Deitel - Python, How To Program.pdf

pythonhtp1_02.fm Page 43 Wednesday, December 12, 2001 12:12 PM
Chapter 2 Introduction to Python Programming 43
integer1 "45"
Fig. Fig. 2.9 2.9 Memory location showing value of a variable and the name bound to
the value.
integer1
Fig. Fig. 2.10 2.10 Memory location showing the name and value of a variable.
Returning to our addition program, when the statements
integer2 = raw_input( "Enter second integer:\n" )
integer2 = int( integer2 )
execute, suppose the user enters the string "72". After the program converts this value to
the integer value 72 and places the value into a memory location to which integer2 is
bound, memory appears as in Fig. 2.11. Note that the locations of these objects are not necessarily
adjacent in memory.
Once the program has obtained values for integer1 and integer2, the program
adds these values and assigns the sum to variable sum. After the statement
sum = integer1 + integer2
"45"
performs the addition, memory appears as in Fig. 2.12. Note that the values of integer1
and integer2 appear exactly as they did before they were used in the calculation of
sum. These values were used, but not modified, as the computer performed the calculation.
Thus, when a value is read out of a memory location, the value is not changed.
Fig. Fig. 2.11 2.11 Memory locations after values for two variables have been input.
45
integer1 45
integer2 72