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Evaluate sum of values without clicking a button

I really don't know how to program in Javascript, yet I've tried to do some stuff. I have values to be summed if some are selected. The following function does sum operation and show result at once without clicking a button only for checkbox fields, not for the dropdown list and text fields.
However, I need to involve the selected values as well from the dropdown lists and entered both text fields in the same manner of the checkboxes and their immediate responses(output).
I think change method can be used in the same manner.
#Edit:
Sorry, I forgot the tell but, I need to multiply the m2 entered value by before summing with 3134,204499 and Bina Yasi with -566,2202888.
function calcAndShowTotal() {
var total = 55435.04798; // it is constant value which must be the beginning value.
$('#housepriceguesslist :checkbox:checked').each(function() {
total += parseFloat($(this).attr('price')) || 0;
});
$('#total').val(total);
}
$('#housepriceguesslist :checkbox').change(calcAndShowTotal).change();
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="housepriceguesslist">
<select id="ilce">
<option value="0">Choose a state</option>
<option value="63999.50712318567">Adalar</option>
<option value="6259.918435398593">Ataşehir</option>
<option value="46285.35729304834">Beykoz</option>
<option value="54258.92953697798">Kadıköy</option>
<option value="-3500.620905653726">Kartal</option>
<option value="12889.41219909602">Maltepe</option>
<option value="-33711.83156847153">Pendik</option>
<option value="-34577.42485251657">Sancaktepe</option>
<option value="-52183.1608811019">Sultanbeyli</option>
<option value="-34760.71735007784">Tuzla</option>
<option value="-27379.87966213786">Çekmeköy</option>
<option value="-5332.203990418937">Ümraniye</option>
<option value="26485.33532820411">Üsküdar</option>
<option value="-18732.62070553167">Şile</option>
</select>
<select id="odasalon">
<option value="0">Choose room number</option>
<option value="5940.027623576261">1+0</option>
<option value="-1399.625767475593">1+1</option>
<option value="-3033.638999060459">2+1</option>
<option value="11519.83954756076">3+1</option>
<option value="-7018.52116545078">4+1</option>
<option value="-6008.081239150166">5+1</option>
</select>
m2: <input type="text" id="m2" /> Bina Yasi: <input type="text" id="binayasi" />
<br/>
<br/> Features:
<br/>
<br/>
<input type="checkbox" id="Bati" value="Bati" price="-783.4452683566533" /> <label for="Bati">Bati</label>
<br/>
<input type="checkbox" id="Dogu" value="Dogu" price="-4895.732009084263" /> <label for="Dogu">Dogu</label>
<br/>
<input type="checkbox" id="Guney" value="Guney" price="-2599.001349652765" /> <label for="Guney">Guney</label>
<br/>
<input type="checkbox" id="Kuzey" value="Kuzey" price="3832.013070771234" /> <label for="Kuzey">Kuzey</label>
<br/>
<input type="checkbox" id="Amerikan Mutfak" value="Amerikan Mutfak" price="2346.347889828301" /> <label for="Amerikan Mutfak">Amerikan Mutfak</label>
<br/>
<input type="checkbox" id="Dusakabin" value="Dusakabin" price="-176.5092353740256" /> <label for="Dusakabin">Dusakabin</label>
<br/>
<input type="checkbox" id="Akilli Ev" value="Akilli Ev" price="-4756.206384200719" /> <label for="Akilli Ev">Akilli Ev</label>
<br/>
<input type="checkbox" id="Ebeveyn Banyosu" value="Ebeveyn Banyosu" price="10135.50644456019" /> <label for="Ebeveyn Banyosu">Ebeveyn Banyosu</label>
<br/>
<input type="checkbox" id="Kiler" value="Kiler" price="-10693.49979821809" /> <label for="Kiler">Kiler</label>
<br/>
<input type="checkbox" id="Kuvet" value="Kuvet" price="-941.0629741243247" /> <label for="Kuvet">Kuvet</label>
<br/>
<input type="checkbox" id="Parke Zemin" value="Parke Zemin" price="-8453.847199707092" /> <label for="Parke Zemin">Parke Zemin</label>
<br/>
<input type="checkbox" id="Seramik Zemin" value="Seramik Zemin" price="1545.025601000643" /> <label for="Seramik Zemin">Seramik Zemin</label>
<br/>
<input type="checkbox" id="Sauna" value="Sauna" price="8677.708176289747" /> <label for="Sauna">Sauna</label>
<br/>
<input type="checkbox" id="Spor Salonu" value="Spor Salonu" price="1159.884679563385" /> <label for="Spor Salonu">Spor Salonu</label>
<br/>
<input type="checkbox" id="Tenis Kortu" value="Tenis Kortu" price="16065.26794172257" /> <label for="Tenis Kortu">Tenis Kortu</label>
<br/>
</div>
<input type="text" id="total" value="0" />

