title: Replacing letters with numbers in a MATLAB array
body: <p>I am trying to write a function to mark the results of a test. The answers given by participants are stored in a nx1 cell array. However, theses are stored as letters. I am looking for a way to convert (a-d) these into numbers (1-4) ie. a=1, b=2 so these can be compared the answers using logical operations.  </p>
  
  <p>What I have so far is:<br>
       [num,txt,raw]=xlsread('FolkPhysicsMERGE.xlsx', 'X3:X142');<br>
       FolkPhysParAns=txt;</p>
  
  <p>I seem to be able to find how to convert from numbers into letters but not the other way around. I feel like there should be a relatively easy way to do this, any ideas?</p>