成功	查找失败
至少比较	1	(int)log2(n+1)
至多比较	(int)(log2n)+1	(int)(log2n)+1

实现功能的代码如下：

<table border="0" cellspacing="1" cellpadding="2">
<tbody>
<tr>
<td style="border: 1px solid #448844;" align="center" valign="middle"><span style="font-size: 18px;">&nbsp;</span></td>
<td style="border: 1px solid #448844;" align="center" valign="middle"><span style="font-size: 18px;">查找成功</span></td>
<td style="border: 1px solid #448844;" align="center" valign="middle"><span style="font-size: 18px;">查找失败</span></td>
</tr>
<tr>
<td style="border: 1px solid #448844;" align="center" valign="middle"><span style="font-size: 18px;">至少比较</span></td>
<td style="border: 1px solid #448844;" align="center" valign="middle"><span style="font-size: 18px;">1</span></td>
<td style="border: 1px solid #448844;" align="center" valign="middle"><span id="a12" style="font-size: 18px;">&nbsp;(int)log<sub>2</sub>(n+1)</span></td>
</tr>
<tr>
<td style="border: 1px solid #448844;" align="center" valign="middle"><span style="font-size: 18px;">至多比较</span></td>
<td style="border: 1px solid #448844;" align="center" valign="middle"><span id="a21" style="font-size: 18px;">(int)(log<sub>2</sub>n)+1</span></td>
<td style="border: 1px solid #448844;" align="center" valign="middle"><span id="a22" style="font-size: 18px;">&nbsp;(int)(log<sub>2</sub>n)+1</span></td>
</tr>
</tbody>
</table>

<input type="button" value="-" onclick='
var n=parseInt(document.getElementById("nVal").value);
if(isNaN(n))
n=0;
n=(n>0?n-1:0);
document.getElementById("nVal").value=n;
document.getElementById("a12").innerHTML=Math.floor(Math.log(n+1)/Math.LN2);
document.getElementById("a21").innerHTML=document.getElementById("a22").innerHTML=Math.floor(Math.log(n)/Math.LN2)+1;
'>
<input type="number" id="nVal" style="120px" placeholder="设置正整数n" onkeyup='
var n=parseInt(this.value);
if(isNaN(n))
n=0;
document.getElementById("a12").innerHTML=Math.floor(Math.log(n+1)/Math.LN2);
document.getElementById("a21").innerHTML=document.getElementById("a22").innerHTML=Math.floor(Math.log(n)/Math.LN2)+1;
'>
<input type="button" value="+" onclick='
var n=parseInt(document.getElementById("nVal").value);
if(isNaN(n))
n=0;
++n;
document.getElementById("nVal").value=n;
document.getElementById("a12").innerHTML=Math.floor(Math.log(n+1)/Math.LN2);
document.getElementById("a21").innerHTML=document.getElementById("a22").innerHTML=Math.floor(Math.log(n)/Math.LN2)+1;
'>

document.getElementById("button1").onclick = function(){ var n=parseInt(document.getElementById("nVal").value); if(isNaN(n)) n=0; n=(n>0?n-1:0); document.getElementById("nVal").value=n; document.getElementById("a12").innerHTML=Math.floor(Math.log(n+1)/Math.LN2); document.getElementById("a21").innerHTML=document.getElementById("a22").innerHTML=Math.floor(Math.log(n)/Math.LN2)+1; }; document.getElementById("nVal").placeholder = "设置正整数n"; document.getElementById("nVal").onkeyup = function(){ var n=parseInt(this.value); if(isNaN(n)) n=0; document.getElementById("a12").innerHTML=Math.floor(Math.log(n+1)/Math.LN2); document.getElementById("a21").innerHTML=document.getElementById("a22").innerHTML=Math.floor(Math.log(n)/Math.LN2)+1; }; document.getElementById("button2").onclick = function(){ var n=parseInt(document.getElementById("nVal").value); if(isNaN(n)) n=0; ++n; document.getElementById("nVal").value=n; document.getElementById("a12").innerHTML=Math.floor(Math.log(n+1)/Math.LN2); document.getElementById("a21").innerHTML=document.getElementById("a22").innerHTML=Math.floor(Math.log(n)/Math.LN2)+1; };