You can access the elements in a list-of-lists by first specifying which list you&#39;re interested in and then specifying which element of that list you want. For example, `17` is element `2` in list `0`, which is `list1[0][2]`:

    &gt;&gt;&gt; list1 = [[10,13,17],[3,5,1],[13,11,12]]
    &gt;&gt;&gt; list1[0][2]
    17

So, your example would be

    50 - list1[0][0] + list1[0][1] - list1[0][2]

You can use `itertools.cycle`:

    &gt;&gt;&gt; from itertools import cycle
    &gt;&gt;&gt; lis = [[10,13,17],[3,5,1],[13,11,12]]
    &gt;&gt;&gt; cyc = cycle((-1, 1))
    &gt;&gt;&gt; 50 + sum(x*next(cyc) for x in lis[0])   # lis[0] is [10,13,17]
    36

Here the generator expression inside `sum` would return something like this:

    &gt;&gt;&gt; cyc = cycle((-1, 1))
    &gt;&gt;&gt; [x*next(cyc) for x in lis[0]]
    [-10, 13, -17]

You can also use `zip` here:

    &gt;&gt;&gt; cyc = cycle((-1, 1))
    &gt;&gt;&gt; [x*y for x, y  in zip(lis[0], cyc)]
    [-10, 13, -17]



This code will print each individual number:

    for myList in [[10,13,17],[3,5,1],[13,11,12]]:
        for item in myList:
            print(item)


Or for your specific use case:

    ((50 - List1[0][0]) + List1[0][1]) - List1[0][2]



    List1 = [[10,-13,17],[3,5,1],[13,11,12]]

    num = 50
    for i in List1[0]:num -= i
    print num

    50 - List1[0][0] + List[0][1] - List[0][2]

`List[0]` gives you the first list in the list (try out `print List[0]`). Then, you index into it again to get the items of that list. Think of it this way: `(List1[0])[0]`.


    for l in list1:
        val = 50 - l[0] + l[1] - l[2]
        print &quot;val:&quot;, val

Loop through list and do operation on the sublist as you wanted.

    new_list = list(zip(*old_list)))

*old_list unpacks old_list into multiple lists and zip picks corresponding nth element from each list and list packs them back.

Consider the following scenario. 

    List&lt;String&gt; list = new ArrayList&lt;&gt;();
Now I added the `String` values for this list. 

I used following ways to go each and every element in the list.

Option one- use `for-each`
   
    for (String i : list) {
            System.out.println(i);
        } 

Option two- use `Iterator`

  

 

    Iterator it=list.iterator();
    while (it.hasNext()){
       System.out.println(it.next());
    }

I just want to know is there any performance advantage if I use `for-each` instead of `Iterator`. And also is it a bad practice to use Iterator now a days in Java?

For-each vs Iterator. Which will be the better option

Say I have a column in a dataframe that has some numbers and some non-numbers

    &gt;&gt; df[&#39;foo&#39;]
    0       0.0
    1     103.8
    2     751.1
    3       0.0
    4       0.0
    5         -
    6         -
    7       0.0
    8         -
    9       0.0
    Name: foo, Length: 9, dtype: object

How can I convert this column to `np.float`, and have everything else that is not float convert it to `NaN`?

When I try:

    &gt;&gt; df[&#39;foo&#39;].astype(np.float)

or

    &gt;&gt; df[&#39;foo&#39;].apply(np.float)

I get `ValueError: could not convert string to float: -`





Pandas: Converting to numeric, creating NaNs when necessary

If I have a list of lists and just want to manipulate an individual item in that list, how would I go about doing that?

For example: 

    List1 = [[10,13,17],[3,5,1],[13,11,12]]

What if I want to take a value (say 50) and look just at the first sublist in `List1`, and subtract 10 (the first value), then add 13, then subtract 17?

Access item in a list of lists

If I have a list of lists and just want to manipulate an individual item in that list, how would I go about doing that?
For example:
<pre><code class="hljs language-lua">List1 = [[10,13,17],[3,5,1],[13,11,12]]
</code></pre>
What if I want to take a value (say 50) and look just at the first sublist in <code>List1</code>, and subtract 10 (the first value), then add 13, then subtract 17?

