Computer Programming1

Problem Solving and Software Engineering

Computer Programming2

Objectives

Review Structure of a program Basic phases of software life cycle

– Object-centered design

Practice sessions

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Review

Computer

Software

Hardware

Drivers

Applications OS

CPU

ALU

Registers RAMSecond memory

Bus

User programs

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Review

A program– A sequence of instructions that specifies for the

computer how to solve a problem.– Remember that computers are dumb!– Computers only understands machine language:

A sequence of ones and zeros Cause the computer to perform a particular action, such as

add, subtract, multiply, ... Write machine languages is very hard to do, to understand,

and correct. Thus the invention of high-level languages such as C++.

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Compiler Versus Interpreter

Program in high-level language

Compiler Machine language

A Compiler translates a file written in a high-level language into a machine language file that can be executed on the computer.

An interpreter executes each high-level statement directly. On fly, the interpreter translates the statement to the corresponding machine language instructions and executes/runs them.

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Contrast Assembler and HLL Compiler

Assembler translates one mnemonic into one ML statement

Compiler translates one HL statement into several ML statements

1010110011110101

0000000000010000

0010111010110101

0000000000010010

0010111011111101

0000000000010100

z = x + y; Compiler

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First program: Hello World!

#include <iostream>

int main() { std::cout << “Hello World!”<<std::endl; return 0;}What is the difference between this program

and the one given in the previous lecture?

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Structure of program

Preprocessor directives– #include – #if– …

Namespaces to use New namespaces

– variables– Functions– Classes

The main function

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Structure of a program

#include <…>#include “xxx.h”…

using namespace xxx;….

namespace ns1 { xxx function1(xxxx) { … } … public class }…int main() {

…..return 0;

}

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Variables, Functions, and Classes

Variables– In C++ all variables must be declared before they are used. Furthermore,

variables must be used in a manner consistent with their associated type.– E.g. double radius;

Functions– All C++ functions must be declared before being used, their return type

defaults to int. However, it is considered good style to fully declare all functions. Use void keyword to specify that a function does not return a value.

– E.g. int power(double x, int y) { … return …;} Classes

– Combines data objects and functions to provide an Abstract Data Type (ADT).

– E.g. class Sphere { …}

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From problems to programs

Given a problem, how to write a program that solves it?

A simplified version of the main steps:

DesignerProblem Computer

program Resultsprogrammer

design

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Problem Solving through Software Engineering Phases (V model)

Requirements &Specifications

Design

Implementation

Testing

Maintenance

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Problem Solving through Software Engineering Phases

Requirements & specifications– Formally specify and document the problem we are solving

Design– Analysis, specify algorithms to solve problem

Implementation– Write solution in syntax of language

Testing – Execution and Debugging– Get rid of “bugs”

Maintenance– Update, modify to meet changing needs

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Problem

World’s largest ball of twine found in Cawker City, Ks.– How much does the ball weigh?– What is the length of

the twine when unrolled?

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Object-Centered Design Steps

State how you want the program to behave Identify and categorize the objects involved in the

problem Identify operations needed to solve the problem Develop algorithms by arranging objects and

operations in an order which solves the problem Implement your algorithm using C++

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Behavior

To find the weight of a ball of string:Enter radius of sphere : 9

Enter the density of twine: 1

Now computing . . .

Weight of ball of string = 999.99

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Objects

DescriptionSoftware Objects

Type Kind Name

prompt for radius of sphere

string constant none

screen ostream variable cout

radius of sphere double variable radius

Twine density double variable density

keyboard istream variable cin

weight of ball double variable weight

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Operations

Output prompt for radius of sphere to cout Input real value from cin Store it in radius Output prompt for density of twine to cout Input real value from cin Store it in density Compute weight Output weight to cout

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More Objects

Computation of weight requires additional objects

DescriptionSoftware Objects

Type Kind Name

density of sphere double variable density

4.0 double constant

π double constant PI

3 integer constant

3.0 double constant

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Algorithm (Recipe!)

