I've got python where an assignment to a specific 2d list in python is updating multiple elements and I don't know why.  I'm not looking for a better way to do things as much as I'm trying to understand what is going on here so I get my mistake before just doing something else.  Below is the code and beneath it snippet of what gets printed out after a couple loops.  Notice that with each assignment to an index, the tuple being assigned to that index is written to multiple locations in the 2d list. I don't even know how that is possible.  num_list = [] for num in range(32, 43, 1):     num_list.append((str(num), chr(num) + " ")) print print "num_list:" print num_list print  cols = 4 rows = len(num_list) / cols  arry2d = [[("a","b")] * cols] * (rows+1) print ("arry2d after initialization") print arry2d print   for i, item in enumerate(num_list):         print "i: " + str(i),         print "row: ", i / cols,         print "column: ", i % cols,         print "contents before update: ", arry2d[i / cols][i % cols]         print "item: ", item         print "array 2d after update with item: ", item         arry2d[i / cols][i % cols] = item         print arry2d ----------------------------------------------------   Output  num_list [('32', '  '), ('33', '! '), ('34', '" '), ('35', '# '), ('36', '$ '), ('37', '% '), ('38', '&amp; '), ('39', "' "), ('40', '( '), ('41', ') '), ('42', '* ')]  arry2d after initialization [[('a', 'b'), ('a', 'b'), ('a', 'b'), ('a', 'b')], [('a', 'b'), ('a', 'b'), ('a', 'b'), ('a', 'b')], [('a', 'b'), ('a', 'b'), ('a', 'b'), ('a', 'b')]]  i: 0 row:  0 column:  0 contents before update:  ('a', 'b') item:  ('32', '  ') array 2d after update with item:  ('32', '  ') [[('32', '  '), ('a', 'b'), ('a', 'b'), ('a', 'b')], [('32', '  '), ('a', 'b'), ('a', 'b'), ('a', 'b')], [('32', '  '), ('a', 'b'), ('a', 'b'), ('a', 'b')]] i: 1 row:  0 column:  1 contents before update:  ('a', 'b') item:  ('33', '! ') array 2d after update with item:  ('33', '! ') [[('32', '  '), ('33', '! '), ('a', 'b'), ('a', 'b')], [('32', '  '), ('33', '! '), ('a', 'b'), ('a', 'b')], [('32', '  '), ('33', '! '), ('a', 'b'), ('a', 'b')]]
I needed to create a list of lists in Python, so I typed the following: my_list = [[1] * 4] * 3  The list looked like this: [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1, 1]]    Then I changed one of the innermost values: my_list[0][0] = 5  Now my list looks like this: [[5, 1, 1, 1], [5, 1, 1, 1], [5, 1, 1, 1]]    which is not what I wanted or expected. Can someone please explain what's going on, and how to get around it?
The help pages in R assume I know what those numbers mean, but I don't. I'm trying to really intuitively understand every number here. I will just post the output and comment on what I found out. There might (will) be mistakes, as I'll just write what I assume. Mainly I'd like to know what the t-value in the coefficients mean, and why they print the residual standard error.   Call: lm(formula = iris$Sepal.Width ~ iris$Petal.Width)  Residuals:      Min       1Q   Median       3Q      Max  -1.09907 -0.23626 -0.01064  0.23345  1.17532    This is a 5-point-summary of the residuals (their mean is always 0, right?). The numbers can be used (I'm guessing here) to quickly see if there are any big outliers. Also you can already see it here if the residuals are far from normally distributed (they should be normally distributed).  Coefficients:                  Estimate Std. Error t value Pr(&gt;|t|)     (Intercept)       3.30843    0.06210  53.278  &lt; 2e-16 *** iris$Petal.Width -0.20936    0.04374  -4.786 4.07e-06 *** --- Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1    Estimates $\hat{\beta_i}$, computed by least squares regression. Also, the standard error is $\sigma_{\beta_i}$. I'd like to know how this is calculated.  I have no idea where the t-value and the corresponding p-value come from. I know $\hat{\beta}$ should be normal distributed, but how is the t-value calculated?  Residual standard error: 0.407 on 148 degrees of freedom   $\sqrt{ \frac{1}{n-p} \epsilon^T\epsilon }$, I guess. But why do we calculate that, and what does it tell us?  Multiple R-squared: 0.134,  Adjusted R-squared: 0.1282    $ R^2 = \frac{s_\hat{y}^2}{s_y^2} $, which is $ \frac{\sum_{i=1}^n (\hat{y_i}-\bar{y})^2}{\sum_{i=1}^n (y_i-\bar{y})^2} $. The ratio is close to 1 if the points lie on a straight line, and 0 if they are random.  What is the adjusted R-squared?  F-statistic: 22.91 on 1 and 148 DF,  p-value: 4.073e-06    F and p for the whole model, not only for single $\beta_i$s as previous. The F value is $ \frac{s^2_{\hat{y}}}{\sum\epsilon_i} $. The bigger it grows, the more unlikely it is that the $\beta$'s do not have any effect at all. I came across the following question:     If 3 boys can paint a fence in 2 days what part of the job can     be completed by two boys in 1 day?   The answer to this problem is $\frac13$.  I can't manage to tackle this problem. Could anyone tell me what would be the quickest way to solve this problem (This question is a standardized test based question). Any help would be appreciated. What should I do if a user posts sensitive information as part of a question or answer? I've observed several incidents where users inadvertently posted detailed, sensitive information. For example, a Server Fault user who posted a question with working login credentials to his actual production server, or the Stack Overflow user who posted actual medical records as example data for his question.   What should be the procedure for handling such cases? Where does Google Chrome automatically save pages? I have uploaded an excel sheet into static resource of my environment. I have it as a demo for my product how to create a excel sheet for import records into sf. Can any body guide me how can i provide link on the VF page so that user can download the record at single click.I want to into a managed package and want users to be able to click the link on the Vf page to download. Right now i have my demo excel sheet in static resource. I am open if you have any other ideas to get the process done. "Disappear" as a transitive verb A white number inside a black circle. How to make a white number inside a black circle (the picture below)? 