Implement exercise 42

[jlmitch5]

It says that exercise 42 is about learning how to retry after errors, but it's actually just a copy of the "Distinct Until Changed Input" exercise (40). Verified this was true in the index.html in this repo as well.

[elgerlambert]

Came here to report the same thing.

[morenoh149]

some code for anyone that wants to write a lesson. The following is probably just a copy of lesson 42.

    <p>You'll notice in the previous exercise that if you pressed your arrow keys while inside the textbox, the query
-           will still fire, regardless of whether the text actually changed or not. How do we prevent that? The
-           distinctUntilChanged filters out successive repetitive values.</p>
-       <pre>
-           seq([1,,,1,,,3,,,3,,,5,,,1,,,]).distinctUntilChanged() ===
-           seq([1,,,,,,,3,,,,,,,5,,,1,,,]);
-       </pre>
-
-       <textarea class="code" rows="60" cols="80">
-           function (keyPresses, isAlpha) {
-
-               return keyPresses.
-                   map(function (e) { return String.fromCharCode(e.keyCode); }).
-
-                   // Ensure we only have alphabetic characters
-                   filter(function (character) { return isAlpha(character); }).
-
-                   // TODO: Filter out successive repetitive keys
-                   // Building up a string of all the characters typed.
-                   scan('', function (stringSoFar, character) {
-                       return stringSoFar + character;
-                   });
-           }
-       </textarea>
-
-       <button class="go">Run</button>
-
-       <label for="inputName">Enter Keys</label>
-       <input type="text" class="inputName">
-
-       <div>Keys filtered by distinctUntilChanged</div>
-       <div class="filteredKeysByDistinct"></div>
-
-       <button class="showAnswer">Show Answer</button>
-
-        <pre class="verifier">
-           function(str, lesson) {
-               preVerifierHook();
-               var $inputName = $('.inputName', lesson),
-                   $filtered = $('.filteredKeysByDistinct', lesson);
-
-               var keyups = Rx.Observable.fromEvent($inputName[0], 'keyup');
-
-               var isAlpha = function (x) {
-                   return 'abcdefghijklmnopqrstuvwxyz'.indexOf(x.toLowerCase()) !== -1;
-               };
-
-               var code =  eval("(" + str + ")")
-               var code = code(keyups, isAlpha);
-
-               code
-                   .subscribe(function (text) {
-                       $filtered.text(text);
-                   });
-           }
-       </pre>
-        <pre class="answer">
-           function (keyPresses, isAlpha) {
-
-               return keyPresses.
-                   map(function (e) { return String.fromCharCode(e.keyCode); }).
-                   filter(function (character) { return isAlpha(character); }).
-                   distinctUntilChanged().
-                   scan('', function (stringSoFar, character) {
-                       return stringSoFar + character;
-                   });
-           }
-        </pre>
-       <div class="post">
-           <p>Now that we know how to get only the distinct input, let's see how it applies to our autocomplete example...</p>
-
-           <h1>A Work in Progress</h1>
-
-           <p>Congratulations! You've made it this far, but you're not done. Learning is an on-going process. Go out and start
-               using the functions you've learned in your day-to-day coding. Over time, I'll be adding more exercises to this
-               tutorial. If you have suggestions for more exercises, send me a pull request!
-           </p>
-       </div>