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1、Variables, Expressions, and StatementsChapter 2Python for Informatics: Exploring IUnless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution 3.0 License.htp:/creativecommons.org/licenses/by/3.0/.Copyright 2010- Charles R. SeveranceConstantsFixed valu
2、es such as numbers, letters, and strings are caled “constants” - because their value does not changeNumeric constants are as you expectString constants use single-quotes ()or double-quotes () print 123123 print 98.698.6 print Hello worldHello worldVariablesA variable is a named place in the memory w
3、here a programmer can store data and later retrieve the data using the variable “name”Programmers get to choose the names of the variablesYou can change the contents of a variable in a later statement12.2x14 yx = 12.2y = 14100x = 100Python Variable Name RulesMust start with a letter or underscore _
4、Must consist of letters and numbers and underscoresCase SensitiveGood: spam eggs spam23 _speedBad: 23spam #sign var.12Different: spam Spam SPAMReserved WordsYou can not use reserved words as variable names / identifiersand del for is raise asert elif from lambda return break else global not try clas
5、 except if or while continue exec import pas yield def finaly in print Sentences or Linesx = 2x = x + 2print xVariable OperatorConstant Reserved WordAssignment StatementAssignment with expressionPrint statementAssignment StatementsWe asign a value to a variable using the asignment statement (=)An as
6、ignment statement consists of an expression on the right hand side and a variable to store the resultx = 3.9 * x * ( 1 - x )x = 3.9 * x * ( 1 - x )0.6xRight side is an expression. Once expression is evaluated, the result is placed in (asigned to) x.0.60.60.40.93A variable is a memory location used t
7、o store a value (0.6).x = 3.9 * x * ( 1 - x )0.6 0.93xRight side is an expression. Once expression is evaluated, the result is placed in (asigned to) the variable on the left side (i.e. x).0.93A variable is a memory location used to store a value. The value stored in a variable can be updated by rep
8、lacing the old value (0.6) with a new value (0.93).Numeric ExpressionsBecause of the lack of mathematical symbols on computer keyboards - we use “computer-speak” to express the clasic math operationsAsterisk is multiplicationExponentiation (raise to a power) loks different from in math.Operator Oper
9、ation+ Addition- Subtraction* Multiplication/ Division* Power% RemainderNumeric Expressions xx = 2 xx = xx + 2 print xx4 yy = 440 * 12 print yy5280 zz = yy / 1000 print zz5 jj = 23 kk = jj % 5 print kk3 print 4 * 364Operator Operation+ Addition- Subtraction* Multiplication/ Division* Power% Remainde
10、r5234 R 3203Order of EvaluationWhen we string operators together - Python must know which one to do firstThis is caled “operator precedence”Which operator “takes precedence” over the othersx = 1 + 2 * 3 - 4 / 5 * 6Operator Precedence RulesHighest precedence rule to lowest precedence ruleParenthesis
11、are always respectedExponentiation (raise to a power)Multiplication, Division, and RemainderAddition and SubtractionLeft to rightParenthesisPowerMultiplicationAdditionLeft to RightParenthesisPowerMultiplicationAdditionLeft to Right1 + 2 * 3 / 4 * 51 + 8 / 4 * 51 + 2 * 51 + 1011 x = 1 + 2 * 3 / 4 * 5
12、 print x11 ParenthesisPowerMultiplicationAdditionLeft to Right x = 1 + 2 * 3 / 4 * 5 print x11 1 + 2 * 3 / 4 * 51 + 8 / 4 * 51 + 2 * 51 + 1011Note 8/4 goes before 4*5 because of the left-right rule.Operator PrecedenceRemember the rules top to bottomWhen writing code - use parenthesisWhen writing cod
13、e - keep mathematical expressions simple enough that they are easy to understandBreak long series of mathematical operations up to make them more clearParenthesisPowerMultiplicationAdditionLeft to RightExam Question: x = 1 + 2 * 3 - 4 / 5Python Integer Division is Weird!Integer division truncatesFlo
14、ating point division produces floating point numbers print 10 / 25 print 9 / 24 print 99 / 1000 print 10.0 / 2.05.0 print 99.0 / 100.00.99This changes in Python 3.0Mixing Integer and FloatingWhen you perform an operation where one operand is an integer and the other operand is a floating point the r
15、esult is a floating pointThe integer is converted to a floating point before the operation print 99 / 1000 print 99 / 100.00.99 print 99.0 / 1000.99 print 1 + 2 * 3 / 4.0 - 5-2.5 What does “Type” Mean?In Python variables, literals, and constants have a “type”Python knows the difference between an integer number and a stringFor example “+” means “adition” if something is a number and “concatenate” if something is a string dd = 1 + 4 print dd5 eee = hello + there print eeehello thereconcatenate = put togethernt