I&#39;d like to create a stacked histogram.  If I have a single 2-D array, made of three equal length data sets, this is simple.  Code and image below:

    import numpy as np
    from matplotlib import pyplot as plt

    # create 3 data sets with 1,000 samples
    mu, sigma = 200, 25
    x = mu + sigma*np.random.randn(1000,3)

    #Stack the data
    plt.figure()
    n, bins, patches = plt.hist(x, 30, stacked=True, density = True)
    plt.show()

![enter image description here][1]


However, if I try similar code with three data sets of a different length the results are that one histogram covers up another.  Is there any way I can do the stacked histogram with mixed length data sets?

    ##Continued from above
    ###Now as three separate arrays
    x1 = mu + sigma*np.random.randn(990,1)
    x2 = mu + sigma*np.random.randn(980,1)
    x3 = mu + sigma*np.random.randn(1000,1)

    #Stack the data
    plt.figure()
    plt.hist(x1, bins, stacked=True, density = True)
    plt.hist(x2, bins, stacked=True, density = True)
    plt.hist(x3, bins, stacked=True, density = True)
    plt.show()

![enter image description here][2]


  [1]: http://i.stack.imgur.com/4aBSy.png
  [2]: http://i.stack.imgur.com/sZ92m.png

Create stacked histogram from unequal length arrays

Is there a built-in/quick way to use a list of keys to a dictionary to get a list of corresponding items?

For instance I have:

    &gt;&gt;&gt; mydict = {&#39;one&#39;: 1, &#39;two&#39;: 2, &#39;three&#39;: 3}
    &gt;&gt;&gt; mykeys = [&#39;three&#39;, &#39;one&#39;]

How can I use `mykeys` to get the corresponding values in the dictionary as a list?

    &gt;&gt;&gt; mydict.WHAT_GOES_HERE(mykeys)
    [3, 1]


Dictionary: Get list of values for list of keys

I have the following data set.  I would like to use Python or Gnuplot to plot the data.  The tuples are of the form `(x, y)`.  The Y-axis should be a log axis, that is, `log(y)`.  A scatter plot or line plot would be ideal.

How can this be done?

     [(0, 6.0705199999997801e-08), (1, 2.1015700100300739e-08), 
     (2, 7.6280656623374823e-09), (3, 5.7348209304555086e-09), 
     (4, 3.6812203579604238e-09), (5, 4.1572516753310418e-09)]



How do I plot list of tuples in Python?

Given two ndarrays

    old_set = [[0, 1], [4, 5]]
    new_set = [[2, 7], [0, 1]]

I&#39;m looking to get the mean of the respective values between the two arrays so that the data ends up something like:

    end_data = [[1, 4], [2, 3]]

basically it would apply something like 

    for i in len(old_set):
        end_data[i] = (old_set[i]+new_set[i])/2

But I&#39;m unsure what syntax to use.. Thanks for the help in advance!


Average values in two Numpy arrays

The [Cython documentation on typed memory views](http://docs.cython.org/src/userguide/memoryviews.html) list three ways of assigning to a typed memory view:

1. from a raw C pointer,
2. from a `np.ndarray` and
3. from a `cython.view.array`.

Assume that I don&#39;t have data passed in to my cython function from outside but instead want to allocate memory and return it as a `np.ndarray`, which of those options do I chose? Also assume that the size of that buffer is not a compile-time constant i.e. I can&#39;t allocate on the stack, but would need to `malloc` for option 1.

