Best way to handle formulas?

[kilbey1]

I have been presented with a project, to make a special 'calculator'. There are formulas to calculate based on state and federal criteria. For example, we need to find totals for someone being paid $5000 a month in California, and spit back results:

SSA (Social Security)
Medicare
SUI
etc. etc.

The formula for SSA is as follows:
IF Salary x 12 < $87,000
then result = Salary x 0.062 / 12
ELSE
87000 x .062 / 12

And there folows a variety of other formulas.

I started by putting the above SSA formula inside the page as code. Then I got to thinking how many codes would be in each page!

I have not written a real function, nor classes. I am guessing this would be the best way to go about it. The only function I have written never returned anything; can anyone point me to a primer on functions, and/or classes?

Perhaps a barebones example from which I can take off?

[AstroTeg]

I'd say functions. They're pretty easy to work with once you get the hang of it...

Your example might look like this:

function calc($salary)   // don't know name of formula so this one will do
{
  if(($salary * 12) < 87000)
    return ($salary * .062 / 12);
  return (87000 * .062 / 12);
}  // end function calc($salary)


// to actually use this function:
echo calc(5000);
// or
$value = calc(1000);

You're pseudo code you have is really good. Two thumbs up on that! It will make writing the functions a lot easier...

[kilbey1]

Ok, I implemented the function, changing it just slightly.

		function calc($ssa)   // don't know name of formula so this one will do 
		{ 
		  if(($_POST['salary'] * 12) < 87000) 
			return ($_POST['salary'] * .062 / 12); 
		  return (87000 * .062 / 12); 
		}  // end function calc($ssa) 
		echo calc($ssa);

I installed a new PHP after a system crash, in which I decided to keep register globals OFF. So now, obnoxiously, I have to $_POST everything.

At any rate, the new PHP may have something to do with this notice I recieve:

Notice: Undefined variable: ssa in C:\xitami\webpages\recruit\index.php on line 204
1.7205

It still does return the correct calculation, as you can see (based on monthly $333 salary).

[kilbey1]

Here's some test data:

CALCULATOR RESULTS

State: CA
Salary: 1000 (assumed monthly for test purposes)

Number format: 1,000.00
String replace: 1000

Non-function:
YES, it's less than 87,000
5.16666666667

Function:
Notice: Undefined variable: ssa in C:\xitami\webpages\recruit\index.php on line 204
5.16666666667

[leatherback]

Hope!

You ONLY need to use $POST[] if you are having the var's POST-ed (Through form eg). If you need to get variables from the url, you use $GET[].

In your case, the variables are defined.

In any case: You are using a function.

In the function definition (function calc($ssa)) it is defined that when the function is called, the values for the variable $SSA wil be between the brackets. So in the function you can just keep using the variable.

J.

[kilbey1]

Leatherback...

Yep, I am posting the information through a form.

Any way I can rid of those "NOTICE" messages, if everything appears to be in working order?

[leatherback]

Then when you use the function (Leave as it was suggested by astrotech), you have to call it with hte post:

echo calc($_POST[salary]);

IN the function the VALUE of $_POST[salary] is used.

J.

[AstroTeg]

Ok - its very good register globals are off. It should break some bad habits 🙂.

Let's take a look at your code:

function calc($ssa)   // don't know name of formula so this one will do
        { 
          if(($_POST['salary'] * 12) < 87000) 
            return ($_POST['salary'] * .062 / 12); 
          return (87000 * .062 / 12); 
        }  // end function calc($ssa)
        echo calc($ssa);

You're passing in $ssa. This is good. But the idea is $ssa contains the salary you are going to calculate (basically, $ssa needs to swap places with $_POST['salary']).

I've found it not very groovy to use $GET and $POST directly in fuctions. One, they're hard to type (I'm lazy). Two, they lock you into those two arrays and your function isn't overly portable. By passing the salary variable to the function, you can now use that function ANYWHERE - either from a form submit, a database, whatever you like. And you don't have to worry about recoding it to fit either.

So...

function calc($ssa)   // don't know name of formula so this one will do
        { 
          if(($ssa * 12) < 87000) 
            return ($ssa * .062 / 12); 
          return (87000 * .062 / 12); 
        }  // end function calc($ssa)

echo calc($_POST['salary']);  // here we grab the form variable and pass it to the function

When you go to call calc() from the echo statement, you'll pass in the form variable that contains the salary. Then the calc() function will go off in its own little world and return answer back which echo will then display. Make sense?

[kilbey1]

This all makes sense. But as the requirements progress, I am faced with two other issues. Is a function still the best choice here, or is a class what I need?

