numerical python tom lefebvre. sept. 24-27, 2002numerical python2 since python is an interpreted...

Numerical PythonNumerical Python

Tom LeFebvre

Sept. 24-27, 2002 Numerical Python 2


Since Python is an interpreted language, we need better performance particularly when crunching numbers.

Advantages: High-level language allows very fast development

Disadvantages: Not always intuitive



Numerical Python operates on “multiarrays”One line of Numerical Python processes one

or more whole arrays, not just one number.Currently NumPy is implemented as an

“extension” to Python. But soon it will be an intrinsic part of Python


Importing NumericImporting Numeric

You must import the Numeric module before using Numerical Python

import Numeric or

>>> from Numeric import *>>> from Numeric import *


Making Numeric ArraysMaking Numeric Arrays

>>> a = array ([0, 1, -2, 6, -5])>>> a = array ([0, 1, -2, 6, -5])>>> a>>> a

>>> b = arrayrange(10)>>> b = arrayrange(10)>>> b>>> b


Multi-dimensional ArraysMulti-dimensional Arrays

>>> a = array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])>>> a = array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

>>> a>>> a

>>> a.shape>>> a.shape


Creating Special ArraysCreating Special Arrays

>>> BunchaOnes = ones((4, 4))>>> BunchaOnes = ones((4, 4))>>> BunchaOnes>>> BunchaOnes

>>> BunchaZeros = zeros((4, 4))>>> BunchaZeros = zeros((4, 4))>>> BunchaZeros>>> BunchaZeros


Manipulating ArraysManipulating Arrays

>>> BunchaThrees = BunchaOnes * 3>>> BunchaThrees = BunchaOnes * 3>>> BunchaThrees>>> BunchaThrees

>>> f = arrayrange(0, 100, 5)>>> f = arrayrange(0, 100, 5)>>> c = (f - 32) * 5 / 9>>> c = (f - 32) * 5 / 9>>> c>>> c

>>> c = (f - 32) * 5 / 9.0>>> c = (f - 32) * 5 / 9.0>>> c>>> c


Array SlicingArray Slicing

•Slicing Operator “[:]”Slicing Operator “[:]”

array[start : stop : increment]array[start : stop : increment]

array - a Numeric Python arrayarray - a Numeric Python arraystart - begin here including this indexstart - begin here including this indexstop - end here but do not include this indexstop - end here but do not include this indexincrement - skip this manyincrement - skip this many


Slicing ArraysSlicing Arrays

>>> a = arrayrange(10)>>> a = arrayrange(10)>>> a>>> a>>> a[2]>>> a[2]>>> a[0:4]>>> a[0:4]>>> a[-1]>>> a[-1]>>> a[:-1]>>> a[:-1]>>> a[2] = -999>>> a[2] = -999>>> a>>> a


Slicing Multi-dimensional ArraysSlicing Multi-dimensional Arrays

>>> b = reshape(arrayrange(9), (3, 3))>>> b = reshape(arrayrange(9), (3, 3))>>> b>>> b

>>> b[0, 0]>>> b[0, 0]

>>> b [2, 1]>>> b [2, 1]

>>> b[-1, -1]>>> b[-1, -1]


Slicing Multi-dimensional ArraysSlicing Multi-dimensional Arrays

>>> b>>> b>>> b[0:1, 0]>>> b[0:1, 0]

>>> b[0:2, 0:2]>>> b[0:2, 0:2]

>>> b [:]>>> b [:]

>>> b[::2]>>> b[::2]

>>> b[::-1]>>> b[::-1]

>>> b[::-1, ::-1]>>> b[::-1, ::-1]


Some Useful FunctionsSome Useful Functions

>>> a = arrayrange(10)>>> a = arrayrange(10)>>> a>>> a>>> add.reduce(a)>>> add.reduce(a)


Logical Functions - makes 0Logical Functions - makes 0ss or 1 or 1ss

>>> b = reshape(arrayrange(25), (5, 5))>>> b = reshape(arrayrange(25), (5, 5))>>> b>>> b

>>> less (b, 12)>>> less (b, 12)

>>> greater(b, 7)>>> greater(b, 7)>>> less_equal(b, 10)>>> less_equal(b, 10)


Array Functions - where()Array Functions - where()

where(condition, true value, false value)where(condition, true value, false value)

>>> b>>> b>>> c = where (less(b, 12), 0, b)>>> c = where (less(b, 12), 0, b)>>> c>>> c

Note: 0 is false, everything else is true

“where” is the Numerical Python “if” statement


Array Functions - clip()Array Functions - clip()

clip(array, min, max)clip(array, min, max)

>>> b>>> b>>> d = clip(b, 5, 15) >>> d = clip(b, 5, 15) >>> d>>> d

numerical python tom lefebvre. sept. 24-27, 2002numerical python2 since python is an interpreted...

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