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diff --git a/arrays.org b/arrays.org deleted file mode 100644 index 8e3030f..0000000 --- a/arrays.org +++ /dev/null @@ -1,187 +0,0 @@ -* Arrays -*** Outline -***** Introduction -******* What do we want to do -********* We shall use arrays (mentioned before) which -********* shall be used for matrices in future -******* Arsenal Required -********* working knowledge of lists -***** Initializing matrices -******* Entering element-wise -******* using special functions -***** Accessing and Changing elements -******* Slicing?? -******* Striding?? -***** {{Simple operations??}} -*** Script - Welcome to the Tutorial on arrays. - - As mentioned in [the previous tutorial] arrays are much faster and - more efficient. In this tutorial we shall look at creating arrays, - accessing elements and changing them. - - --- - - Let's start with creating simple arrays. We've already seen how to - convert lists to arrays. Inputting a new array is similar to that. - - In []: a = array([5, 8, 10, 13]) - - Type /a/, to see what it is. - - In []: a - - We enter a multi-dimensional array this way - - - In []: c = array([[11,12,13], - [21,22,23], - [31,32,33]]) - - To see what c is, we just type c in the prompt. - - In []: c - - To see the dimensions of the array c, we use c.shape - In []: c.shape - - Now let us look at some special methods of creating an - array. There are various functions that allow us to create special - arrays. - - The first one we shall look at is, /arange/. /arange/ is similar to - the range command, except that it returns an array and accepts - float arguments. - - In []: a = arange(10) - - In []: a - This is the array we just created. - - In []: a.shape - Note that /a/ is one dimensional and has 10 elements, as expected. - - We could also use a.shape to change the shape of the array a. - In []: a.shape = 2,5 - Note that the total size of new array must be unchanged. - - We type a, to see what it looks like - In []: a - - ones command can be used to get an array with all the entries as - ones. We pass it the shape of the array that we require. - - In []: b = ones((3, 4)) - - Look at b, by printing it out. - In []: b - - To create an array with all entries as ones, with it's shape - similar to an already existing array, we use the ones_like - command. - In []: b = ones_like(a) - - zeros and zeros_like are similar commands that can give you arrays - with all zeros. empty and empty_like give you empty arrays (arrays - with no initialization done.) - - In []: b = zeros((3, 4)) - In []: b = zeros_like(a) - - The identity command can be used to obtain a square array with - ones on the main diagonal. - - In []: identity(3) - - To obtain a 2-D array, that is not necessarily square, eye command - can be used. Look at the documentation of eye (using eye?) for - more info. - - --- - - Now that we have learnt how to create arrays, let's move on to - accessing elements and changing them. - - Let's work with the c, array which we had already created. - - In []: c - - Let's say we want to access the element 23 in c, we say - - In []: c[1][2] - Note that this is similar to accessing an element inside a list of - lists. Also, note that counting again starts from 0. - - But arrays provide a more convenient way to access the elements. - In []: c[1, 2] - - Now, we can also change the element using a simple assignment. - In []: c[1, 2] = -23 - - Let's look at accessing more than one elements at a time. We begin - with accessing rows. - In []: c[1] gives us the second row. (counting starts from 0) - - To get a column, we use a syntax that is similar to the one used - to access a single element. - In []: c[:,1], gives us the first column. - - The colon specifies that we wish to obtain all elements in that - dimension from the array. - - So, we could use a more explicit way to access the second row of - the array. - In []: c[1,:] - - The colon can be used to access specific portions of the array, - similar to the way we do with lists. - In []: c[1,1:3] - Observe that we get the second and third columns from the second - row. As with lists, the number after the colon is excluded when - slicing a portion of the array. - - In []: c[1:3,1] - Now, we get the second and third rows from the first column. - - In []: c[1:3,1:3] - We get the second and third rows and the second and third - columns. - - The numbers before and after the colons are optional. If the - number before the colon is omitted, it is assumed to be zero by - default. If the element after the colon is omitted, it is assumed - to be until the end. - - In []: c[1:, 1:] - This is essentially similar to the previous example. We are using - the default value i.e, the end, instead of specifying 3, - explicitly. - - In []: c[:2, :2] - We have omitted specifying the zero before the colon, explicitly. - - --- - - You may have observed the similarity of the semi-colon notation to - the notation used in lists. As expected, the semi-colon notation - also provides a way to specify a jump. This {concept/idea} is - termed as Striding. - - To get every alternate row of c, starting from the first one, we say - In []: c[::2,:] - - To get every alternate row of c, starting from the second one, we - say - In []: c[1::2,:] - - - In []: c[:,::2] - In []: c[::2,::2] - - --- - - We come to the end of this tutorial on arrays. In this tutorial, - you've learnt how to create arrays and access, change elements. - - Thank you. - -*** Notes |