To better understand programs, and often to debug them, programmers use pen and pencil tracing.
To trace, step through a program line-by-line, showing the value of each variable.
When a new variable is introduced, create a memory cell for it and place its value in it:
x = 5 x
5
When variables change, cross out the old value and put the new value underneath.
x = 5 x
x=x+1 5
6
In this way, the trace shows the history of the program execution, how each variable changes over time. Note, however, that each (scalar) variable represents a single cell with one value. In terms of program execution, the crossed out values are gone forever.
Instructor Demo: Interest Rate Programprincipal = 5000
interestRate = .1
oneYearInterest = principal * interestRate
oneYearPrincipal = principal+oneYearInterest
twoYearPrincipal = oneYearPrincipal+(oneYearPrincipal*interestRate)In-Class Worksheet: Programmer Milestone 2-- total of a list
list = [4,2,19,3]
i=0
total=0
while i<len(list):
total = total+list[i]
i=i+1
print total
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