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On the other hand this strategy is not desirable for both of them because they do not reach their goals. As there was mentioned, this deals with undecidability of the problem. On the other hand, if you imagine yourself to be this lady/monster, you can be interested in the strategy which is not optimal. What is it? If there are such strategies in the game theory?  Edited2: My question is more general in fact. If we have a game with one parameter $v$ when two players $P_1, P_2$ are enemies and if $v&gt;0$ then for any strategy of $P_2$ the player $P_1$ wins. If $v\leq 0$ then for any strategy of $P_2$ there is a strategy of $P_1$ such that $P_2$ does not win and vice versa. I am interested in this case. From the mathematical point of view as I have understood the problem is undecidable since there is no an ultimate strategy neither for $P_1$ nor for $P_2$. But we are solving somehow these problem IRL. Imagine that you are a lady in this game - then you would like to win anyway even while knowing that your strategy can be covered by the strategy of the monster. On the other hand, the monster knows that if he will cover all strategies of the lady she will never reach the shore and he will never catch her. I mean they have to develop some non-optimal strategies. I hope now it's more clear. For example block cipher AES-128, Key size is 128bit and it is used to make a 10 round key which is total 320bit.  Question 1. If i use another Key schedule algorithm in AES, then security decreased or remain same? In other words, if the key schedule algorithm of AES has some relationship with AES encryption itself, then it will be decreased, or it will be same if the new key schedule algorithm is safe enough.  Question 2. Why we expand key? brute force will take O(2^128) time regardless of the key schedule is strong or not. what is the advantage of making key expend? I was curious about this when I was learned about 3DES. Which developed because the former DES has weak key length 56bit so vulnerable to brute force attack, i was wondering why doesn't they just remove key schedule process and use the whole round key as a key. Why does the air flow faster over the top of an airfoil? I understand the common explanation of lift, which describes the airflow over the top of the wing as moving faster than the air below the wing.  However, I don't quite understand why the air moves faster.  I've read some explanations citing a circulation effect among others, but haven't found a good, clear explanation for the exact cause.  Can someone help me out? Change date format [YYYYMMDD] to [MM/DD/YYYY] I need to convert one into 1, two into 2 and so on.  Is there a way to do this with a library or a class or anything? Short story where time is accelerated and characters have to get back to ship? Looking for a short story where time is accelerated on a planet.  The characters are actually born at the beginning of the story and then grow up quickly and inadvertently learn that their forebears came from a rocket ship nearby.  After having aged to "teenagers" they try to reach the ship?  At some point there is a war between confused aging survivors.  I think it was in a compilation which included a story called "Cousins". Let $G$ be a matrix Lie group and let $H$ be a Lie subgroup of $G$. My understanding is that all $g \in G$ are matrices, since $G$ is a subgroup of the general linear group. Then aren’t all elements $h \in H$ matrices, since $h \in G$?  From what I can gather $H$ is a matrix Lie group if $H$ is a closed subset of $G$. But if $H$ is not a closed subset of $G$, why is it not a matrix Lie group? Is the reasoning in the first paragraph wrong? How to make text aligned left/center/right in the same line? I am trying to align some text to left/center/right in the same line. 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I can kind of imagine 2 of Newton's 3 laws of motion to be incapsulated within Einstein's Special Relativity, but the one about "for every action there is an equal and opposite reaction", I'm struggling to find a place for that within Special Relativity.   Does this sit outside Special Relativity's explanatory power or is it inferred within somehow? Double line spacing I would like to typeset a draft of my document with double linespacing so that my collaborators have space to write in their edits (with a pen).  How do I do it? When I used  pip install ipython   or  pip install "ipython[notebook]"   I get the following error:  Downloading/unpacking ipython   Downloading ipython-2.3.1-py27-none-any.whl (2.8MB): 2.8MB downloaded Installing collected packages: ipython Cleaning up... Exception: Traceback (most recent call last):   File "/usr/lib/python2.7/dist-packages/pip/basecommand.py", line 122, in main     status = self.run(options, args)   File "/usr/lib/python2.7/dist-packages/pip/commands/install.py", line 283, in run     requirement_set.install(install_options, global_options, root=options.root_path)   File "/usr/lib/python2.7/dist-packages/pip/req.py", line 1435, in install     requirement.install(install_options, global_options, *args, **kwargs)   File "/usr/lib/python2.7/dist-packages/pip/req.py", line 671, in install     self.move_wheel_files(self.source_dir, root=root)   File "/usr/lib/python2.7/dist-packages/pip/req.py", line 901, in move_wheel_files     pycompile=self.pycompile,   File "/usr/lib/python2.7/dist-packages/pip/wheel.py", line 206, in move_wheel_files     clobber(source, lib_dir, True)   File "/usr/lib/python2.7/dist-packages/pip/wheel.py", line 193, in clobber     os.makedirs(destsubdir)   File "/usr/lib/python2.7/os.py", line 157, in makedirs     mkdir(name, mode) OSError: [Errno 13] Permission denied: '/usr/local/lib/python2.7/dist-packages/IPython'  Storing debug log for failure in /home/username/.pip/pip.log   What does this error mean? How do I fix it?