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C++ Programming BasicsChapter 2 Lecture

CSIS 10A

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Agenda

Review An Intermediate Program

Making a Program Interactive

Assignment Statements

Standard Program Structure

File output

Debugging

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Review –what does this show?

int x;x = 8; cout << "x=" <<x<<endl;cout << x << endl << "=x";

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Review –Identify each piece#include <iostream>using namespace std;

int main(){

// this program stores data in a variableint m;cout << “Enter the value of m:”;cin >> m;cout << “m = “ << m << endl;

system(“pause”); return 0;}

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Variables and AssignmentsVariables are like small blackboards

We can write a number on them

We can change the number

We can erase the number

C++ variables are names for memory locationsWe can write a value in them

We can change the value stored there

We cannot erase the memory location• Some value is always there

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Review –what does this show?

int x;x=6;x=8;cout << "x = " << x;

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IdentifiersVariable names are called identifiersChoosing variable names

Use meaningful names that represent data to be storedFirst character must be

• a letter• the underscore character _

Remaining characters must be• letters• Numbers• underscore character

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Which identifiers are legal?

feet Average score

1993tax Average_score

Sum Average.score

Valid?

FINAL

final

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Keywords

Keywords (also called reserved words)Are used by the C++ language

Must be used as they are defined in the programming language

Cannot be used as identifiers

Examples (75 altogether):• main, while, if, new, for, do, class, struct, cin, cout

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Declaring VariablesBefore use, variables must be declared

Tells the compiler the type of data to store

Examples: int one_weight, total_weight;

int represents whole numbers• Could store 1, 4, -345, etc.

• one_weight and total_weight are both of type int

• Many other types, but for now, just use int!!!

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Declaring Variables (Part 2)

int main(){

int score1, score2, sum; cin>> … etc … sum=score1+score2; … etc …

return 0; }

Put all your variable declarations at the top of main

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Declaring Variables (Part 3)

Declaration syntax:Type_name Variable_1 , Variable_2, . . . ;

Declaration Examples:int m_score, total_score;

float moon_distance;

int age, num_students;

char grade;

ONLY USE TYPE int FOR NOW!!!

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Agenda

Review

An Intermediate Program Making a Program Interactive

Arithmetic

Standard Program Structure

Debugging

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An intermediate C++ program:// height.cpp// Convert height in feet to inches.#include <iostream>using namespace std;int main(){int feet, inches;feet = 6;inches = feet * 12;cout << "Height is " << inches << " in.";return 0;}

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Execution – what you see

Height is 72 in.

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Execution – what really happensint feet, inches;

feet

int

int

?inches

?

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Execution – what really happensfeet=6;

feet

int

int

?inches

6

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Execution – what really happensinches = feet * 12;

feet

6*12

int

int

72inches

6.0

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Execution – what really happenscout << "Height is " << inches << " in.";

feet

int

int

72inches

6.0

Height is 72 in.

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Agenda

Review

An Intermediate Program

Making a Program Interactive Arithmetic

Standard Program Structure

Debugging

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Interactive Program

The previous program always runs exactly the same way (Height is 72 in)

We’d like to be able to “put in” the data we want to process

So we add two more lines. One asking for data: cout<<“What is height in feet?”<<endl;

The other takes input: cin >> feet;

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An intermediate C++ program:// height.cpp// Convert height in feet to inches.#include <iostream>using namespace std;int main(){int feet, inches;cout << “What is height in feet?” << endl; //********cin >> feet; //********inches = feet * 12; cout << "Height is " << inches << " in.";return 0;}

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Basic Interactive Program

int main()

{

1) Declare Variables

2) Ask (prompt) for data

3) Input data

4) Calculate result

5) Display result with message

return 0;

} Go back a slide and number the lines

Then open Lab1Basics.cpp and do prob1

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A clever use of comments

Inside Lab1Basics.cpp are 10 programming problems…how to keep separate?

