Sequences

Lists are mutable heterogeneous sequences (anything that can be modified is called mutable). A list can be a sequence of strings, integers, floats, or any combination of these. In fact, a list can contain any type of object; for example,

>>> myIntegerList = [7,6,5,4]
>>> myFloatList  = [7.1,6.5,5.3,4.8]

You can refer to individual items from a sequence using the index of the item. Indices start at zero. Negative indices count backward from the end of a sequence.

>>> myList = [1,2,3]
>>> myList[0]
1
>>> myList[1] = 9
>>> myList
[1, 9, 3]
>>> myNewList = [1.0,2.0,myList]
>>> myNewList
[1.0, 2.0, [1, 9, 3]]
>>> myNewList[-1]
[1, 9, 3]

Lists are heterogeneous, which means they can contain objects of different type.

>>> myList=[1,2.5,'steel']

A list can contain other lists.

>>> myList=[[0,1,2],[3,4,5],[6,7,8]] 
>>> myList[0]
[0, 1, 2]
>>> myList[2]
[6,7,8]

myList[1][2] refers to the third item in the second list. Remember, indices start at zero.

>>> myList[1][2]
5

Python has built-in methods that allow you to operate on the items in a sequence.

>>> myList
[1, 9, 3]
>>> myList.append(33)
>>> myList
[1, 9, 3, 33]
>>> myList.remove(9)
>>> myList
[1, 3, 33]

The following are some additional built-in methods that operate on lists:

count()

Return the number of times a value appears in the list.

>>> myList = [0,1,2,1,2,3,2,3,4,3,4,5]
>>> myList.count(2)
3

index()

Return the index indicating the first time an item appears in the list.

>>> myList.index(5)
11
>>> myList.index(4)
8

insert()

Insert a new element into a list at a specified location.

>>> myList.insert(2,22)   
>>> myList
[0, 1, 22, 2, 1, 2, 3, 2, 3, 4, 3, 4, 5]

reverse()

Reverse the elements in a list.

>>> myList.reverse()
>>> myList
[5, 4, 3, 4, 3, 2, 3, 2, 1, 2, 22, 1, 0]

sort()

Sort the elements in a list.

>>> myList.sort()
>>> myList
[0, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 5, 22]

Tuples are very similar to lists; however, they are immutable heterogeneous sequences, which means that you cannot change them after you create them. You can think of a tuple as a list that cannot be modified. Tuples have no methods; you cannot append items to a tuple, and you cannot modify or delete the items in a tuple. The following statement creates an empty tuple:

myTuple = ()

The following statement creates a tuple with one element:

myTuple = (5.675,)

You can use the tuple() function to convert a list or a string to a tuple.

>>> myList = [1, 2, "stress", 4.67]
>>> myTuple = tuple(myList)
>>> print(myTuple)
(1, 2, 'stress', 4.67)
>>> myString = 'Failure mode'
>>> myTuple = tuple(myString)
>>> print(myTuple)
('F', 'a', 'i', 'l', 'u', 'r', 'e', ' ', 'm', 'o', 'd', 'e')

The following statements create a tuple and then try to change the value of an item in the tuple. An AttributeError error message is generated because a tuple is immutable.

>>> myTuple = (1,2,3,4,5)
>>> type(myTuple)
<type 'tuple'>
>>> myTuple[2]=3
Traceback (innermost last):
  File "", line 1, in ?
AttributeError: __setitem__

Strings are immutable sequences of characters. Strings are defined by single or double quotation marks. You can use the + operator to concatenate two strings and create a third string; for example,

>>> odbString = "Symbol plot from "
>>> odb = 'load1.odb'
>>> annotationString = odbString + odb
>>> print(annotationString)
Symbol plot from load1.odb

>>> annotationString
'Symbol plot from load1.odb'
>>> annotationString.upper()
'SYMBOL PLOT FROM LOAD1.ODB'
>>> annotationString.split()
['Symbol', 'plot', 'from', 'load1.odb']

As with all sequences, you use negative indices to index backward from the end of a string.

>>> axis_label = 'maxstrain'
>>> axis_label[-1]
'n'

Use the built-in str function to convert an object to a string.

>>> myList = [8, 9, 10]
>>> str(myList)
'[8, 9, 10]'

Look at the standard Python documentation on the official Python website (http://www.python.org) for a list of common string operations. String functions are described in the String Services section of the Python Library Reference.

Arrays are mutable homogeneous sequences. The numpy module allows you to create and operate on multidimensional arrays. Python determines the type of elements in the array; you do not have to declare the type when you create the array. For more information about the numpy module, see http://numpy.scipy.org.

>>> from numpy import array
>>> myIntegerArray = array([[1,2],[2,3],[3,4]])
>>> myIntegerArray
array([[1, 2], 
       [2, 3], 
       [3, 4]])
>>> myRealArray =array([[1.0,2],[2,3],[3,4]])
>>> myRealArray
array([[1., 2.], 
       [2., 3.], 
       [3., 4.]])
>>> myRealArray * myIntegerArray
array([[  1.,   4.],
       [  4.,   9.],
       [  9.,  16.]])