Page - 122 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Volume Second Edition

122 5 SomeMorePythonEssentials 5.5 Files:ReadandWrite Input data for a program often come from files and the results of the computations are often written to file. To illustrate basic file handling, we consider an example where we readx andy coordinates from two columns in a file, apply a functionf to the y coordinates, and write the results to a new two-column data file. The first lineof the inputfile is a heading thatwecan just skip: # x and y coordinates 1.0 3.44 2.0 4.8 3.5 6.61 4.0 5.0 Therelevant Python lines for reading the numbersandwriting out a similar file are givenin 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: # Read one line at a time words = line.split() # Split line into words x.append(float(words[0])) y.append(float(words[1])) infile.close() # 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’.format(xi, yi)) outfile.close() If you have problems understanding the details here, make your own copy and insertprintoutsoflineand thewordelements in the (first) loop. Withzip, in thefirst iteration,xiandyiwill represent thefirstelementofxand y, respectively. In thesecond iteration,xiandyiwill represent thesecondelement ofxandy, andso on.11 11 Generally,zipallowsrunning overmultiple listsat thesame time,endingwhen theshortest list isfinished.