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NumPy - Array from Existing Data
NumPy - Array From Existing Data
In this chapter, we will discuss how to create an array from existing data.
numpy.asarray
This function is similar to numpy.array except for the fact that it has fewer parameters. This routine is useful for converting Python sequence into ndarray.
numpy.asarray(a, dtype = None, order = None)
The constructor takes the following parameters.
| Sr.No. | Parameter & Description |
|---|---|
| 1 | a Input data in any form such as pst, pst of tuples, tuples, tuple of tuples or tuple of psts |
| 2 | dtype By default, the data type of input data is appped to the resultant ndarray |
| 3 | order C (row major) or F (column major). C is default |
The following examples show how you can use the asarray function.
Example 1
# convert pst to ndarray import numpy as np x = [1,2,3] a = np.asarray(x) print a
Its output would be as follows −
[1 2 3]
Example 2
# dtype is set import numpy as np x = [1,2,3] a = np.asarray(x, dtype = float) print a
Now, the output would be as follows −
[ 1. 2. 3.]
Example 3
# ndarray from tuple import numpy as np x = (1,2,3) a = np.asarray(x) print a
Its output would be −
[1 2 3]
Example 4
# ndarray from pst of tuples import numpy as np x = [(1,2,3),(4,5)] a = np.asarray(x) print a
Here, the output would be as follows −
[(1, 2, 3) (4, 5)]
numpy.frombuffer
This function interprets a buffer as one-dimensional array. Any object that exposes the buffer interface is used as parameter to return an ndarray.
numpy.frombuffer(buffer, dtype = float, count = -1, offset = 0)
The constructor takes the following parameters.
| Sr.No. | Parameter & Description |
|---|---|
| 1 | buffer Any object that exposes buffer interface |
| 2 | dtype Data type of returned ndarray. Defaults to float |
| 3 | count The number of items to read, default -1 means all data |
| 4 | offset The starting position to read from. Default is 0 |
Example
The following examples demonstrate the use of frombuffer function.
import numpy as np s = Hello World a = np.frombuffer(s, dtype = S1 ) print a
Here is its output −
[ H e l l o W o r l d ]
numpy.fromiter
This function builds an ndarray object from any iterable object. A new one-dimensional array is returned by this function.
numpy.fromiter(iterable, dtype, count = -1)
Here, the constructor takes the following parameters.
| Sr.No. | Parameter & Description |
|---|---|
| 1 | iterable Any iterable object |
| 2 | dtype Data type of resultant array |
| 3 | count The number of items to be read from iterator. Default is -1 which means all data to be read |
The following examples show how to use the built-in range() function to return a pst object. An iterator of this pst is used to form an ndarray object.
Example 1
# create pst object using range function import numpy as np pst = range(5) print pst
Its output is as follows −
[0, 1, 2, 3, 4]
Example 2
# obtain iterator object from pst import numpy as np pst = range(5) it = iter(pst) # use iterator to create ndarray x = np.fromiter(it, dtype = float) print x
Now, the output would be as follows −
[0. 1. 2. 3. 4.]Advertisements