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updatePrices.js

function onOpen() {
  SpreadsheetApp.getActiveSpreadsheet().addMenu("Update", [
    {name: "Prices", functionName: "updatePrices"}
  ]);
}

function updatePrices() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var cell = sheet.getRange('D2');
  while (/^...\/...$/.test(cell.getValue())) {
    var ticker = cell.getValue().replace('/', '_').toLowerCase();
    var valueCell = sheet.getRange(cell.getRow(), cell.getColumn() + 1);
    var url = 'https://btc-e.com/api/2/' + ticker + '/ticker';
    Logger.log('Fetching last price for ' + ticker);
    var lastPrice = JSON.parse(UrlFetchApp.fetch(url)).ticker.last;
    Logger.log('Last price for ' + ticker + ' = ' + lastPrice);
    valueCell.setValue(lastPrice);
    cell = sheet.getRange(cell.getRow() + 1, cell.getColumn());
  }
}