1. Class 2

2. The Universal Machine What is a computer program?
A detailed, step-by-step set of instructions telling a computer what to do.
If we change the program, the computer performs a different set of actions or a different task.
The machine stays the same, but the program changes!
Python Programming, 3/e

3. Hardware Basics
Python Programming, 3/e

4. Hardware Basics Fetch-Execute Cycle
First instruction retrieved from memory
Decode the instruction to see what it represents
Appropriate action carried out.
Next instruction fetched, decoded, and executed.
Lather, rinse, repeat!
Python Programming, 3/e

5. Programming Languages
High-level language c = a + b
This needs to be translated into machine language that the computer can execute.
Compilers convert programs written in a high-level language into the machine language of some computer.
Python Programming, 3/e

6. Programming Languages
Interpreters simulate a computer that understands a high-level language.
The source program is not translated into machine language all at once.
An interpreter analyzes and executes the source code instruction by instruction.
Python Programming, 3/e

7. Review what we saw in lab 1
# This is a comment 2 an integer literal a floating point literal "Syracuse" a string literal

8. The Python shell prompt
In the Python shell >>> The Python shell prompt >>> We type commands here and press the enter key Some statements show results >>> Some statements do something without producing output. >>> length = 5

9. Variables Can store values – ints, floats, strings,...
Naming rules for identifiers:
UPPER case and lower case are different
can begin with _underscore or letter
followed by a sequence of letters, digits, or underscores
Python keywords can't be variable names (p32)

10. identifiers
Legal names: sk8er given_name houseNumber version3 Illegal names: 4kLift month/day day of week "variable" Legal but bad names: O o l

11. expressions: fragments of code that produce or calculate new data values
not and expression:
literals
age = 15
variables
3+5
"Falk " + "100"

12. Assignment statement <variable> = <expr>
Compute the expression on the right side. Put the result in the variable on the left side.
examples:
nonexamples:
fish = "tuna"
"weight" = 20
norm = x*x + y*y
= change

13. Operations
On numbers. (ints, floats) () ** (exponentiation) * / + - On ints (discuss later) // % On strings +

14. the Python Shell
Interprets commands as we go. Commands go at shell prompts: >>> >>> 2**5 32

15. the Python Shell
Anything in a line after # is a comment, ignored by the computer >>> #Here is a comment >>> 8+45 # Add two numbers 53 >>> # + 3 3
Why not 6?

16. the Python Shell remembers values till they are replaced
till the shell is “restarted”
like a long calculator tape