1. Python Programming, 2/e1 Python Programming: An Introduction to Computer Science Chapter 1 Computers and Programs

2. Objectives To understand the respective roles of hardware and software in a computing system. To learn what computer scientists study and the techniques that they use. To understand the basic design of a modern computer. 2Python Programming, 2/e

3. Objectives (cont.) To understand the form (syntax) and function (semantics) of computer programming languages. To begin using the Python programming language. 3Python Programming, 2/e

4. The Universal Machine A modern computer can be defined as “a machine that stores and manipulates information under the control of a changeable program.” Two key elements: Computers are devices for manipulating information. Computers operate under the control of a changeable program. 4Python Programming, 2/e

5. 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! (Von Neumann model) 5Python Programming, 2/e

6. The Universal Machine Programs are executed, or carried out by the CPU(s). 6Python Programming, 2/e

7. Program Power Software (programs) rule the hardware (the physical machine). The process of creating this software is called programming. Why learn to program? Fundamental part of computer science Having an understanding of programming helps you have an understanding of the strengths and limitations of computers. 7Python Programming, 2/e

8. Program Power Helps you become a more intelligent user of computers It can be fun! Creative form of expression Helps the development of problem solving skills, especially in analyzing complex systems by reducing them to interactions between simpler systems. Programmers are in great demand! 8Python Programming, 2/e

9. What is Computer Science? It is not the study of computers! “Computers are to computer science what telescopes are to astronomy.” – E. Dijkstra The question becomes, “What processes can be described?” This question is really, “What can be computed?” 9Python Programming, 2/e

10. What is Computer Science? Design One way to show a particular problem can be solved is to actually design a solution. This is done by developing an algorithm, a step-by-step process for achieving the desired result. 10Python Programming, 2/e

11. Euclid’s Algorithm for GCD Finding the greatest common divisor, one of the 1 st known algorithms 1. Get two numbers, call them num1 and num2 2. While num2 is not zero 1. If num1 > num2, num1 = num1 – num2 2. otherwise num2 = num2 - num1 3. Show num1 as the GCD Python Programming, 2/e11

12. What is Computer Science? Analysis Analysis is the process of examining algorithms and problems mathematically. Some seemingly simple problems are not solvable by any algorithm. These problems are said to be unsolvable (halting problem) Problems can be intractable if they would take too long or take too much memory to be of practical value (traveling salesman). 12Python Programming, 2/e

13. Alan Turing First to formalize the concept of “computation” using a hypothetical device called the “Turing Machine” In the 30s, he laid the foundations of theoretical computer science Python Programming, 2/e13

14. Finite State Machine Input States Transitions Python Programming, 2/e14 Basically, a simple Turing Machine.

15. Python Programming, 2/e15 Example FSM that recognizes the word “nice”

16. Turing Machine More complicated version of an FSM using infinite amount of tape, actions (move head left, right) To this day, it is the model of computation of choice for theoretical computer scientists Python Programming, 2/e16

17. Python Programming, 2/e17 Graphic rendition of a Turing Machine

18. Undecidability In any axiomatic mathematical system, there are statements that cannot be proven true or false (Gödel’s Theorem) E.g., proof by Turing: determine if a program halts (ends) at some point or continues to run forever Python Programming, 2/e18

19. On the other hand… There exist problems that are intractable There are algorithms to solve them, but they take prohibitive amount of time or space to solve them E.g.: traveling salesperson problem, knapsack problem, many more These problems become intractable as the size of the input (e.g.: number of cities) increases Python Programming, 2/e19

20. Implementation Computer scientists DO write code See web page for Python implementation of Euclid’s algorithm Python Programming, 2/e20

21. What is Computer Science? Experimentation Some problems are too complex for precise mathematical analysis. Instead, you can implement a system that simulates the problem and then study its behavior. 21Python Programming, 2/e

22. Hardware Basics The central processing unit (CPU) is the “brain” of a computer. The CPU carries out all the basic operations on the data. Examples: simple arithmetic operations, testing to see if two numbers are equal, I/O requests sent to peripherals. 22Python Programming, 2/e

