[Bookstore](https://github.com/rgbkrk/bookstore) ([post on rackspace](http://developer.rackspace.com/blog/bookstore-for-ipython-notebooks.html)) is the only storage backend that supports multiple checkpoints for now. We hope that someone will write a git backend at some point. It will just not come from the core team soon.

If you are motivated to do so, we&#39;ll be happy to give you pointers. 


maybe you can try this, I add multiple checkpoint for every time of saving . https://github.com/jupyter/notebook/issues/3563


**EDIT**: For anyone new to this question, I have posted an answer clarifying what was going on. The accepted answer is the one I feel best answers my question as originally posted, but for further details please see my answer.

**NOTE**: This problem was originally pseudocode and used lists. I have adapted it to Java and arrays. So while I&#39;d love to see any solutions that use Java-specific tricks (or tricks in any language for that matter!), just remember that the original problem is language-independent.

The Problem
-----------

Let&#39;s say that there are two unsorted integer arrays `a` and `b`, with element repetition allowed. They are identical (with respect to contained elements) *except* one of the arrays has an extra element. As an example:

    int[] a = {6, 5, 6, 3, 4, 2};
    int[] b = {5, 7, 6, 6, 2, 3, 4};
Design an algorithm that takes as input these two arrays and outputs the single unique integer (in the above case, 7).

The Solution (So Far)
---------------------

I came up with this:

    public static int getUniqueElement(int[] a, int[] b) {
    	int ret = 0;
    	for (int i = 0; i &lt; a.length; i++) {
    		ret ^= a[i];
    	}
    	for (int i = 0; i &lt; b.length; i++) {
    		ret ^= b[i];
    	}
    	return ret;
    }
The &quot;official&quot; solution presented in class:

    public static int getUniqueElement(int[] a, int[] b) {
    	int ret = 0;
    	for (int i = 0; i &lt; a.length; i++) {
    		ret += a[i];
    	}
    	for (int i = 0; i &lt; b.length; i++) {
    		ret -= b[i];
    	}
    	return Math.abs(ret);
    }
So, both are conceptually doing the same thing. And given that `a` is of length m and `b` is of length n, then both solutions have running time of O(m + n).

The Question
------------

I later got to talking with my teacher and he hinted that there was an even *faster* way of doing it. Honestly I don&#39;t see how; to find out whether an element *is* unique it seems you&#39;d have to at least look at every element. At that&#39;s at least O(m + n)...right?

So is there a faster way? And if so, what is it?

Faster algorithm to find unique element between two arrays?

There are lots of posts about regexs to match a _potentially_ empty string, but I couldn&#39;t readily find any which provided a regex which _only_ matched an empty string.

I know that `^` will match the beginning of any line and `$` will match the end of any line as well as the end of the string. As such, `/^$/` matches far more than the empty string such as &quot;\n&quot;, &quot;foobar\n\n&quot;, etc.

I would have thought, though, that `/\A\Z/` would match just the empty string, since `\A` matches the beginning of the string and `\Z` matches the end of the string. However, my testing shows that `/\A\Z/` will also match &quot;\n&quot;. Why is that?

What is a regex to match ONLY an empty string?

I see that IPython Notebook has a menu item: `File &gt; Revert to Checkpoint`, but this never contains more than a single entry for any of my notebooks.

Is there a way to allow this menu to hold multiple checkpoints? I can&#39;t find documentation about how to do this anywhere on the web.
Thanks.

Also, I put in the green box as a highlight.

![image showing only a single entry to &#39;Revert to Checkpoint&#39; list.][1]


  [1]: http://i.stack.imgur.com/UKMOp.png

IPython Notebook Multiple Checkpoints

I see that IPython Notebook has a menu item: <code>File > Revert to Checkpoint</code>, but this never contains more than a single entry for any of my notebooks.
Is there a way to allow this menu to hold multiple checkpoints? I can't find documentation about how to do this anywhere on the web.
Thanks.
Also, I put in the green box as a highlight.
<img src="http://i.stack.imgur.com/UKMOp.png" alt="image showing only a single entry to &#x27;Revert to Checkpoint&#x27; list.">

I have a `dataframe df`:
```
20060930  10.103       NaN     10.103   7.981
20061231  15.915       NaN     15.915  12.686
20070331   3.196       NaN      3.196   2.710
20070630   7.907       NaN      7.907   6.459
```
Then I want to select rows with certain sequence numbers which indicated in a list, suppose here is [1,3], then left:
```                       
20061231  15.915       NaN     15.915  12.686
20070630   7.907       NaN      7.907   6.459
```
How or what function can do that?

Select Pandas rows based on list index

I&#39;m trying to find out why the use of `global` is considered to be bad practice in python (and in programming in general). Can somebody explain? Links with more info would also be appreciated.

