title: how to ask the user to enter account number and then return his balance from a text file
body: <p>I have a text file that has two columns, one for account numbers and the other for balances.
  I would like to ask the user for his account number and get his balance from the text.
  I have methods like deposit and withdraw, which I want to apply to the balances of the user, and then update the text file. What is the best way to do it?
  Should I use an array? or there are easier ways to do? </p>
  
  <p>The text file would be like this</p>
  
  <pre><code>1001 50.67
  1002 500.32
  1003 63.63
  1004 953.53
  1005 735.22
  </code></pre>