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MiamiDolphins asked on

How to compare the values in two list boxes

Hey there,
im using javascript and i would like to compare the values in two listboxes (list box A, and list box B)

when the compare all the items in listbox B which also appear in list box A need to be SELECTED in listbox A.

can anyone help  me to do this - (using asp.net c# and javascript - new to both)

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Last Comment

8/22/2022 - Mon

Please see this PAQ for some ideas on looping through a Listbox with Javascript:  https://www.experts-exchange.com/Programming/Languages/Scripting/JavaScript/Q_21509625.html

Can you show the code you have already and what problems you are having?  Also, please explain your second line a little better.  Are you wanting to search through confirm all items in List box B are selected in List box A?

Please define the comparison test(s) to be made.

For example, if item #1 on List A has the same text as item #12 on List B, but the "value" attributes don't match, is that "close enough"?

the matches will be identical matches (value matches)

so the value and text attributes will be the same in both cases as they are sourced from the same database tables.

sadly im not at work so i cant supply any sample codes at the moment :-(

but just imagine two listboxes (asp listboxes - selectmode - multiple) and they both have items

i just want to be able to loop through listbox 2 on an event (i.e button click) and then at the end event i want to be able to see which values matched in listbox 1 because they would be selected
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Can a value from list A exist multiple times in list B?

no it can only exist once


something like this?
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<title>compare selection lists</title>
<script type='text/javascript'>
  function field( id ) {
    var ele = document.getElementById( id );
    if ( !ele ) {
      alert( 'Specified document element not found.  id="' + id + '"' );
    return ele;
  function selCheck( sel ) {
    var result = ( sel.nodeName == 'SELECT' )
    var id = sel.getAttribute( 'id' );
    if ( !result ) {
      alert( 'document element is not a SELECT.  id="' + id + '"' );
    return result;
  function compare( id1, id2 ) {
    var sel1 = field( id1 );
    var sel2 = field( id2 );
    var matches = 0;
    if ( sel1 && sel2 && selCheck( sel1 ) && selCheck( sel2 ) ) {
      for ( var j = 0; j < sel2.options.length; j++ ) {
        sel2.options[ j ].selected = false;
      for ( var i = 0; i < sel1.options.length; i++ ) {
        var opt = sel1.options[ i ];
//      alert( 'opt: ' + opt.value );
        for ( var j = 0; j < sel2.options.length; j++ ) {
          if ( sel2.options[ j ].value == opt.value ) {
            sel2.options[ j ].selected = true;
      alert( 'Number of matching entries: ' + matches );
  <table border='1'>
      <th>List A</th>
      <th>List B</th>
          <select id='A'>
          <select id='B' multiple>
        <td colspan='2' align='center'>
          <input type='button' value='compare' onclick='compare("A","B")'>

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ill try and test it out later tonight

thanks buddy will let you know the results

fantastic - great stuff

Great.  I'm glad to have been able to help
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