Javascript-如果任意两个选定下拉列表值相同，如何显示消息框
如果任意两个下拉列表的值相同，我可以知道如何显示消息框吗 以下是我对这些下拉列表的编码：Javascript-如果任意两个选定下拉列表值相同，如何显示消息框,javascript,Javascript,如果任意两个下拉列表的值相同，我可以知道如何显示消息框吗 以下是我对这些下拉列表的编码： <form action="checkresult2.php"> <table class="p1" bgcolor="#FFFFCC" bordercolor="#000000" align="center" width="771" border="2"> <tr> <td><div align=
<form action="checkresult2.php">
<table class="p1" bgcolor="#FFFFCC" bordercolor="#000000" align="center" width="771" border="2">
<tr>
<td><div align="center"><strong>No.</strong></div></td>
<td><div align="center"><strong>Subject Name</strong></div></td>
<td><div align="center"><strong>Grade</strong></div></td>
</tr>
<?php
for($i=1; $i<=10; $i++)
{?>
<tr>
<td width="42"><div align="center"><?php echo $i; ?></div></td>
<td width="539">
<select id="subject_opt" name="subj[]">
<option value="">--- Please choose a subject ---</option>
<?php
$result = mysql_query("SELECT * FROM spm_subject");
while($s = mysql_fetch_assoc($result))
{?>
<option value="<?php echo $s['name']; ?>"><?php echo $s["name"]; ?></option>
<?php } ?>
</select>
</td>
<td width="166"><div align="center">
<select id="grade_opt" name="grad[]">
<option value="">- Select grade -</option>
<?php
$result2 = mysql_query("SELECT * FROM spm_grade");
while($g = mysql_fetch_assoc($result2))
{?>
<option value="<?php echo $g['grade']; ?>"><?php echo $g["grade"]; ?></option>
<?php } ?>
</select>
</div>
</td>
</tr>
<?php }?>
<tr>
<td colspan="3">
<div align="center">
<input type="submit" value="Submit">
</div>
</td>
</tr>
</table>
</form>
否。
主题名称
等级
---请选择一个主题---
函数验证（）
{
var dropdown1=document.getElementById（“grade_opt”）；
var dropdown2=document.getElementById（“subject_opt”）；
if（dropdown1.value==dropdown2.value）{
警报（“不能有重复值”）；
}
}
然后将属性onchange添加到每个selectonchange=“validate（）”
如果您使用的是jquery，请使用下面的代码
function validateDropdowns(){
var selectBox1 = $("#subject_opt option:selected").val();
var selectBox2 = $("#grade_opt option:selected").val();
if((selectBox1 != "" && selectBox2 !="") &&(selectBox1 == selectBox2)){
alert("duplicate values are not accepted");
}
}
在每个下拉列表中调用validateDropdowns（）
函数我认为'id'不应该相同，但'name'可以相同，然后可以使用getElementById进行检查。您应该能够在“id”之后添加$i以对元素进行四舍五入
id="subject_opt<?php echo $i; ?>"
id=“主题选择”
以下是您的示例：
<script>
function checkSubjectAndGrade(num){
for(i=0;i<2;i++){
if(i!=num && document.getElementById("subject_opt"+num).value === document.getElementById("subject_opt"+i).value){
alert("dropdown lists' value are same");
}
}
}
</script>
<form action="checkresult2.php">
<table class="p1" bgcolor="#FFFFCC" bordercolor="#000000" align="center" width="771" border="2">
<tr>
<td><div align="center"><strong>No.</strong></div></td>
<td><div align="center"><strong>Subject Name</strong></div></td>
<td><div align="center"><strong>Grade</strong></div></td>
</tr>
<tr>
<td width="42"><div align="center">1</div></td>
<td width="539">
<select id="subject_opt0" name="subj[]" onChange="checkSubjectAndGrade(0)">
<option value="">--- Please choose a subject ---</option>
<option value="aaa">English</option>
<option value="bbb">Math</option>
</select>
</td>
<td width="166"><div align="center">
<select id="grade_opt0" name="grad[]">
<option value="">- Select grade -</option>
<option value="aaa">A</option>
<option value="bbb">B</option>
</select>
</td>
</tr>
<tr>
<td width="42"><div align="center">2</div></td>
<td width="539">
<select id="subject_opt1" name="subj[]" onChange="checkSubjectAndGrade(1)">
<option value="">--- Please choose a subject ---</option>
<option value="aaa">English</option>
<option value="bbb">Math</option>
</select>
</td>
<td width="166"><div align="center">
<select id="grade_opt1" name="grad[]">
<option value="">- Select grade -</option>
<option value="aaa">A</option>
<option value="bbb">B</option>
</select>
</td>
</tr>
<tr>
<td colspan="3">
<div align="center">
<input type="submit" value="Submit">
</div>
</td>
</tr>
</table>
</form>
函数检查SubjectAndGrade（num）{
对于（i=0；i如果值相等则检查更改如果两个“主题选项”值相同怎么办？我只能在那些“主题选项”下拉列表上进行验证吗？是的，只需更改函数中的元素ID即可
<script>
function checkSubjectAndGrade(num){
for(i=0;i<2;i++){
if(i!=num && document.getElementById("subject_opt"+num).value === document.getElementById("subject_opt"+i).value){
alert("dropdown lists' value are same");
}
}
}
</script>
<form action="checkresult2.php">
<table class="p1" bgcolor="#FFFFCC" bordercolor="#000000" align="center" width="771" border="2">
<tr>
<td><div align="center"><strong>No.</strong></div></td>
<td><div align="center"><strong>Subject Name</strong></div></td>
<td><div align="center"><strong>Grade</strong></div></td>
</tr>
<tr>
<td width="42"><div align="center">1</div></td>
<td width="539">
<select id="subject_opt0" name="subj[]" onChange="checkSubjectAndGrade(0)">
<option value="">--- Please choose a subject ---</option>
<option value="aaa">English</option>
<option value="bbb">Math</option>
</select>
</td>
<td width="166"><div align="center">
<select id="grade_opt0" name="grad[]">
<option value="">- Select grade -</option>
<option value="aaa">A</option>
<option value="bbb">B</option>
</select>
</td>
</tr>
<tr>
<td width="42"><div align="center">2</div></td>
<td width="539">
<select id="subject_opt1" name="subj[]" onChange="checkSubjectAndGrade(1)">
<option value="">--- Please choose a subject ---</option>
<option value="aaa">English</option>
<option value="bbb">Math</option>
</select>
</td>
<td width="166"><div align="center">
<select id="grade_opt1" name="grad[]">
<option value="">- Select grade -</option>
<option value="aaa">A</option>
<option value="bbb">B</option>
</select>
</td>
</tr>
<tr>
<td colspan="3">
<div align="center">
<input type="submit" value="Submit">
</div>
</td>
</tr>
</table>
</form>