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Chapter 4 Numbers

Chapter 4 Numbers. CSC1310 Fall 2008. volume.py:. Python Program Structure. Program Module Statement Expression. import math r=int(raw_input()) print 4.0/3*math.pi*r**3. Python Built-in types. Numbers 3.1415, 1234, 999L, 3+4j Strings “Hello World”, ‘spam’

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Chapter 4 Numbers

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  1. Chapter 4 Numbers CSC1310 Fall 2008

  2. volume.py: Python Program Structure • Program • Module • Statement • Expression import math r=int(raw_input()) print 4.0/3*math.pi*r**3

  3. Python Built-in types • Numbers 3.1415, 1234, 999L, 3+4j • Strings “Hello World”, ‘spam’ • Lists [1,2,3,4], [1,2,[3,4]] • Dictionaries {‘test’: ‘yum’} • Tuples (1, 2, 3, 4) • Files input=open(‘file.txt’, ‘r’).read()

  4. Numbers • Integer 222, -24, 0 (32 bit) [-231 , 231- 1] • Long integer 99999999L ,33657l • Floating-point 1.23, 123e-2, 0.123e+2 • Octal literals 0177, 01234, 0456 (0-8) • Hex literals 0x9ff, 0XFf (0-9, A,B,C,D,E,F) • Complex number literals 3+4j, 3j, 4J+3, complex(3,4)(real,imag) realpart+imaginarypart j realpart+imaginarypart J

  5. Assignment • Variables – a name that is used to keep some information • To create variable, assign a value to it. • Variable in expression is replaced by its value. • Variable should be assigned before it is used in expression. >>> a=3 #Name Created >>> b=4 >>>s=d=0 What will happen if I ask to print variable c at this point?

  6. Mathematical Operators • Addition (a + b) • Subtraction (a - b) • Multiplication (a * b) • Division (a / b, a // b) (integer, floating-point; floor division) (from 3.0: true and floor division) • Remainder/format (a % b) • Binary power (a ** b) • Unary negation (- b) • Identity (+ b)

  7. Variable _ • In interactive mode, the last printed expression is assigned to the variable _. >>> tax = 12.5 / 100 >>> price = 100.50 >>> price * tax >>> price + _ >>>_*tax

  8. Comparison and Value Equality Operators • Value equality (a == b, a <> b, a != b) • Comparison operators (a < b, a > b, a <=b, a >= b)

  9. Mixed Operators, Types • Operator Precedence • unary negation, identity, binary power • multiplication, remainder/ format, division • addition, subtraction • comparison and value equality operators • Parentheses to group sub-expressions • Different numeric types in expression: • Python converts operand up to the type of the most complicated operand and then calculates result. • Integer -> Long integer-> Floating-point numbers-> Complex numbers.

  10. Numeric Representation • Interactive prompt shows more digits than print >>> b / (2.0 + a) >>> print b/(2.0+a) • String formatting >>> num=0.33333333 >>> “%e” % num >>> “%2.2f” % num >>> str(num) >>> repr(num)

  11. Long Integers • Arbitrarily big • Convert integer to long integer if overflow occurs (after 2.2) • More precision vs lower performance

  12. Hexadecimal and Octal Notations • Octal notation (base 8) • leading zero (0) (!!!!!) • octal digits 0-7 (each represents 3 bits) >>> oct(64) • Hexadecimal notation (base 16) • leading 0X or 0x • hex digits 0-9 (each represents 4 bits) • upper- or lowercase A-F (each represents 4 bits) >>> hex(64) • String to integer and vise-verse >>> int(’64’), int (‘0xf’,16), int (‘034’,8) >>> eval(’64’), eval(‘0xf’), eval(‘034’) #slower >>> “%o %x %X” % (64,64,255)

  13. Built-in functions and modules >>>import math >>>math.pi, math.e >>>math.cos(2*math.pi/3) #sin, tan, acos, asin, atan >>>math.exp(4) #math.e**4 >>>math.pow(math.e,4), pow(math.e,4) >>>math.log (math.e**4) >>>math.sqrt(4) >>>math.ceil(2.0/3), math.floor(2.0/3) >>>int(2.567), round(2.567), round(2.4) #float,long >>>print round(2.567,2) >>>abs(-2), abs(2.0) #available without import >>>divmod(3,4)

  14. Dynamic Typing • Types are determined at runtime (no need for declaration as in C++, JAVA) • Creation: variable is created when it is first assigned a value. • Types: there is no type information associated with variable. Variable refers to some object of a particular type. • Use: when a variable is in expression, it is immediately replaced with a referenced object. Unassigned variable results in error.

  15. >>>a = 3 • Create an object • to represent the value of 3 2. Create the variable a, if it does not yet exist 3. Link the variable a to the new object 3 • Variables always link to objects • Larger objects may be link to other objects.

  16. Shared Reference • Objects are allocated memory • Variables are entries in search table with space for a link to an object (pointers, not labels) >>>a=3 >>>b=a >>>a=“change” “change”

  17. Garbage Collection >>>a=3 >>>a=5 5

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