Robson » Little Red Book » Currency Conversion #1

Chapter 2 - Question 4

Write programs to convert pounds sterling to other major currencies.

  1. United States Dollars
  2. French Francs
  3. Dutch Guilders
  4. Austrian Schillings
  5. Danish Krona

Input is to be a number representing a quantity in pounds, sterling; output is to be equivalent amount and the currency unit involved.

Solution 1

I decided to expand this problem to make it more challenging. I'd already written a C++ Measurements Converter, so I had no wish to just write another one. That would be too simple.

I wanted to obtain the exchange rates from the web, so writing this solution in PHP seemed ideal. To be polite I should cache the results to avoid using up loads of someone's bandwidth. After stumbling around various policy-filled sites, I eventually found the Bank of England. They allows you to build a list of currencies and then download them in a variety of formats. Very useful!

Here's my PHP solution:

   // filename to use for storing exchange rates
   define('RATES_FILE', $_SERVER['DOCUMENT_ROOT'] . '/littleredbook/rates-cache.csv');
   // check if the rates file is a day old
   // content is cached to avoid hammering the server
   // with this method we only make one request every 24 hours
   if (!file_exists(RATES_FILE) || @filemtime(RATES_FILE) < strtotime('yesterday'))
       // make up the static part of the url
       $url = ''
            . 'Travel=NIxIRxSUx&FromSeries=1&ToSeries=50&DAT=RNG&'
            . 'csv.x=20&csv.y=27&CSVF=TN&FN=N&C=ECH&C=C8J&C=C8N&C=ECU&C=C8P';
       // add yesterdays date
       $url .= '&FD=' . date('d', strtotime('yesterday'))
             . '&FM=' . date('M', strtotime('yesterday'))
             . '&FY=' . date('Y', strtotime('yesterday'));
       // add todays date
       $url .= '&TD=' . date('d') . '&TM=' . date('M') . '&TY=' . date('Y');
       // get the exchange rates and store them
       $csv = file_get_contents($url);
       // store locally
       $fp = fopen(RATES_FILE, 'wb');
       // write the rates to a file
       fwrite($fp, $csv);
       // close the file so anything else can use it
       // if it's still new, we can use the cached version
       $csv = file_get_contents(RATES_FILE);
   // make up a random amount of great british pounds
   $gbp = mt_rand(1, 100);
   // these are the currency names
   $currency = array('Danish Krone', 'Euro', 'Japanese Yen', 'Swiss Franc', 'American Dollar');
   // extract all the values into an array and return the rates from the end
   $rates = array_slice(explode(',', $csv), -count($currency));
   // show how many great british pounds we have
   echo $gbp . ' GBP is:<ul>';
   // loop through all available currency
   for ($n = 0; $n < count($currency); $n++)
       // show how much this is when converted
       echo '<li>' . round($rates[$n] * $gbp, 2) . ' ' . $currency[$n] . '</li>';
   // end the list
   echo '</ul>';    

The amount is generated randomly because I don't want people to use my site as a currency converter. Here's the script in action:

14 GBP is:
  • 0 Danish Krone
  • 0 Euro
  • 0 Japanese Yen
  • 0 Swiss Franc
  • 0 American Dollar

You can refresh for a different amount.


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