Why are global variables evil?

I have an integer

    {% set curYear = 2013 %}

In `{% if %}` statement I have to compare it with some string. I can&#39;t set `curYear` to string at the beginning because I have to decrement it in loop.

How can I convert it?

Convert integer to string Jinja

I&#39;m pretty new in `numpy` and I am having a hard time understanding how to extract from a `np.array` a sub matrix with defined columns and rows:

    Y = np.arange(16).reshape(4,4)
If I want to extract columns/rows 0 and 3, I should have:

    [[0 3]
     [12 15]]
I tried all the reshape functions...but cannot figure out how to do this.  Any ideas?

Numpy extract submatrix

I have some points and I am trying to fit curve for this points. I know that there exist `scipy.optimize.curve_fit` function, but I do not understand documentation, i.e how to use this function.

My points: `np.array([(1, 1), (2, 4), (3, 1), (9, 3)])`

Can anybody explain how to do that?

python numpy/scipy curve fitting

I usually have to rerun (most parts of) a notebook when reopen it, in order to get access to previously defined variables and go on working. 

However, sometimes I&#39;d like to skip some of the cells, which have no influence to subsequent cells (e.g., they might comprise a branch of analysis that is finished) and could take very long time to run. These cells can be scattered throughout the notebook, so that something like &quot;Run All Below&quot; won&#39;t help much.

Is there a way to achieve this?


Ideally, those cells could be tagged with some special flags, so that they could be &quot;Run&quot; manually, but would be skipped when &quot;Run All&quot;.

**EDIT**

`%%cache` (ipycache extension) as suggested by @Jakob solves the problem to some extent.

Actually, I don&#39;t even need to load any variables (which can be large but unnecessary for following cells) when re-run, only the stored output matters as analyzing results.

As a work-around, put `%%cache folder/unique_identifier` to the beginning of the cell. The code will be executed only once and no variables will be loaded when re-run unless you delete the unique_identifier file.

&lt;del&gt;**Unfortunately**, all the output results are lost when re-run with `%%cache`...&lt;/del&gt;

**EDIT II** (Oct 14, 2013)

The master version of ipython+ipycache now pickles (and re-displays) the codecell output as well.

For rich display outputs including Latex, HTML(pandas DataFrame output), remember to use IPython&#39;s display() method, e.g., `display(Latex(r&#39;$\alpha_1$&#39;))`

How to (intermittently) skip certain cells when running IPython notebook?

The following code allows me to view a `png` image in an iPython notebook. Is there a way to view `pdf` image? I don&#39;t need to use IPython.display necessarily. I am looking for a way to print a pdf image in a file to the iPython notebook output cell.

    ## This is for an `png` image
    from IPython.display import Image
    
    fig = Image(filename=(&#39;./temp/my_plot.png&#39;))
    fig

Thank you.

View pdf image in an iPython Notebook

If I have multiple images (loaded as NumPy arrays) how can I display the in one IPython Notebook cell?

I know that I can use `plt.imshow(ima)` to display *one* image… but I want to show more than one at a time.

I have tried:

     for ima in images:
         display(Image(ima))

But I just get a broken image link:

![enter image description here][1]


  [1]: http://i.stack.imgur.com/YXFUq.png

Display multiple images in one IPython Notebook cell?

Is there a way to access variables in the current python kernel from within a `%%bash` or other `%%script` cell?

Perhaps as command line arguments or environment variable(s)?

Can I access python variables within a `%%bash` or `%%script` ipython notebook cell?

 I am new to Python to bear with me.

1. I installed Anaconda, works great.
2. I setup a Python 3 environment following the Anaconda [cmd line instructions][1], works great.
3. I [setup Anaconda&#39;s Python 3 environment as Pycharm&#39;s interpreter][2], works great.
4. I launched the [Anaconda &quot;launcher.app&quot;][3] and launched IPython Notebook. However, iPython Notebook is running Python 2 not 3.

Over three hours of Googling later, I cannot figure out how to set IPython Notebook to run Python 3 instead of 2.


  [1]: http://continuum.io/blog/anaconda-python-3
  [2]: http://docs.continuum.io/anaconda/ide_integration.html
  [3]: http://docs.continuum.io/_images/Anaconda_Launcher.png

Content Type	Original Author	Original Content on Stackoverflow
Question	zbinsd	View Question on Stackoverflow
Solution 1 - Python	Matt	View Answer on Stackoverflow
Solution 2 - Python	orange	View Answer on Stackoverflow

IPython Notebook Multiple Checkpoints

Python Problem Overview

Python Solutions

Solution 1 - Python

Solution 2 - Python

What is a regex to match ONLY an empty string?

Faster algorithm to find unique element between two arrays?

Attributions