How to Wrote Simple Calculations by Using Custom Fields in Loop?

Question

I am building a website with whole bunch of custom fields and need to implement several custom fields that will auto calculate values by using values from other custom fields. All files which I need to use in these calculation are in loop, so I would love to use hooks in functions.php.

Can somebody give me few examples on how to wrote filters with some simple calculations so I could continue writing filters for different problems I need to solve, like:

---

Example 1: Sum 2 Numbers

Custom Field 1: product_price
Custom Field 2: addon_price
Custom Field 3: price_total

Formula:

price_total = product_price + addon_price

---

Example 2: Add commision on price

Custom Field 1: product_price
Custom Field 2: tax_commision - [%]
Custom Field 3: price_total

Formula:

price_total = product_price + ( tax_commision / 100 ) * product_price

Answer 1

Define your own template tag

One idea is to use the template tag:

<?php the_price();?>

or

<?php the_price( '%.1f' );?>

to change the format to your needs.

Then you can add filters to it as you like.

In your functions.php file you could add

// setup the price filters:
add_filter( 'the_price' , 'xyz_add_product_price',      1, 1 );
add_filter( 'the_price' , 'xyz_add_commission',         2, 1 );
add_filter( 'the_price' , 'xyz_add_shipping',           3, 1 );

where you use the priority to control the order of the calculations.

The xyz_ is just a prefix that you can change.

The definition of the template tag could look like this:

/**
* Template tag to display the price
* @param  string $d Output format
* @return void
*/
function the_price( $d = '' ){
    echo get_the_price( $d );
}

where:

/**
* Template tag to return the price
* @param  string $d     Output format
* @return number $price Price
*/
function get_the_price( $d = '' ){

        $price = apply_filters( 'the_price', 0 );

        if( empty( $d ) )
                $d =  '%.2f'; // with 2 decimals

        return sprintf( $d, $price );
}

It might be a good idea to check if these template tags already exists with function_exists() or use an additional custom prefix.

Add the price filters

Your first price filter might be to add the product price:

/**
* Price filter #1
* Add product price 
* @param number $price
* @return number $price
*/
function xyz_add_product_price( $price ) {

        $product_price  = get_post_meta( get_the_ID(), 'product_price', TRUE );

        if( ! empty(  $product_price ) ){

                // convert the values from string to int/float:
                $product_price = 1 * $product_price;

                // add product price:
                return ( $price + $product_price );
        }else{
                return $price;
        }
}

and the second one for the tax commmision:

/**
* Price filter #2
* Add tax commission to the product price
* @param number $price
* @return number $price
*/
function xyz_add_commission( $price = 0 ) {

    $tax_commision  = get_post_meta( get_the_ID(), 'tax_commision', TRUE );

    if( ! empty(  $tax_commision ) ){
            // convert the values from string to int/float:
            $tax_commision = 1 * $tax_commision;

            // calculate the commision and add to the price:
            return ( $price  + ( $tax_commision / 100 ) * $price ) ;
    }else{
            return $price;
    }
}

and finally the third one for the shipping fee:

/**
* Price filter #3
* Add shipping fee to the product price
* @param number $price
* @return number $price
*/

function xyz_add_shipping( $price ) {
       $shipping_fee = 10;
       return ( $price + $shipping_fee );
}

... and you can add more later on.

Answer 2

I don't really understand why you have a custom field with the total price (do you fill in these by hand?). Would you like to calculate the total price in the admin area? I don't know how to do that, but I assume you would like to display the total price on the front-end.

If so, then I'd write a function which has 2 input arguments, which can be $product and $addon (for example 2 it's $product and $tax). Then you can use the function to calculate the total price and display it on the front-end.

You can write a function in functions.php which would look something like this:

Example 1

function sum_2_numbers($product, $addon){
    $price_total = $product + $addon;
    return $price_total;
}

On the page where you would like to display the total price:

<?php echo sum_2_numbers(get_post_custom('product_price'), get_post_custom('addon_price')); ?>

Example 2

function commission($product, $tax){
    $price_total = $product + ($tax / 100) * $product;
    return $price_total;
}

On the page where you would like to display the total price:

<?php echo commission(get_post_custom('product_price'), get_post_custom('tax_commission')); ?>