Seite - 67 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Band Second Edition

3.2 ThewhileLoop 67 <code line 2> # 2nd line in loop body ... ... # This is the first line after the loop Thefirst linehere is thewhile loopheader. Itcontains the reservedwordwhileand ends with a colon, both are compulsory. The indented lines that follow the header (i.e.,<code line 1>,<code line 2>, etc.) constitute a block of statements, the loop body. Indentation is done as with for loops, i.e., 4 spaces by convention. In ourexampleabovewith the ball, there was onlya single line in the loopbody(i.e., i = i + 1). Aswithfor loops,one run-throughof the loopbody is referred to as an iteration. Once the indentation is reversed, the loop body has ended. Here, the first lineafter the loop is# This is the first line after the loop. Between while and the colon, there is some_condition, which is a boolean expression that evaluates to eitherTrueorFalse. The boolean expression may be acompoundexpressionwithand,or, etc. When a while loop is encountered by the Python interpreter, it evaluates some_condition the first time. If True, one iteration of the loop body is carried out. After this first iteration, some_condition is evaluated once again (meaning that program execution goes back up to the top of the loop). If True again, there is another iteration, and so on, just like we saw above with ball_time.py. Once some_conditionevaluates toFalse, the loop is finished and executioncontinues with the first line after the loop. Note that ifsome_conditionevaluates toFalse the very first time, the statements inside the loop will not be executed at all, and executionsimplycontinues immediatelywith thefirst lineafter the loop. Compared to afor loop, the programmer does not have to specify the number of iterationswhencodingawhile loop. It simply runsuntil thebooleanexpression becomesFalse. Remember that if you want to use a variable analogously to the loop variable of a for loop, you have to explicitly update that variable inside the while loop (as we did with i in ball_time.py above). This differs from the automaticupdateofa loopvariable infor loops. Justas infor loops, theremightbe(arbitrarily)manycodelinesinawhile loop. Also,nested loopswork just likenestedfor loops.Havingfor loops insidewhile loops,andviceversa, isstraightforward.Anyfor loopmayalsobeimplementedas awhile loop,butwhile loopsaremoreflexible,sonotallof themcanbeexpressed asafor loop. Infinite Loops It is possible to have a while loop in which the condition never evaluates to False, meaning that program execution can not escape the loop! This is referred to as an infinite loop. Sometimes, infinite loops are just what you need, for example, in surveillance camera systems. More often, however, they are unintentional,andwhen learningtocode, it isquitecommontounintentionallyend upwithaninfinite loop(justwaitandsee!). Ifyouaccidentallyenteraninfinite loop and theprogramjust hangs“forever”,pressCtrl+c to stop the program. To check that you have gained a basic understanding of the while loop construction,youare recommendedtodoExercise3.4.