You can loop over all the <select> boxes and text inputs with:
function calcAndShowTotal() {
var total = 55435.04798; // it is constant value which must be the beginning value.
$('#housepriceguesslist :checkbox:checked').each(function() {
total += parseFloat($(this).attr('price')) || 0;
});
$("#housepriceguesslist select, #housepriceguesslist :text").each(function() {
let multiplier = 1;
if (this.id == "m2") {
multiplier = 3134.204499;
}
else if (this.id == "binayasi") {
multiplier = -566.2202888;
}
if (this.value) {
total += parseFloat(this.value) * multiplier;
}
});
$('#total').val(total);
}
And if you change the checkboxes to use value instead of price, you can do them all in one loop:
function calcAndShowTotal() {
var total = 55435.04798; // it is constant value which must be the beginning value.
$('#housepriceguesslist :checkbox:checked').each(function() {
total += parseFloat($(this).attr('price')) || 0;
});
$("#housepriceguesslist select, #housepriceguesslist :text, #housepriceguesslist :checkbox:checked").each(function() {
let multiplier = 1;
if (this.id == "m2") {
multiplier = 3134.204499;
else if (this.id == "binyasi") {
multiplier = -566.2202888;
}
if (this.value) {
total += parseFloat(this.value) * multiplier;
}
});
$('#total').val(total);
}
And call the function when any of the inputs are changed:
$('#housepriceguesslist input, #housepriceguesslist select').change(calcAndShowTotal).change();

