Seite - 16 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python

16 1 TheFirstFewSteps real = 12.89643 integer = 42 string = ’some message’ print ’real=%.3f, integer=%d, string=%s’ % (real, integer, string) print ’real=%9.3e, integer=%5d, string=%s’ % (real, integer, string) Theoutput ofprint is a string, specified in termsof text and a set of variables tobe inserted in the text. Variables are inserted in the text at places indicatedby%. After%comesaspecificationof the formatting, e.g,%f (realnumber),%d (integer), or%s (string). The format%9.3fmeans a real number in decimalnotation,with 3 decimals,written in afieldofwidth equal to9characters. Thevariant%.3fmeans that thenumber iswrittenascompactlyaspossible, indecimalnotation,with three decimals. SwitchingfwitheorE results in thescientificnotation,here1.290e+01 or1.290E+01.Writing%5dmeansthatan integer is tobewritten inafieldofwidth equal to 5 characters. Real numbers can also be specifiedwith %g, which is used toautomaticallychoosebetweendecimalorscientificnotation, fromwhatgives the mostcompactoutput (typically, scientificnotation isappropriateforverysmall and very largenumbersanddecimalnotation for the intermediate range). A typical example of when printf formatting is required, arises when nicely alignedcolumnsofnumbers are tobeprinted. Supposewewant to print a column of t values together with associated function values g.t/ D t sin.t/ in a second column.Thesimplest approachwouldbe from math import sin t0 = 2 dt = 0.55 # Unformatted print t = t0 + 0*dt; g = t*sin(t) print t, g t = t0 + 1*dt; g = t*sin(t) print t, g t = t0 + 2*dt; g = t*sin(t) print t, g withoutput 2.0 1.81859485365 2.55 1.42209347935 3.1 0.128900053543 (Repeating the same set of statementsmultiple times, as done above, is not good programmingpractice-oneshoulduseafor loop,asexplainedlater inSection2.3.) Observe that the numbers in the columns are not nicely aligned. Using the printf syntax’%6.2f %8.3f’ % (t, g) fortandg,wecancontrol thewidthof each column and also the number of decimals, such that the numbers in a column are