THE PROCESS OF WRITING SOFTWARE

Python: System Engineering

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Computing Systems

A computing system is composed of hardware: the physical components of the system software: the instructions that control the hardware

Hardware components include: Motherboard

Central processing unit (CPU) Memory (RAM / Drives)

IO Devices Video Card Sound Card Network Card

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Hardware: CPU

Central Processing Unit An electronic chip that performs instructions

The “brains” of a computing system

A CPU can only perform very simple tasks: Can add/subtract/multiply/divide two numbers Can compare two numbers to see if one is smaller/larger Can copy/move data from one place to another

CPU’s appear more powerful than this since these tasks are done very quickly

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Software and Programming

Tell the CPU to add two numbers Load the number from memory location 2001 into CPU

register R1 Load the number from memory location 2002 into CPU

register R2 Add the numbers in registers R1 and R2 Copy the result into memory location 2003

Programming in such a low-level fashion would not be good. Would like to write something like c = a + b

Programming languages make it easier for a programmer to talk to the CPU

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Overview of Computation

Natural-language : a language used by people If it is written, it is defined by a system of symbols and

notations that communicate the content of the language

Examples: English, Spanish, French, Arabic, Tamil, etc…

Programming language : a language used by computers A system of symbols and notations that communicate

computational content to the CPU.

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Low/High level languages

Low-level language Any language that is “close” to machine instructions Not meant for human readability Specific to the CPU

High-level language Any language that is closer to “natural” language Easier to read and understand Portable

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Languages

Object code, bytecode

Assembly

Fortran, C

Algol, Pascal, Ada

Python, Java, Ruby

microcode

High Level

Low Level

Natural LanguageEnglish, Chinese, Japanese, …

Semantic Gap

Machine Language Example (Pentium III Linux Box)

01111111 0100010101001100 0100011000000001 0000000100000000 00000000…

Low Level Example

Can you understand this code?

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Assembly Example (Pentium III Linux Box).file “hello.c”.version “01.01”.section .rodata

.LCO:.string “Hello World\n”

.text.align 4

.globl main.type main,@function

main:pushl %ebpmovl %esp,%ebppushl $.LCOcall printfaddl $4,%espxorl %eax,%eaxjmp .L1.p2align 4,,7

.L1:leaveret

Intermediate Level Example

Can you understand this code?

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C Language Example (machine independent)

#include <stdio.h>

void main() {printf(“Hello World\n”);

}

High Level Example

Can you understand this code?

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Python Example

print "Hello World"

High Level Example

Can you understand this code?

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Compiled vs. Interpreted12

CompilerSource Program

Executable Code

(THE CPU)Input Output

Interpreter

Source Program Output

Input CPU

Compilation: The language is brought down to the level of the machine using a translator. The translator is often called a compiler.

Interpretation: The machine is brought up to the level of the language by the use of a virtual machine. This machine is often known as an interpreter.

Waterfall model

The waterfall model involves the following processes Analysis: Figure out exactly what the problem is. Understand

as much of the problem as possible Requirements: Describe exactly what your program will do

(not necessarily how). Design: Describe the variables, functions and classes of the

solution. Describe the algorithms that will be used in your software.

Implement: Translate the design into a programming language and enter it into the computer. This course will use Python.

Test/Debug: Execute your implementation and devise tests that are designed to expose errors (bugs).

Maintenance: Continue to improve/develop/fix the program in response to user needs.

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Waterfall14

Example: Temperature Conversion

Narrative: Susan is spending a year of study in China to learn advanced computer science programming. She speaks Chinese fluently and listens to the radio

every morning to figure out what the temperature will be so that she can dress appropriately.

The temperatures are reported in units of Celsius but she thinks only in terms of Fahrenheit.

The analysis is pretty easy in this case: She wants to write a program to convert Celsius to

Fahrenheit for her.

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Example: Temperature Conversion

Requirements The program will allow her to type in a number that

represents the temperature in degrees Celsius. The program will then print out the corresponding

temperature in degrees Fahrenheit. The relationship between input and output must be

well defined. F = 9/5 * C + 32 F is degrees in Fahrenheit (the output) C is degrees Celsius (the input)

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Example: Temperature Conversion

Design This is usually written in fake code (psuedo-code)

This is not a programming language Describes the essentials without worrying about detail For example:

Input the temperature in degrees C (call it celsius)Calculate Fahrenheit as (9/5)*celsuis + 32Output Fahrenheit

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Example: Temperature Conversion

Implementation Must take the design and translate it into a program.

# convert.py# A program to convert Celsius temps to Fahrenheit# by: Susan Computewell

def main(): celsius = eval(input(“What is the Celsius temperature? ”)) fahrenheit = 9 / 5 * celsius + 32 print(“The temperature is”, fahrenheit, “degrees Fahrenheit)

main()

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Example: Temperature Conversion

Testing How would you test this program?

# convert.py# A program to convert Celsius temps to Fahrenheit# by: Susan Computewell

def main(): celsius = eval(input(“What is the Celsius temperature? ”)) fahrenheit = 9 / 5 * celsius + 32 print(“The temperature is”, fahrenheit, “degrees Fahrenheit)

main()

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