This solution is simple, just the get the list of all the inputs (i.e. checkbox, and dropdown) using the .find() method and the selector 'input:checkbox, input:text, select'. Then verify if each one of these inputs elements has the attribute checked or select for the checkbox and select input types respectively, or it is a multiplier for the text inputs. This verification ca be done inside of the each() method callback. Finally, just convert then the value to a number, using the parseFloat() method and add it to the total accumulator.
Finally, sum the value to the total input. Like this:
let container;
let inputs;
let total;
function calcAndShowTotal() {
let sum = 55435.04798; // it is constant value which must be the beginning value.
let value;
inputs.each(function() {
if ($(this).is('input:checkbox:checked')) {
// If is a checkbox
value = parseFloat($(this).attr('price')) || 0;
sum += value;
} else if ($(this).is('input:text:not(#total)')) {
// If is a text wait until you move to another element
let factor = 1;
if (this.id == 'm2') {
factor = 3134.204499;
} else if (this.id == 'binayasi') {
factor = -566.2202888;
}
value = parseFloat($(this).val()) * factor || 0;
sum += value;
} else if ($(this).is('select')) {
// If is a option from the select element
value = parseFloat($('option:selected', this).val()) || 0;
sum += value;
}
});
total.val(sum);
}
function initHandlers() {
inputs.on('keydown', function(e) {
let focusable, next;
if (e.keyCode == 13) {
focusable = inputs.filter(':visible');
next = focusable.eq(focusable.index(this) + 1);
if (next.length) {
next.focus();
}
return false;
}
});
inputs.change(calcAndShowTotal);
calcAndShowTotal();
}
$(function() {
console.log('ready!');
total = $('#total');
container = $('#housepriceguesslist');
inputs = container.find('input:checkbox, input:text, select');
initHandlers();
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="housepriceguesslist">
Total: <input type="text" id="total" value="0" />
<br/>
<br/>
<select id="ilce">
<option value="0">Choose a state</option>
<option value="63999.50712318567">Adalar</option>
<option value="6259.918435398593">Ataşehir</option>
<option value="46285.35729304834">Beykoz</option>
<option value="54258.92953697798">Kadıköy</option>
<option value="-3500.620905653726">Kartal</option>
<option value="12889.41219909602">Maltepe</option>
<option value="-33711.83156847153">Pendik</option>
<option value="-34577.42485251657">Sancaktepe</option>
<option value="-52183.1608811019">Sultanbeyli</option>
<option value="-34760.71735007784">Tuzla</option>
<option value="-27379.87966213786">Çekmeköy</option>
<option value="-5332.203990418937">Ümraniye</option>
<option value="26485.33532820411">Üsküdar</option>
<option value="-18732.62070553167">Şile</option>
</select>
<select id="odasalon">
<option value="0">Choose room number</option>
<option value="5940.027623576261">1+0</option>
<option value="-1399.625767475593">1+1</option>
<option value="-3033.638999060459">2+1</option>
<option value="11519.83954756076">3+1</option>
<option value="-7018.52116545078">4+1</option>
<option value="-6008.081239150166">5+1</option>
</select>
m2: <input type="text" id="m2" />
Bina Yasi: <input type="text" id="binayasi" />
<br/>
<br/> Features:
<br/>
<br/>
<input type="checkbox" id="Bati" value="Bati" price="-783.4452683566533" /> <label for="Bati">Bati</label>
<br/>
<input type="checkbox" id="Dogu" value="Dogu" price="-4895.732009084263" /> <label for="Dogu">Dogu</label>
<br/>
<input type="checkbox" id="Guney" value="Guney" price="-2599.001349652765" /> <label for="Guney">Guney</label>
<br/>
<input type="checkbox" id="Kuzey" value="Kuzey" price="3832.013070771234" /> <label for="Kuzey">Kuzey</label>
<br/>
<input type="checkbox" id="Amerikan Mutfak" value="Amerikan Mutfak" price="2346.347889828301" /> <label for="Amerikan Mutfak">Amerikan Mutfak</label>
<br/>
<input type="checkbox" id="Dusakabin" value="Dusakabin" price="-176.5092353740256" /> <label for="Dusakabin">Dusakabin</label>
<br/>
<input type="checkbox" id="Akilli Ev" value="Akilli Ev" price="-4756.206384200719" /> <label for="Akilli Ev">Akilli Ev</label>
<br/>
<input type="checkbox" id="Ebeveyn Banyosu" value="Ebeveyn Banyosu" price="10135.50644456019" /> <label for="Ebeveyn Banyosu">Ebeveyn Banyosu</label>
<br/>
<input type="checkbox" id="Kiler" value="Kiler" price="-10693.49979821809" /> <label for="Kiler">Kiler</label>
<br/>
<input type="checkbox" id="Kuvet" value="Kuvet" price="-941.0629741243247" /> <label for="Kuvet">Kuvet</label>
<br/>
<input type="checkbox" id="Parke Zemin" value="Parke Zemin" price="-8453.847199707092" /> <label for="Parke Zemin">Parke Zemin</label>
<br/>
<input type="checkbox" id="Seramik Zemin" value="Seramik Zemin" price="1545.025601000643" /> <label for="Seramik Zemin">Seramik Zemin</label>
<br/>
<input type="checkbox" id="Sauna" value="Sauna" price="8677.708176289747" /> <label for="Sauna">Sauna</label>
<br/>
<input type="checkbox" id="Spor Salonu" value="Spor Salonu" price="1159.884679563385" /> <label for="Spor Salonu">Spor Salonu</label>
<br/>
<input type="checkbox" id="Tenis Kortu" value="Tenis Kortu" price="16065.26794172257" /> <label for="Tenis Kortu">Tenis Kortu</label>
<br/>
</div>
Note:
Remember to round the numbers to get more precision. The text modification will apply after you tab, or move to another element. So I added an enter listener so when you press enter is gonna move to the next element like when you press tab. Also, I moved the total to the top so you can see the results in the preview.
Update:
I recommend you to use the $(function(){}); to get the document when is ready. Inside you can assign the values of your elements. I notice that there was a bug in my code whenever ever you uncheck a checked element the Total should be 55435.04798, but it shows the double it is because total is also a text input.

