Page - 62 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Volume Second Edition

62 3 LoopsandBranching Other for Loop Structures In this book, you will occasionally meet for loops witha differentstructure,andasanexample, take a lookat this: for i, j, k in (1, 2, 3), (4, 5, 6), (6, 7, 8): print(i, j, k) Here, in the first iteration, the loop variables i, j and kwill become 1, 2 and 3, respectively. In the second iteration, they will become 4, 5 and 6, respectively, and soon.Thus, theprintout reads 1 2 3 4 5 6 6 7 8 Asusual, eachofi,jandkcanbe used inanydesirablewaywithin the loop. 3.1.3 CombiningforLoopandArray Often, loopsareusedincombinationwitharrays,soweshouldunderstandhowthat works. To reach this understanding, it is beneficial to do an example with just a small array. Assumethecase is tocomputetheaverageheightof familymembers ina family of5.We maychoose to storeall the heights in anarray,whichwe then run through byuseofafor looptocomputetheaverage.Thecode(average_height.py)may look like this: import numpy as np N = 5 h = np.zeros(N) # heights of family members (in meter) h[0] = 1.60; h[1] = 1.85; h[2] = 1.75; h[3] = 1.80; h[4] = 0.50 sum = 0 for i in [0, 1, 2, 3, 4]: sum = sum + h[i] average = sum/N print(’Average height: {:g} meter’.format(average)) When executed, the code gives 1.5m as the average height, which compares favorably to a simple handcalculation.What happenshere, is that we first sum up3 all theheightsbeingstored in thearrayh, beforewedividebythenumberof family membersN (i.e., just likewewoulddobyhand).Observehowsum is initialized to0 beforeentering the loop,and that with each iteration, a newheight is added tosum. Note that the loop variablei takes on the integer index values of the array, which startwith 0andendwithN−1. 3 Note thisway of using a loop to compute a sum, it is a standard technique inprogramming.