Lists, tuples, and arrays are ordered sets of objects. The individual objects are inserted, accessed, and processed according to their place in the list. On the other hand, dictionaries are unordered sets of pairs. One accesses dictionary data by keys.
For example, we may create a dictionary containing the data of a rigid body in mechanics, as follows:
truck_wheel = {'name':'wheel','mass':5.7,
'Ix':20.0,'Iy':1.,'Iz':17.,
'center of mass':[0.,0.,0.]}A key/data pair is indicated by a colon, :. These pairs are comma separated and listed inside a pair of curly brackets, {}.
Individual elements are accessed by their keys:
truck_wheel['name'] # returns 'wheel' truck_wheel['mass'] # returns 5.7
New objects are added to the dictionary by creating a new key:
truck_wheel['Ixy'] = 0.0
Dictionaries are also used to provide parameters to a function (refer to section Parameters and arguments in Chapter 7, Functions...