1. Initialize constant PI2. Output prompt for radius to cout3. Input real value from cin, store in radius4. Output prompt for density to cout5. Input real value from cin, store in density6. Compute

7. Output weight to cout

34.0

3.0

density radiusweight

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Coding

First, create a program stub that contains opening documentation– Compiler directives that add items in libraries needed

for some of the objects and operations– An empty main function

Convert each step of the algorithm into code – If it uses a software object that hasn’t already been

declared, add a declaration statement that specifies the object’s type and name.

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Coding

/* sphereWeight.cpp computes the weight of a sphere.** Input: The radius (feet) and* the density (pounds/cubic foot) of a sphere* Output: The weight of the sphere (pounds)************************************************/#include <iostream> // cin, cout, <<, >>#include <cmath> // pow()using namespace std;int main(){}

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Coding

int main(){

const double PI = 3.14159;cout << "Enter the sphere's radius (feet): ";double radius;cin >> radius;cout << "Enter its density (pounds/cubic feet): ";double density;cin >> density;double weight = density * 4.0 * PI * pow(radius, 3) / 3.0;cout << "\nThe weight of the sphere is approximately "<< weight << " pounds.\n";

}

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Testing

Enter radius of sphere (feet) : 6.5

Enter its density (pounds/cubic feet) : 14.6

The weight of the sphere is approximately 16795 pounds

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Testing, Execution, Debugging

Common error sources Syntax errors

– Violations of grammar rules of the high level language

Semantics/run-time errors– Errors that occur during execution

Logic errors– Errors in the design of the algorithm

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Syntax Errors

Example:double radius

– Missing semi-colon

Compiler finds most of this kind of errors Different compilers give varying degrees of

helpful diagnostics Do many but learn from them!

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Run-time Errors

Not detected until program runs Examples

– Division by zero– Taking square root of negative numbers

Program must be modified to prevent such run-time errors from happening

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Logic Errors

Program compiles, runs without crashing, but gives incorrect results

These are the hardest errors to find Find by using sample data and hand

calculating the correct results, comparing Note: Testing grows increasingly more difficult

with larger programs– Some will run for years without logic error appearing

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Maintenance

Student programs run only a few times Real-world programs are used for many

years– Due to significant investment of resources– They may need to be upgraded or/and changed

New features may be required during life of program usage

Upgrading is called “maintenance”

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Problem SessionA maintenance question?

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Problem

Upgrade the program we previously wrote to solve the following problem:

What is the length of the twine when unrolled?

Follow the OCD steps: State how you want the program to behave Identify and categorize the objects involved in your problem Identify operations needed to solve the problem Develop algorithms by arranging objects and operations in an

order which solves the problem

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Describe Behavior of Program

In addition to the behavior already described, Program should display prompt for the weight of 1 foot of the twine

User enters values from keyboard Program computes, displays on screen the

length (in feet) of the twine when unrolled

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Behavior Envisioned

…

Enter the weight of 1 foot of twine: xxx

...

The length of the twine when unrolled is: xxxx

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Objects

Use description to fill in chart for objects.

DescriptionSoftware Objects

Type Kind Nameprompt for mass density per foot

string constant none

screen ostream variable cout

Density per foot double variable lengthDensity

keyboard istream variable cin

Length of the twine when unrolled

double variable length

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Operations

Description Name

Predefined? Library Operator

Display string using cout

Yes iostream <<

Read the density per foot using cin

Yes iostream >>

Compute the length using weight of ball

divided by the density per foot

Yes /

Output the result using cout

Yes Iostream <<

Use description to fill in chart for operations

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Algorithm and Coding

Work together to determine the steps necessary to have the objects manipulated by the operations

Write the program– Compile and link to fix syntax errors– Test the program to fix run-time and logical errors.