Similar to Barmar's, I added all elements to the .each() loop but also added the .change() handler to all elements:
var context = $('#housepriceguesslist');
var elems = $('input:checkbox, input:text, select', context);
function calcAndShowTotal() {
var total = 55435.04798; // it is constant value which must be the beginning value.
elems.each(function() {
if ( $(this).is('input:checkbox:checked') ) {
total += parseFloat($(this).attr('price')) || 0;
} else if ( $(this).is('#m2') ) {
total += parseFloat($(this).val()) * 3134.204499 || 0;
} else if ( $(this).is('#binayasi') ) {
total += parseFloat($(this).val()) * -566.2202888 || 0;
} else if ( $(this).is('select') ) {
total += parseFloat($('option:selected',this).val()) || 0;
}
});
$('#total').val(total);
}
elems.change(calcAndShowTotal); // handlers
calcAndShowTotal(); // first run
See example here: https://jsfiddle.net/qgvsern4/1/

How not to open a window if the form fields are not filled?

I have this submit button on my form with a jQuery action to open a window depending on the users choice. However, I just want the window to open if the fields are filled. I have this code and I want to merge it with an if.
$(function() {
$('#chkveg').multiselect({
includeSelectAllOption: true
});
$('#btnget').click(function() {
window.open($('#chkveg').val());
})
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form action="http://formmail.kinghost.net/formmail.cgi" method="POST">
<input name="nome" type="text" class="nome" id="nome" required width="100%" placeholder="Nome:">
<input name="cpf" type="text" class="cpf" id="cpf" placeholder="CPF:">
<div style="clear:both"></div><br/>
<input name="nascimento" type="text" class="nascimento" id="nascimento" placeholder="Data de nascimento:">
<select id="chkveg">
<option value="https://pag.ae/7ULKPL7TH">Associados Ancord + C.Dados = R$700,00</option>
<option value="https://pag.ae/7ULKQ8Zm2">Associados Ancord = R$800,00</option>
<option value="https://pag.ae/7ULKQLB9m">Associados Entidades Apoiadoras + C.Dados = R$800,00</option>
</select>
<input id="btnget" class="submit-btn" type="submit" value="INSCREVER-SE">
</form>
For exemple:
IF (#FORM).REQUIRED = TRUE {
(#BUTTON).WINDOWOPEN
}
Thanks

Because you using a submit button you will need to return false, in case if you don't want to do anything. Before that, you need also to check if your required field are empty or not. (i.e. $(your field).val() === "" then it's empty, if all you need have, then call the window.open() function.
Note: you can merge multiple fields for checking ie: $(".your_field1, .your_field2, .your_field3").val() === "" however this is an OR operation.
One possible solution:
$(function() {
$('#btnget').click(function() {
let isEmpty = false;
$('#data_form input,textarea,select').filter(':visible').each(function(i) {
if ($(this).val() === "") {
isEmpty = true;
return false;
}
});
if (isEmpty) {
return false;
}
window.open($('#chkveg').val());
})
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form id="data_form" action="http://formmail.kinghost.net/formmail.cgi" method="POST">
<input name="nome" type="text" class="nome" id="nome" required width="100%" placeholder="Nome:">
<input name="cpf" type="text" class="cpf" id="cpf" placeholder="CPF:">
<div style="clear:both"></div><br/>
<input name="nascimento" type="text" class="nascimento" id="nascimento" placeholder="Data de nascimento:">
<select id="chkveg">
<option value="https://pag.ae/7ULKPL7TH">Associados Ancord + C.Dados = R$700,00</option>
<option value="https://pag.ae/7ULKQ8Zm2">Associados Ancord = R$800,00</option>
<option value="https://pag.ae/7ULKQLB9m">Associados Entidades Apoiadoras + C.Dados = R$800,00</option>
</select>
<input id="btnget" class="submit-btn" type="submit" value="INSCREVER-SE">
</form>
If you want only for the required fields, than use filter('[required]:visible') instead of filter(':visible').

