Seeking some JavaScript help

Tony, i took a look and found the problem. How do you go about calculating gallons per year given miles/gallon and miles/year? I'll give you a hint, it's not multiplication!

Change the last 4 lines of math there to division (citymiles/city + highwaymiles/highway, i believe is what you want), and i think you'll be a lot closer to what you want

 

PS. the top section seems to calculate everything correctly.

 

Try these 2 changes:

form.yourcargallons.value =
(((1.0 * form.citymiles.value) / (1.0 * form.city.value)) +
((1.0 * form.highwaymiles.value) / (1.0 * form.highway.value)));

form.priusgallons.value = (((1.0 * form.citymiles.value) / 60.0 ) +
((1.0 * form.highwaymiles.value) / 51.0));

And you might want to add these four lines just before the return:

<!-- Round to 2 decimal points -->
form.yourcargallons.value = (Math.round(form.yourcargallons.value * 100.0) / 100.0);
form.yourcarcost.value = (Math.round(form.yourcarcost.value * 100.0) / 100.0);
form.priuscargallons.value = (Math.round(form.priuscargallons.value * 100.0) / 100.0);
form.priuscost.value = (Math.round(form.priuscost.value * 100.0) / 100.0);

 

I couldn't remember if there was a method in javascript for that, however a quick search at Google found this:

Formatting numbers in JavaScript - mredkj.com

I haven't tried it yet, so I won't vouch for how well it works.

 

That same site recommends using num.toFixed() for rounding rather than multiplying by a factor of ten then rounding and then dividing by the same factor of ten. That seems more elegant than what I posted earlier.

 

I think one of the reasons why it's a little difficult to debug the problem is because the javascript, in its current form is a little hard to read. Hopefully the below would not only solve your immediate issue, but also make the script easier to understand and maintain. Of note is that I'm using getElementById() insted of form.controlName because it's a little more resiliant (you no longer need to know about any "form" and your code won't break if you move, add or rename forms). This requires that an id attribute be added to all the controls, though (which only takes a minute).

If you want to make the utility awesome (and because I'm in the mood to give unsolicited advice), it would be even better if you added these features to the interface:

1. Instead of having the user enter in their MPG rating (which requires them to look it up), have them enter in the year/make/model of their car. You can do this with cascading drop downs and AJAX (which is easy with ASP.NET if you want to play around with it). You can download all of the data from: Download the Fuel Economy Database then store it locally for use in your system.

2. Default the annual miles driven to the national average 12,000 miles.

3. Use drop down lists with increments of 10% for percentage city/highway driving. Default to 50%/50% and change the other drop down list's selection based on the selection of its peer so it always equals 100%. If you're adventurous, use sliders and do the same.

4. Automatically lookup the average price of gas daily/weekly and default to that.