Page - 61 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Volume Second Edition

3.1 TheforLoop 61 This means that if we had, e.g., switched the print command inside our loop with print(’Hello!’) (i.e., so that i does not appear explicitly within the loop), i wouldstill run throughthenumbers1 to10asbefore,butHello!wouldbeprinted 10 times instead. The loopvariablei takeson the values1 to 10 in theorder listed, andanyorder wouldbeacceptable to Python.Thus, if we (for some reason)would like to reverse theorderof theprintouts,wecouldsimplyreversethe listofnumbers,writing[10, 9, 8, 7, 6, 5, 4, 3, 2, 1] instead. It shouldbenoted that the loopvariable isnot restricted to runover integers.Our nextexample includes loopingalsooverfloatingpointvalues. IndentationandNestedLoops Inoursimple times tableexampleabove, theprint command inside the loop was indented 4 spaces, which is in accordance with the official style guideofPython.2 Strictly speaking, the style guide recommends an indent of 4 spaces per indentation level. What this means, should become clear if we demonstrate how a for loop may appear within another for loop, i.e., if we show an arrangement withnested loops. for i in [1, 2, 3]: # First indentation level (4 spaces) print(’i = {:d}’.format(i)) for j in [4.0, 5.0, 6.0]: # Second indentation level (4+4 spaces) print(’ j = {:.1f}’.format(j)) # First line AFTER loop over j # First line AFTER loop over i The meaning of indentation levels should be clear from the comments (see code), and it is straight forward to use more nested loops than shown here (see, e.g., Exercise 5.7). Note that, together with the colon, indenting is part of the syntax also for other basic programmingconstructions in Python (e.g., inif-elif-else constructionsand functions). Whenexecuting thenested loopconstruction,weget thisprintout: i = 1 j = 4.0 j = 5.0 j = 6.0 i = 2 j = 4.0 j = 5.0 j = 6.0 i = 3 j = 4.0 j = 5.0 j = 6.0 From the printout, we may infer how the execution proceeds. For each value of i, the loop over j runs through all its values before i is updated (or the loop is terminated). To test your understanding of nested loops, you are recommended to doExercise5.1. 2 https://www.python.org/dev/peps/pep-0008/.