How to use a checkbox to show more options of a form and change the action attribute of the form?

Hi I am creating a website that has a form to search rooms. I wanted to make it so a checkbox, when clicked, show new options AND change the action ="" attribute of the form.
<form class="form-horizontal" action="reservation?action=listRooms" method="POST">
<label for="">Date </label>
<div class="datepicker ll-skin-nigran hasDatepicker">
<input class="form-control" type="text" placeholder="14/03/2016" name="dateReservation" id="date" required="required"/>
</div>
<br />
<div>
<input type="checkbox" name="choice-for" id="choice-form">
<label for="choice-for">Show More Options.</label>
<div class="reveal-if-active">
<label for="">Slots</label>
<select name="slot" name ="slot" id="slot" >
<option value="">Choose a slot </option>
<option value="8h-9h30">8h00-9h30</option>
<option value="9h30-11h">9h30-11h00</option>
<option value="11h-12h30h">11h00-12h30h</option>
<option value="12h30-14h">12h30-14h00</option>
<option value="14h-15h30">14h00-15h30</option>
<option value="15h30-17h">15h30-17h00</option>
<option value="17h-18h30">17h00-18h30</option>
</select>
<br />
<label for="">Display Screens</label>
<input class="form-control" type="text" placeholder=" 26 pouces" name="screen" id="screen" />
<br />
<label for="">CPU</label>
<input class="form-control" type="text" placeholder="Intel Core i5 " name="processor" id="processor" />
<br />
<label for="">RAM</label>
<input class="form-control" type="text" placeholder=" 2Go de RAM ?" name="ram" id="ram" />
<br />
<input type="submit" value="Réserver" class="btn btn-primary" />
</div>
I tried then to use a javascript(JQuery) script to satisfy my expectations:
<script>
$(function() {
$( "#date" ).datepicker({ dateFormat: 'dd/mm/yy' });
});
$("#choice-form").change(function() {
//there i need to know when the checkbox is changed dynamically so the attribute can change too.
$("#form-horizontal).attr("reservation?action=listRooms");
});
var FormStuff = {
init: function() {
this.applyConditionalRequired();
this.bindUIActions();
},
bindUIActions: function() {
$("input[type='radio'], input[type='checkbox']").on("change", this.applyConditionalRequired);
},
applyConditionalRequired: function() {
$(".require-if-active").each(function() {
var el = $(this);
if ($(el.data("require-pair")).is(":checked")) {
el.prop("required", true);
} else {
el.prop("required", false);
}
});
}
};
FormStuff.init();
</script>

Try this:
$("#choice-form").change(function() {
// If checkbox checked
if ( $('#choice-form').is(':checked') ) {
// Set new form action
$('#form-horizontal').attr('action', 'reservation?action=listRooms');
// Reveal additional options
$('.reveal-if-active').show(); // or call .css() with appropriate options
}
});

Ok, let's say you want to add the id(you can use name as well) and value to the action parameter from the form...
$(document).on('click','.checkboxClass',function(){
var id = $(this).attr('id');
var val = $(this).val();
var selectedVal = '&'+id+'='+val;
var methodUrl = $('form.form-horizontal').attr('action');
if($(this).is(':checked')){
methodUrl+= selectedVal;
}
else{
methodUrl = methodUrl.replace(selectedVal,'');
}
$('form.form-horizontal').attr({'action':methodUrl});
});
Now let's say you have a checkbox with the id="myId" and value="myValue", if you check it, the action parameter will become action="reservation?action=listRooms&myId=myValue"
Is this what you asked for?

