Seite - 14 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python

14 1 TheFirstFewSteps variable (thewordfloat is just computer language for a real number). In any case, Python thinks of x as an object, of type int or float. Another common type of variable is str, i.e. a string, needed when youwant to store text. When Python interpretsx = "This is a string", it stores the text (inbetween thequotes) in thevariablex. Thisvariable is thenanobjectof typestr. Youmayconvertbetween variable types if itmakes sense. If, e.g.,x is an int object,writingy = float(x) willmakey afloatingpoint representationofx. Similarly, youmaywriteint(x) to produce an int if x is originally of type float. Type conversionmay also occur automatically, as shownjust below. Names of variables should be chosen so that they are descriptive. When com- puting amathematical quantity that has some standard symbol, e.g.˛, this should be reflected in thenameby letting thewordalphabepart of thenamefor the cor- responding variable in the program. If you, e.g., have a variable for counting the numberof sheep, thenoneappropriatenamecouldbeno_of_sheep. Suchnaming makes itmucheasier for a human to understand thewritten code. Variable names mayalsocontainanydigit from0to9,orunderscores,butcannotstartwithadigit. Lettersmaybe lowerorupper case,which toPython is different. Note that certain namesinPythonarereserved,meaningthatyoucannotusetheseasnamesforvari- ables. Some examples are for, while, if, else, global,return and elif. If youaccidentallyusea reservedwordasavariablenameyouget anerrormessage. Wehaveseen that, e.g.,x = 2will assign thevalue2 to thevariablex. Buthow dowewrite it ifwewant to increasex by4? Wemaywrite an assignment likex = x + 4, or (givinga faster computation)x += 4. NowPython interprets this as: takewhatever value that is inx, add4, and let the result become the newvalue of x. Inotherwords, theold valueofx is usedon the right hand side of=, nomatter howmessy theexpressionmightbe, and the result becomes thenewvalueofx. In asimilarway,x -= 4 reduces thevalueofxby4,x *= 4multipliesxby4,andx /= 4dividesxby4,updating thevalueofxaccordingly. What if x = 2, i.e., an object of type int, andwe add 4.0, i.e., a float? Then automatic type conversion takes place, and the newxwill have the value 6.0, i.e., anobjectof typefloat as seenhere, In [1]: x = 2 In [2]: x = x + 4.0 In [3]: x Out [3]: 6.0 NotethatPythonprogrammers,andPython(inprintouts),oftenwrite,e.g.,2.which bydefinition is the integer2 representedasafloat. 1.5.4 IntegerDivision Another issue that is important to be aware of is integer division. Let us look at a small example, assumingwewant todivideonebyfour.