We have the formula for this SSA. The code below:

		function calc($ssa)   // 
        {  

          if(($ssa * 12) < 87900)
            return ($ssa * .062); //monthly only <= ssa cap
          	return ((87900 * .062) / 12);  //monthly only >= ssa cap
        } // end function calc($ssa) 

	echo "<P>";
	echo calc($_POST['salary']);  // here we grab the form variable and pass it to the function

...calculates the monthly SSA (social security) only.

A couple new things have to occur:

1. SSA percentage (.062) may change, and can be modified by an administrator, therefore amount is now in database
2. SSA cap (87900) may change per federal laws, and also can be modified by an administrator, therefore amount stored in database
3. Also need results for Yearly and Hourly SSA calculations

So then,
a. how does a database field(s) get implemented in a function?
b. how can I broaden this function to allow me to find Yearly and Hourly based on, say, a radio button being selected?

Example database data:
Database: recruiting
tablename: formulas
fields:
name, calculate, cap
row:
SSA, 0.062, 87900

You would then calculate based on the 'calculate' field, with a cap of 'cap'.

[kilbey1]

Ok, a bit further. I have laid out the function like so, in its own page, and included it into the main page.

formulas.inc.php

<?php

function ssacalc($ssa)   // 
        {  

          if(($ssa * 12) < 87900)
            return ($ssa * .062); //monthly only <= ssa cap
          	return ((87900 * .062) / 12);  //monthly only >= ssa cap
        } // end function calc($ssa) 


?>

I then perform the calculations from the main page like so:

    <tr> 
      <td width="25%">SSA</td>
      <td width="27%"> 
        <div align="right"><?php echo number_format(ssacalc($_POST['salary']) * 12, 2); ?></div>
      </td>
      <td width="23%"> 
        <div align="right"><?php echo number_format(ssacalc($_POST['salary']), 2); ?></div>
      </td>
      <td width="25%"> 
        <div align="right"><?php echo number_format(ssacalc($_POST['salary']) / 168, 2, ".",""); ?></div>
      </td>
    </tr>

But I am still at a loss on how to add calls to my database inside that function.

[AstroTeg]

If the function was to be used once in a page, I might get lazy and put the DB code in the function itself.

But since you're using the function at least 3 times, I'd lean towards keeping the DB code outside of this function. The idea is each time you query the database, its additional time and load you're script is using. So if you have the function called 3 times, thats three hits the function will make on the database. But if you call the database before the 3 funtions and then pass the DB values to the function itself, the you just hit the DB once.

It might also be of value to put the DB query code in its own function. Then you could decide how you want to retrieve the admin values and change your mind later. Lets say for example today you want to have the function get the values directly. You put the 2nd function call directly in the function, but later you decide it should be outside the function. You'd only have a few lines of code to update since the 2nd function is all setup (versus trying to untangle the coded mess you might have if you didn't use a function).

so you might have something like (and this is in pseudo code):

function calc($var1, $var2)
list($ssaval1, $ssaval2) = getSSAValues();
do your calculations here - but swap out the hardcoded numbers with the variables that hold those values

--

or if you want to do the function call outside of the function and pass the values on:

list($ssaval1, $ssava2) = getSSAValues();

// values passed in
function calc($var1, $var2, $ssaval1, $ssaval2)
do your calculations here - but swap out the hardcoded numbers with the variables that hold those values

--

How you do it is up to you...

[kilbey1]

I am not following this.

I want to take my function and pull in specific values from the database, for example:

SELECT from formulas where SSA
$result = mysql_query($sql);
(assign $row variable)
$ssa_modifier = $row['calculate']; //this is modifier: .062
$ssa_cap = $row['cap'];

		function ssacalc($ssa)   // 
        {  

	        if(($ssa * 12) < $cap)
    	        return ($ssa * $ssa_modifier); //monthly only <= ssa cap
    	    return (($cap * $ssa_modifier) / 12);  //monthly only >= ssa cap
        } // end function ssacalc($ssa) 

[kilbey1]

Would it look like this?

		function ssacalc($ssa)   // 
        {  

			$sql = "SELECT * FROM formulas WHERE name=SSA";
			$result = mysql_query($sql);s
			list($calculate, $cap) = getSSAValues(); 
	        if(($ssa * 12) < 87900)
				return ($ssa * $calculate);
			return (($cap * $calculate) / 12);
        } // end function ssacalc($ssa) 

Where $cap is field 'cap' from row 'ssa', and $calculate is field 'calculate' from row 'ssa'.

[AstroTeg]

Yes, but...

            $sql = "SELECT * FROM formulas WHERE name=SSA";
            $result = mysql_query($sql);s 

Would be in the function getSSAValues();