Page - 105 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Volume Second Edition

5.1 ListsandTuples:AlternativestoArrays 105 The elements of the list x: hello 4 3.14 6 List Comprehension There is a special construct in Python that allows you to run through all elements of a list, do the same operation on each, and store the new elements in another list. It is referred to as list comprehension and may be demonstratedas follows: In [1]: L1 = [1, 2, 3, 4] In [2]: L2 = [e*10 for e in L1] In [3]: L2 Out[3]: [10, 20, 30, 40] So, we get a new list by the nameL2, with the elements10,20,30 and40, in that order.NoticethesyntaxwithinthebracketsforL2,e*10 for e in L1signals that e is tosuccessivelybeeachof the list elements inL1, andforeache, create thenext element inL2bydoinge*10.Moregenerally, thesyntaxmaybewrittenas L2 = [E(e) for e in L1] whereE(e)meanssomeexpression involvinge. In some cases, it is required to run through 2 (or more) lists at the same time. Pythonhasahandyfunctioncalledzip for thispurpose.Anexampleofhowtouse zip isprovidedinSect.5.5 (file_handling.py). SomePropertiesofTuples Weshouldalsobrieflymentionabout tuples,whichare very much like lists, the main differencebeing that tuples cannot be changed. To a freshman, it may seem strange that such “constant lists” could ever be preferable over lists. However, the property of being constant is a good safeguard against unintentional changes. Also, it is quicker for Python to handle data in a tuple than in a list, which contributes to faster code.With the data fromabove,we may create a tupleandprint thecontentbywriting x = (’hello’, 4, 3.14, 6) print(’The elements of the tuple x:\n’) for e in x: print(e) Tryinginsertorappend for the tuple gives an error message (because it cannot bechanged),stating that the tupleobjecthasnosuchattribute.