Seite - 44 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python

44 2 BasicConstructions # Transform y coordinates from math import log def f(y): return log(y) for i in range(len(y)): y[i] = f(y[i]) # Write out x and y to a two-column file filename = ’tmp_out.dat’ outfile = open(filename, ’w’) # Open file for writing outfile.write(’# x and y coordinates\n’) for xi, yi in zip(x, y): outfile.write(’%10.5f %10.5f\n’ % (xi, yi)) outfile.close() Such a file with a comment line and numbers in tabular format is very com- mon so numpy has functionality to ease reading and writing. Here is the same exampleusing theloadtxtandsavetxt functions innumpy for tabular data (file file_handling_numpy.py): filename = ’tmp.dat’ import numpy data = numpy.loadtxt(filename, comments=’#’) x = data[:,0] y = data[:,1] data[:,1] = numpy.log(y) # insert transformed y back in array filename = ’tmp_out.dat’ filename = ’tmp_out.dat’ outfile = open(filename, ’w’) # open file for writing outfile.write(’# x and y coordinates\n’) numpy.savetxt(outfile, data, fmt=’%10.5f’) 2.7 Exercises Exercise2.1:Errorswithcolon, indent, etc. Write the program ball_function.py as given in the text and confirm that the program runs correctly. Then save a copy of the program and use that program during the followingerror testing. You are supposed to introduce errors in the code, one by one. For each error introduced, save and run the program, and comment howwell Python’s response corresponds to the actual error. When you are finishedwith one error, re-set the programtocorrectbehavior(andcheckthat itworks!) beforemovingonto thenext error. a) Change thefirst line fromdef y(t): todef y(t), i.e., removethecolon. b) Remove the indent in front of the statementv0 = 5 inside the functiony, i.e., shift the text four spaces to the left.