title: How can I guarantee that my Rubik's cube scrambling algorithm doesn't select the same direction twice in a row?
body: <p>I am working on a Rubik's Cube Timer Website, (JavaScript) and it requires a scrambling algorithm. It would be easy if it was just picking random letters from an array, but there are requirements that it needs to meet. Each Letter Represents a move in the Rubik's Cube Algorithmic notation, so for example "L" would mean to move the Left side clockwise. Or "U2" would mean to move the Upper side two times, and " B' " would mean to move the Backside anticlockwise. and so on.</p>
  
  <p>The thing is that there cannot be two of the same letters next to each other, and not even if they are in different directions. For example, U cannot be next to U', or U2 and so on. It must be a different letter. Sometimes, my code generates 2 of the same letter next to each other.</p>
  
  <p>Here's my code:</p>
  
  <pre><code>function generateScramble() {
  
      //Possible Letters
      var array = new Array(" U", " D", " R", " L", " F", " B", " U\'", " D\'", " R\'", " L\'", " F\'", " B\'", " U2", " D2", " R2", " L2", " F2", " B2"); 
  
      var array2 = new Array(); // The Scramble.
  
      var rdArr = new Array(); // The Array of random numbers.
  
      for (var i = 0; i &lt; 20; i++) {
          var random = Math.floor(Math.random() * array.length);
          rdArr.unshift(random);
  
          if (rdArr[1] - rdArr[0] == 0 ||
              rdArr[0] - rdArr[1] == 0 ||
              rdArr[1] - rdArr[0] == 6 ||
              rdArr[0] - rdArr[1] == 6 ||
              rdArr[1] - rdArr[0] == 12 ||
              rdArr[0] - rdArr[1] == 12) { // Check whether a D is next to D' or D2, or if F is next to F' or F2, R next to R' or R2, and so on
              if (random &lt; 17) {
                  random++;
              } else {
                  random--;
              }
          }
  
          array2.push(array[random]); // Get letters in random order in the array.
      }
  
      var scramble = "Scramble: " + array2[0] + array2[1] + array2[2] + array2[3] + array2[4]
                                  + array2[5] + array2[6] + array2[7] + array2[8] + array2[9]
                                  + array2[10] + array2[11] + array2[12] + array2[13] + array2[14]
                                  + array2[15] + array2[16] + array2[17] + array2[18] + array2[19];
  
      document.getElementById("Scramble").innerHTML = scramble; // Display the scramble
  
  }
  </code></pre>