The 3 options would therefore looke something like this:

    from libc.stdlib cimport malloc, free
    cimport numpy as np
    from cython cimport view

    np.import_array()

    def memview_malloc(int N):
        cdef int * m = &lt;int *&gt;malloc(N * sizeof(int))
        cdef int[::1] b = &lt;int[:N]&gt;m
        free(&lt;void *&gt;m)

    def memview_ndarray(int N):
        cdef int[::1] b = np.empty(N, dtype=np.int32)
        
    def memview_cyarray(int N):
        cdef int[::1] b = view.array(shape=(N,), itemsize=sizeof(int), format=&quot;i&quot;)

What is surprising to me is that in all three cases, [Cython generates quite a lot of code](http://htmlpreview.github.com/?https://gist.github.com/kynan/910a2e4ce3332c9bd8c2/raw/a4131b4417d4b93bff0064e7a14b683e27564631/memview_bench.html) for the memory allocation, in particular a call to `__Pyx_PyObject_to_MemoryviewSlice_dc_int`. This suggests (and I might be wrong here, my insight into the inner workings of Cython are very limited) that it first creates a Python object and then &quot;casts&quot; it into a memory view, which seems unnecessary overhead.

A [simple benchmark](http://nbviewer.ipython.org/urls/gist.github.com/kynan/910a2e4ce3332c9bd8c2/raw/c4c935d34eb77e1ec0fb06bfa40ebce54a1c1812/Benchmark+Cython+typed+memory+view+allocation.ipynb) doesn&#39;t reveal much difference between the three methods, with 2. being the fastest by a thin margin.

Which of the three methods is recommended? Or is there a different, better option?

**Follow-up question:** I want to finally return the result as a `np.ndarray`, after having worked with that memory view in the function. Is a typed memory view the best choice or would I rather just use the old buffer interface as below to create an `ndarray` in the first place?

    cdef np.ndarray[DTYPE_t, ndim=1] b = np.empty(N, dtype=np.int32)

What is the recommended way of allocating memory for a typed memory view?

Given a list, is there a way to get the first non-None value? And, if so, what would be the pythonic way to do so?

For example, I have:

 - `a = objA.addreses.country.code` 
 - `b = objB.country.code`
 - `c = None` 
 - `d = &#39;CA&#39;`

In this case, if a is None, then I would like to get b. If a and b are both None, the I would like to get d.

Currently I am doing something along the lines of `(((a or b) or c) or d)`, is there another way?

Getting the first non None value from list

Using LINQ, from a `List&lt;int&gt;`, how can I retrieve a list that contains entries repeated more than once and their values?

C# LINQ find duplicates in List

I have two lists as below

    tags = [u&#39;man&#39;, u&#39;you&#39;, u&#39;are&#39;, u&#39;awesome&#39;]
    entries = [[u&#39;man&#39;, u&#39;thats&#39;],[ u&#39;right&#39;,u&#39;awesome&#39;]]
    
I want to extract entries from `entries` when they are in `tags`:

    result = []
    
    for tag in tags:
        for entry in entries:
            if tag in entry:
                result.extend(entry)

How can I write the two loops as a single line list comprehension?

How to frame two for loops in list comprehension python

I need something like (ansible inventory file):

    [example]
    127.0.0.1 timezone=&quot;Europe/Amsterdam&quot; locales=&quot;en_US&quot;,&quot;nl_NL&quot;

However, ansible does not recognize &#39;locales&#39; as a list. 


Content Type	Original Author	Original Content on Stackoverflow
Question	John	View Question on Stackoverflow
Solution 1 - Python	arshajii	View Answer on Stackoverflow
Solution 2 - Python	Ashwini Chaudhary	View Answer on Stackoverflow
Solution 3 - Python	Phil	View Answer on Stackoverflow
Solution 4 - Python	Joran Beasley	View Answer on Stackoverflow
Solution 5 - Python	Donald Miner	View Answer on Stackoverflow
Solution 6 - Python	rajpy	View Answer on Stackoverflow
Solution 7 - Python	Coddy	View Answer on Stackoverflow

Access item in a list of lists

Python Problem Overview

Python Solutions

Solution 1 - Python

Solution 2 - Python

Solution 3 - Python

Solution 4 - Python

Solution 5 - Python

Solution 6 - Python

Solution 7 - Python

Pandas: Converting to numeric, creating NaNs when necessary

For-each vs Iterator. Which will be the better option

Attributions