Page - 4 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python

4 1 TheFirstFewSteps 1.2.1 TheProgram Let us next look at a Pythonprogramfor evaluating this simple formula. Assume theprogramiscontainedas text inafilenamedball.py. The text looksas follows (fileball.py): # Program for computing the height of a ball in vertical motion v0 = 5 # Initial velocity g = 9.81 # Acceleration of gravity t = 0.6 # Time y = v0*t - 0.5*g*t**2 # Vertical position print y Computer programs and parts of programs are typesetwith a blue background in thisbook.Aslightlydarker topandbottombar,asabove, indicates that thecode isacompleteprogramthatcanbe runas it stands.Without thebars, thecode is just a snippetandwill (normally)needadditional lines to runproperly. 1.2.2 DissectionoftheProgram Acomputer program is plain text, as here in the fileball.py,which contains in- structions to the computer. Humans can read the code and understand what the programis capableofdoing,but theprogramitself doesnot trigger anyactionson a computer before another program, thePython interpreter, reads theprogram text and translates this text into specificactions. Youmustlearntoplaytheroleofacomputer AlthoughPythonisresponsibleforreadingandunderstandingyourprogram,it is of fundamental importance thatyoufullyunderstand theprogramyourself. You have toknowthe implicationof every instruction in theprogramandbe able to figureout theconsequencesof the instructions. Inotherwords,youmustbeable toplaytheroleofacomputer. Thereasonforthisstrongdemandofknowledgeis that errors unavoidably, andquite often,will be committed in theprogram text, and to track down these errors, you have to simulate what the computer does with theprogram.Next,weshall explainall the text inball.py in fulldetail. Whenyou runyourprograminPython, itwill interpret the text inyourfile line by line, fromthe top, readingeach line fromleft to right. Thefirst line it reads is # Program for computing the height of a ball in vertical motion. This line iswhatwecallacomment. That is, the line isnotmeantforPythontoread andexecute,but rather forahumanthat reads thecodeand tries tounderstandwhat is going on. Therefore, one rule in Python says thatwhenever Python encounters