Validating a form with JavaScript and displaying them on the page rather than an alert box

I've created a form, that I need to validate with JavaScript or using Jquery. How would I validate if the dropdown list has the value "Title" and if the text box is empty and display it on the page rather then an alert box :
<form action="" method="post" accept-charset="UTF-8" name="myForm">
<option value="Title" id="title_3-0" disabled selected>Title</option><option value="Mr" id="title_3-1">Mr</option>
<option value="Mrs" id="title_3-2">Mrs</option><option value="Miss" id="title_3- 3">Miss</option>
<option value="Ms" id="title_3-4">Ms</option><option value="Dr" id="title_3-5">Dr</option>
<option value="Professor" id="title_3-6">Professor</option></select></div></div>
TextBox:
<input type="text" class="text" name="firstname_4" id="amf-input-firstname_4" value="" placeholder="First Name">
I also have a button.

Here is a start:
<script>
function validateForm()
{
var return_value = true;
var x=document.forms["myForm"]["firstname_4"].value;
if (x==null || x=="")
{
document.getElementById("error").innerHTML += "First name must be filled out<br />";
return_value = false;
}
var y=document.forms["myForm"]["selectid"];
if(y.options[y.selectedIndex].value == "Title")
{
document.getElementById("error").innerHTML += "You need to select a title<br />";
return_value = false;
}
return return_value;
}
</script>
<span id="error"></span>
<form name="myForm" onsubmit="return validateForm()" method="post">
First name: <input type="text" class="text" name="firstname_4" id="amf-input-firstname_4" value="" placeholder="First Name">
<input type="submit" value="Submit">
</form>
If you like, you can also put an "error span element" above/beneath each field and set each error individually:
<script>
function validateForm()
{
var return_value = true;
var x=document.forms["myForm"]["firstname_4"].value;
if (x==null || x=="")
{
document.getElementById("error1").innerHTML = "First name must be filled out";
return_value = false;
}
var y=document.forms["myForm"]["selectid"];
if(y.options[y.selectedIndex].value == "Title")
{
document.getElementById("error2").innerHTML = "You need to select a title";
return_value = false;
}
return return_value;
}
</script>
<form name="myForm" onsubmit="return validateForm()" method="post">
First name: <input type="text" class="text" name="firstname_4" id="amf-input-firstname_4" value="" placeholder="First Name">
<span id="error1"></span>
Title: <input .... >
<span id="error2"></span>
<input type="submit" value="Submit">
</form>

Since you are new to this, look up these links and read up. It will guide you through it. That's better than us giving you all the code. Because this kind of form validation is important and hence you better learn it from scratch.
Link-1
Link-2
Link-3 (using library)

Html:
<select id="ddl"> <option value="Title" id="title_3-0" selected>Title</option><option value="Mr" id="title_3-1">Mr</option>
<option value="Mrs" id="title_3-2">Mrs</option><option value="Miss" id="title_3- 3">Miss</option>
<option value="Ms" id="title_3-4">Ms</option><option value="Dr" id="title_3-5">Dr</option>
<option value="Professor" id="title_3-6">Professor</option></select>
<input type="text" class="text" name="firstname_4" id="amf-input-firstname_4" value="" placeholder="First Name">
<input type="button" value="Submit" id="btn"/>
<span id="error"/>
JQuery:
$('#btn').click(function (){
if(($('#amf-input-firstname_4').val()=='') || ($("#ddl :selected").val() == 'Title'))
{
$('#error').html('Please select title and enter name.');
}
else
{
$('#error').html('Success');
}
return false;
});
Demo:
http://jsfiddle.net/8h8HF/