Remember // ignore to end of line

Another type of comment works in “blocks” :/* start a block comment

ignored

end a block comment with */

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Combine the two…

/**** Problem 1 ********

// Here are your instructions

(space for your code)

// *****End Problem 1 *****/

/**** Problem 2 ********

(etc..)

Make // to work

on this problemchange back to /

when done

TRY IT NOW! Do problems 1-3 in Lab1Basics.cpp

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Agenda

Review

An Intermediate Program

Making a Program Interactive

Arithmetic Standard Program Structure

Debugging

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Assignment StatementsAn assignment statement changes the value of a variable

total_weight = one_weight + number_of_bars; • total_weight is set to the sum one_weight + number_of_bars

Assignment statements end with a semi-colon

The single variable to be changed is always on the leftof the assignment operator ‘=‘

On the right of the assignment operator can be• Constants -- age = 21;• Variables -- my_cost = your_cost;• Expressions -- area = length * width;

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Assignment Statements and Algebra

The ‘=‘ operator in C++ is not an equal signThe following statement cannot be true in algebra

number_of_bars = number_of_bars + 3;

In C++ it means the new value of number_of_bars is the previous value of number_of_bars plus 3

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Initializing Variables

Declaring a variable does not give it a valueGiving a variable its first value is initializing the variable

Variables are initialized in assignment statements

int miles, area; // declare the variables miles = 26; // initialize the variable

area=0; // initializeDeclaration and initialization can be combined

int miles = 26, area = 0.0;Variables that are going to be input using cin>> or calculated later do not need to be initialized

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Arithmetic in C++

Basic operations + - * /A = r2 area=radius*radius*3.14;

F = (v -a) force=(v-a)/(v+a);

(v+a)

More on arithmetic later. C++ has some interesting properties to be explored next chapter.

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Can you guess what is stored in x?

int a, b=3, c, x;

a=b*2;

c=a+3;

x=c-2;

Program flow is sequential (top to bottom)

a b c x

? 3 ? ?

6 3 ? ?

6 3 9 ?

6 3 9 7

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Your turn…what is stored in x?

int a, b, c, x=3;

a=2;

x=3+a;

x=x+a;

Program flow is sequential, order matters!!

a b c x

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Agenda

Review

An Intermediate Program

Arithmetic

Standard Program Structure Debugging

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Standard Program Structure

To solve most problems, your main() program will generally look like this (conceptually)

1. Declare variables for input, result and intermediate data

2. Ask for data3. Input data (cin)4. Calculate result5. Output result (cout)

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Another interactive C++ program:// Convert input number of nickels and dimes// to cents.#include <iostream>using namespace std;

int main(){ int nickels, dimes, cents; cout << "Enter number of nickels and dimes: "; cin >> nickels >> dimes; cents = 5 * nickels + 10 * dimes; cout << nickels << " nickels and " << dimes << " dimes " << "= " << cents << " cents " << endl; return 0;}

Notice cascading input

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Your Turn

Run the previous code, found in Lab1Basics problem4, and verify it works correctly

Then modify the program to input dimes and quarters and calculate the equivalent cents

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Agenda

Review

An Intermediate Program

Arithmetic

Standard Program Structure

Debugging

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Syntax Errors // errors.cpp#include <iostream> using namespace std;

int main(){ int n; cout << "Hello" << endl; N = 2; cout << "n eqalls " << n cout << "So long << endl; system("pause"); return 0;}

This program has 3 errors

Run Problem 5 and see what they are

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Tracking them down

Syntax—double click on the first error message

it takes you to that line of code

Look at or above that line for possible errors

Fix it and try again

Errors often create other errorsJust fix the first one, then recompile

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That’s a wrap !

What we learned today:How to write an intermediate C++ program

Standard program structure

Assignments and Basic Arithmetic

How to debug your program

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Go back home proud ! You’re a C++ programmer !