Mastering pandas - Second Edition

Comparing R-matrix and NumPy array

Let's look at creation and selection in R:

>r_mat<- matrix(2:13,4,3) 
>r_mat 
     [,1] [,2] [,3] 
[1,]    2    6   10 
[2,]    3    7   11 
[3,]    4    8   12 
[4,]    5    9   13 

To select the first row, we write the following:

>r_mat[1,] 
[1]  2  6 10 

To select the second column, we use the following command:

>r_mat[,2] 
[1] 6 7 8 9 

Let's now look at NumPy array creation and selection:

In [60]: a=np.array(range(2,6)) 
         b=np.array(range(6,10)) 
         c=np.array(range(10,14)) 
In [66]: np_ar=np.column_stack([a,b,c]) 
np_ar 
Out[66]: array([[ 2,  6, 10], 
[ 3,  7, 11], 
[ 4,  8, 12], 
[ 5,  9, 13]]) 

To select the first row, we use the following command:

    In [79]: np_ar[0,]
    Out[79]: array([ 2,  6, 10])
  

Indexing is different in R and pandas/NumPy. In R, indexing starts ...