Page - 85 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Volume Second Edition

4.1 Functions:HowtoWriteThem? 85 andtest it!).This isbecausePythonreadsthe listedargumentsfromleft to right,and does not know that an argument is taken out of the list before it comes to the end (but then it is too late). 4.1.6 AFunctionwithTwoReturnValues Takeaslightvariationofourcasewith theball, assuming that theball isnot thrown straightup,butatanangle,so that twocoordinatesareneededtospecify itsposition atany time. According to Newton’s laws (when air resistance is negligible), the vertical position is given by y(t) = v0yt − 0.5gt2 and, simultaneously, the horizontal positionbyx(t)=v0xt. We can includeboth these expressions in a new versionof ourprogramthatfinds thepositionof theball at a given time: def xy(v0x, v0y, t): """Compute horizontal and vertical positions at time t""" g = 9.81 # acceleration of gravity return v0x*t, v0y*t - 0.5*g*t**2 v_init_x = 2.0 # initial velocity in x v_init_y = 5.0 # initial velocity in y time = 0.6 # chosen point in time x, y = xy(v_init_x, v_init_y, time) print(’Horizontal position: {:g} , Vertical position: {:g}’.format(x, y)) We note that in the returnstatement, the returnedvalues (twohere)areseparated by a comma. Here, the x coordinate will be computed by v0x*t, while the y coordinate will be computed like before. These two calculations produce two numbers that are returned to variablesx andy in main. Note that, the order of the resultsreturnedfromthefunctionmustbethesameastheorderofthecorresponding variables in the assignment statement. Note the use of a comma both in the return statementand theassignment statement. 4.1.7 CallingaFunctionDefinedwithKeywordParameters Let us adjust the previousprogramslightly, introducingkeywordparameters in the definition. For example, if we use 0.6 as a default value fort, and aim to get the sameprintoutasbefore, theprogramreads def xy(v0x, v0y, t=0.6): """Compute horizontal and vertical positions at time t""" g = 9.81 # acceleration of gravity return v0x*t, v0y*t - 0.5*g*t**2 v_init_x = 2.0 # initial velocity in x v_init_y = 5.0 # initial velocity in y x, y = xy(v_init_x, v_init_y) print(’Horizontal position: {:g} , Vertical position: {:g}’.format(x, y))