23. Hardware Basics Memory and secondary storage hold programs and data. CPU can only directly access information stored in main memory (RAM or Random Access Memory). Main memory is fast but volatile, i.e. when the power is interrupted, the contents of memory are lost. Secondary storage provides more permanent storage: magnetic (hard drive, floppy), optical (CD, DVD) but slower 23Python Programming, 2/e

24. Memory (RAM) A memory cell (also called a word) has an address, has contents Contents are expressed in bits / bytes - binary code; a byte is made up of 8 bits Holds a value which may be data, may be an instruction data retrieval = a "copy" not a "cut" getting a value does not erase it

25. Memory Cells AddressContents -27.2 354 0.005 -26 H X 75.62 RTV 001... 0 1 2 3 4 5 6... 999

26. Secondary Storage Files - source files, data file, output file Devices - hard disk, floppy, CD, SD card, flash memory stick Slower than RAM, and cheaper per byte Usually much larger capacity than RAM Permanent – does not change when power is cut off

27. Computer Units Units of capacity - Kilobyte (2^10), Megabyte (2^20), Gigabyte (2^30), Terabyte (2^40), Petabyte (2^50), Exabyte (2^60) Units of speed – MegaHertz, GigaHertz speed of signals from system clock Python Programming, 2/e27

28. Hardware Basics Input devices Information is passed to the computer through keyboards, mice, etc. Output devices Processed information is presented to the user through the monitor, printer, etc. Both are peripherals 28Python Programming, 2/e

29. Programming Languages Natural language has ambiguity and precision problems when used to describe complex algorithms. Programs expressed in an unambiguous, precise way using programming languages. Every structure in programming language has a precise form, called its syntax Every structure in programming language has a precise meaning, called its semantics. (What does the computer do when that structure is executed?) 29Python Programming, 2/e

30. Programming Languages Programming language is a code for writing the instructions the computer will follow. Programmers will often refer to their program as source code. Process of writing an algorithm in a programming language often called coding. 30Python Programming, 2/e

31. Programming Languages High-level computer languages Designed to be used and understood by humans Low-level language Computer hardware can only understand a very low level language known as machine language (actually binary codes) 31Python Programming, 2/e

32. Languages Telling a robot to leave a room In Machine languages take a step, lift arm, grasp knob, turn knob... In Assembly languages LEAVE through DOOR In High-level languages "get outta here!"

33. Programming Languages High-level language c = a + b High-level languages are portable - they can be moved from one kind of CPU to another without rewriting the code. Examples -- FORTRAN, COBOL, Pascal, Python, Ada, Modula-2, C++, Java 33Python Programming, 2/e

34. Programming Languages But high-level languages need to be translated into machine language that the computer can execute. TWO ways of translation: compilers and interpreters Compilers convert an entire program written in a high-level language into the machine language of some computer. A compiler does NOT run the code. 34Python Programming, 2/e

35. 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, one at a time. 35Python Programming, 2/e

36. Programming Languages Compiling vs. Interpreting Once program is compiled, it can be executed over and over without the source code or compiler. If it is interpreted, the source code and interpreter are needed each time the program runs Compiled programs generally run faster since the translation of the source code happens only once. 36Python Programming, 2/e

37. Programming Languages Interpreted languages are part of a more flexible programming environment since they can be developed and run interactively 37Python Programming, 2/e

38. The Translation Process Syntax - the rules governing the formation of statements in the language Spelling Order of words, statements Punctuation Semantics – meaning of the statement What does it DO? What action does the computer do when the statement is executed?

39. Tools for programming Compilers and interpreters for high- level languages IDE (integrated development environment) Translator (compiler or interpreter) editor (creates text format files) loader (get machine code into RAM so it can be run) debugger

40. The Magic of Python When you start Python, you will see something like: Python 3.1.2 (r312:79149, Mar 21 2010, 00:41:52) [MSC v.1500 32 bit (Intel)] on win32 Type "copyright", "credits" or "license()" for more information. >>> 40Python Programming, 2/e

41. The Magic of Python The “>>>” is a Python prompt indicating that Python is ready for us to give it a command. These commands are called statements. >>> print("Hello, world“) Hello, world >>> print(2+3) 5 >>> print("2+3=", 2+3) 2+3= 5 >>> 41Python Programming, 2/e

