Seite - 43 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python

2.6 ReadingfromandWritingtoFiles 43 In some cases, it is required to run through 2 (ormore) lists at the same time. Pythonhasahandyfunctioncalledzip for thispurpose.Anexampleofhowtouse zip isprovided in thecodefile_handling.pybelow. Weshouldalsobrieflymentionabout tuples,whichareverymuch like lists, the main differencebeing that tuples cannot be changed. To a freshman, itmay seem strange that such “constant lists” could ever bepreferable over lists. However, the propertyofbeingconstant is agoodsafeguardagainstunintentionalchanges.Also, it is quicker forPython tohandledata in a tuple than in a list,whichcontributes to faster code.With thedata fromabove,wemaycreate a tuple andprint the content bywriting x = (’hello’, 4, 3.14, 6) for e in x: print ’x element: ’, e print ’This was all the elements in the tuple x’ Tryinginsertorappend for the tuple gives an errormessage (because it cannot bechanged), stating that the tupleobjecthasnosuchattribute. 2.6 ReadingfromandWritingtoFiles Inputdata for aprogramoftencome fromfiles and the results of the computations are oftenwritten to file. To illustrate basic file handling,we consider an example wherewe readx andy coordinates fromtwocolumns in afile, applya functionf to they coordinates, andwrite the results to a new two-columndatafile. Thefirst lineof the inputfile is aheading thatwecan just skip: # x and y coordinates 1.0 3.44 2.0 4.8 3.5 6.61 4.0 5.0 The relevantPython lines for reading thenumbersandwritingout a similarfileare given in thefilefile_handling.py filename = ’tmp.dat’ infile = open(filename, ’r’) # Open file for reading line = infile.readline() # Read first line # Read x and y coordinates from the file and store in lists x = [] y = [] for line in infile: words = line.split() # Split line into words x.append(float(words[0])) y.append(float(words[1])) infile.close()