brackets - Python - what is this syntax? my_2d_list[0:20,1] -

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To find out what's inside the bracket, a difficult time with an interpreter is Google or nude. The reference is the code: matplotlib.mlab import from mlabPCA mlab_pca = mlabPCA (all_samples.T.) In the form of PCA ('in terms of measurement axes' from PC axis '\' standard Increased deviation: \ n ', \ mlab_pca.Wt) plt.plot (mlab_pca.Y [0: 20,0], mlab_pca.Y [0: 20,1],' o ', markers = 7, \ color =' Blue ',' alpha = 0.5, label = 'class1') plt.plot (mlab_pca.Y [20: 40,0], mlab_pca.Y [20: 40,1], '^', marker size = 7, \ Color = 'red', alpha = 0.5, label = 'class2') plt.xlabel ('x_values') plt.ylabel ('y_values') plt.xlim ([- 4,4]) plt. Ylim ([- 4,4]) plt.legend () plt.title ('matplotlib.mlab.PCA (to be replaced with class labels') plt.show ()

I have Tried to create a small 2D array 

  a = [[1,2], [1,2], [1,2], [1,2], [1] , 2]]   
 and evaluation  
  a [0: 2.0]   
 But he did something to me Not even thank you!  
 The code taken from the "using the PCA (class) matplotlib.mlab library" section.   
  After "text" itemprop = "text"> 
 Indigenous Python Lists (which you are creating for  a ) Do not support indexing or skiing, at least in the way you are doing this, to use index and slice, you can use syntax, because you are already using it:  
     
  1.  To use index and slice  
       
    1.   
        np a = np.array ([[1,2], Samples as [1,2], [1,2], [1,2], [1,2] Importing]] [0: 2, 0] # Returns array ([1, 1])   
         
      1.  Your original python list with a different syntax Log in.   
         This method does not really allow you to split the way you are using it, but, you can:  a [0 ] [1]  to access 0, 1 element correctly with  a [0, 1]  list. But again, a piece of action ( one [0: 2] [0]  will produce some unwanted results).  
         It seems that you may have syntax options based on some, if so, then use this great reference guide to reduce the infection:

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