42. The Magic of Python When we exit Python, the functions we’ve defined cease to exist! However, the source code is saved on secondary storage and can be accessed and ran many times Programs are usually composed of functions, modules, or scripts that are saved on disk so that they can be used again and again. A module file is a file created in text editing software (saved as “plain text”) that contains function definitions. 42Python Programming, 2/e

43. The Magic of Python # File: stars.py # this is a sample program # it prints out a string of stars on the screen def main (): stars = "“ num = eval(input("How many stars do you want? ")) for i in range(num): stars = stars + "*" print (stars) main() When we run the program, here’s the output: >>> How many stars do you want? 25 ************************* >>> 43Python Programming, 2/e

44. The Magic of Python We’ll use filename.py when we save our work to indicate it’s a Python program. In this code we’re defining a new function called main. The main() at the end tells Python to run the code. Python Programming, 2/e44

45. Inside a Python Program # File: stars.py # A simple program Lines that start with # are called comments Intended for human readers and ignored by Python Python skips text from # to end of line 45Python Programming, 2/e

46. Inside a Python Program def main(): Beginning of the definition of a function called main Since our program has only this one module, it could have been written without the main function. The use of main is customary, however. 46Python Programming, 2/e

47. Inside a Python Program print (stars) This line causes Python to print the value of a variable. 47Python Programming, 2/e

48. Inside a Python Program num = eval(input(“How many stars do you want? ")) num is an example of a variable A variable is used to assign a name to a value so that we can refer to it later. The quoted information is displayed, and the number typed in response is stored in num. 48Python Programming, 2/e

49. Inside a Python Program for i in range(num): For is a loop control structure A loop tells Python to repeat the same thing over and over. In this example, the following code will be repeated num times. 49Python Programming, 2/e

50. Inside a Python Program stars = stars + “*” This line is the body of the loop. The body of the loop is what gets repeated each time through the loop. The body of the loop is identified through indentation. The effect of the loop is the same as repeating this line num times! 50Python Programming, 2/e

51. Inside a Python Program stars = stars + “*” This is called an assignment statement The part on the right-hand side (RHS) of the “=“ is a mathematical expression. * is used to indicate multiplication In this case, it is a character in a string Once the value on the RHS is computed, it is stored back into (assigned) into stars 51Python Programming, 2/e

52. Inside a Python Program main() This last line tells Python to execute the code in the function main 52Python Programming, 2/e

53. Case Study: Converting Miles to Kilometers Problem Your summer surveying job requires you to study some maps that give distances in kilometers and some that use miles. You and your coworkers prefer to deal in metric measurements. Write a program that performs the necessary conversion.

54. Case Study Analysis The first step in solving this problem is to determine what you are asked to do. You must convert from one system of measurement to another, but are you supposed to convert from kilometers to miles, or vice versa? The problem states that you prefer to deal in metric measurements, so you must convert distance measurements in miles to kilometers.

55. Data Requirements Problem Input milesdistance in miles Problem Output kmsthe distance in kilometers Relevant Formula 1 mile = 1.609 kilometers

56. Design an Algorithm that solves the problem Algorithm (Pseudocode) 1. Get the distance in miles. 2. Convert the distance to kilometers. 3. Display the distance in kilometers. Algorithm Refinement 2.1 The distance in kilometers is 1.609 times the distance in miles Desk check!

57. Implementation in Python # converter.py # converts miles to kilometers def main (): miles = eval(input("distance in miles? ")) kilometers = miles * 1.609 print("The equivalent distance in kilometers is", kilometers) main() Python Programming, 2/e57

58. Testing Test with input data for which you can easily determine the expected results E.g. 10 miles should convert to 16.09 kilometers

59. Ethics Responsibility comes with knowledge Don’t do hacking! Don’t do piracy! Don’t do data theft Protect data privacy You have a responsibility to develop programs without errors or at least let the users know there are flaws

60. Python keywords used in Ch.1 Python Programming, 2/e60 print def # eval, input for, while, range if assignment = +, -, /, *, **