insert an array item in the input box on changing the combo box

i want a particular array item to set as the value of input tag, when the combo1 box's selection changes.
<%# page language="java" import="java.util.*;" pageEncoding="ISO-8859-1"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
</head>
<script language="javascript">
function check()
{
if(!validate())
return false;
}
function validate()
{
if (document.form.amount.value <=0)
{
alert ("Please enter valid loan amount.");
return false;
}
else
return true;
}
var arr = new Array();
arr[0] = new Array("-select-", "10.0","10.5", "12.0");
function change(combo1) {
document.getElementById("rate").style.visibility = "visible";
var sel = document.getElementById("combo1");
var val = document.getElementById("rate").value;
for(var i = 0, j = sel.options.length; i < j; ++i) {
if(sel.options[i].innerHTML == val) {
sel.selectedIndex = i;
break;
}
}
}
}
</script>
<jsp:include page="include.jsp"></jsp:include>
<body>
<br>
<form action="PaymentServlet.do" name="form" method="post"
ONSUBMIT="return check()">
<fieldset>
<legend>
<font style="font-family: sans-serif" size="4px" color="Megenta">Online Loan
Application Page</font>
</legend>
<br />
Loan Type
<select name="combo1" onchange="change(this);">
<option value="0">
-Select-
</option>
<option value="1">
Home Loan
</option>
<option value="2">
Education Loan
</option>
<option value="3">
Car Loan
</option>
</select>
<br />
Interest Rate(%)<input type="text" name="rate" style="visibility: hidden;">
<br />
Loan Amount
<input type="text" name="loanamount" />
<br />
<input type="submit" name="go" value="o Go o" />
</fieldset>
</form>
</body>
</html>
depending on the loan type i want to select the respective interest rate from the array and want this array item to be used as the value of the input tag whose name="rate".

To get the value of a combo box you need to do the following:
var val = sel.options[sel.selectedIndex].value;
sel is your combo box

You can't use document.getElementById("elementid") by giving name as paramater. The paramater must be id of element not name.
Actually you don't need to use document.getElementById because you are returning this as paramater in change function. this refers the actual element.
So your change function should be like this,
function change(combo1) {
document.getElementById("rate").style.visibility = "visible";
document.getElementById("rate").value = arr[combo1.selectedIndex];
}
Your array definition is wrong as well. The right one is here,
var arr = new Array();
arr[0] = new Array("-select-", "10.0","10.5", "12.0");
var arr = new Array("-select-", "10.0","10.5", "12.0");
Finally don't forget to add id attribute for rate input,
<input type="text" id="rate" name="rate" style="visibility: hidden;">
And here is the complete code and a DEMO
<script language="javascript">
function check()
{
if(!validate())
return false;
}
function validate()
{
if (document.form.amount.value <=0)
{
alert ("Please enter valid loan amount.");
return false;
}
else
return true;
}
var arr = new Array("-select-", "10.0","10.5", "12.0");
function change(combo1) {
document.getElementById("rate").style.visibility = "visible";
document.getElementById("rate").value = arr[combo1.selectedIndex];
}
</script>
<body>
<br>
<form action="PaymentServlet.do" name="form" method="post"
ONSUBMIT="return check()">
<fieldset>
<legend>
<font style="font-family: sans-serif" size="4px" color="Megenta">Online Loan
Application Page</font>
</legend>
<br />
Loan Type
<select name="combo1" onchange="change(this);">
<option value="0">
-Select-
</option>
<option value="1">
Home Loan
</option>
<option value="2">
Education Loan
</option>
<option value="3">
Car Loan
</option>
</select>
<br />
Interest Rate(%)<input type="text" id="rate" name="rate" style="visibility: hidden;">
<br />
Loan Amount
<input type="text" name="loanamount" />
<br />
<input type="submit" name="go" value="o Go o" />
</fieldset>
</form>
</body>

if your array definition is like
arr[0] = new Array("-select-", "10.0","10.5", "12.0");
then
function change(combo1) {
document.getElementById("rate").style.visibility = "visible";
var rate = document.getElementById("rate");
rate.value = arr[0][combo1.selectedIndex];
}