PIC10A Lecture 2

Reading

p6-p13

Identifiers

They are names of variables.

Format

must start with either a letter or the underscore ( _ ) symbol
all the rest of the characters must be letters, digits, or the underscore ( _ ) symbol.
cannot be keywords or reserved words. (e.g. int, double, if, for, .....)

Keywords or reserved words

They have predefined meanings in C++ and cannot be used as names for variables or anything else.
Refer to Savitch Appendix 1 for the whole list of keywords.
Some examples are int, double, float, char, if, for, while, .... You will learn more.

Exercises

Which of the following can be identifiers? If it can't be an identifier, specific the reason.

(1) a (2) a2 (3)abc (4) nbc4 (5) 2go 

(6) _abc (7) score (8)myFirstHomeworkScroe (9) a2b3c (10)A_2_B_3  

(11) 1010  (12) 3X (13) %change (14) change% (15) data-1 

(16)int (17) www.ucla.edu (18) program.cpp (19) ucla_edu

Case-sensitive

C++ distinguishes between uppercase and lowercase letters.
Hence the following are three distinct identifiers and could be used to name three distinct variables
```
rate  RATE  Rate
```
It is not a good idea to use them because it may be confusing.

Convention

use meaningful English words e.g. score, income, rent
usually start with lowercase letter
use uppercase letter to indicate "word" boundary. e.g.
```
topSpeed, highestScore, taxRate, timeOfArrival
```
don't use predefined identifiers like cin, cout, main ....

Real Story (you can skip it, it won't be part of your midterm/exam)

When I was a TA, one of my student wrote the following program. Obvious he was a NBA fan.

#include<iostream>
using namespace std;

int main() {
    int lakers, bulls, spurs;
    double kings;
    cin >> lakers >> bulls >> spurs;
    kings = 0.25*lakers+0.1*bulls+0.05*spurs;
    cout << kings << endl;
    return 0;
}

Do you know what the program does? This is what he meant

#include<iostream>

using namespace std;

      

int main() {

    int numOfQuarters, numOfDimes, numOfNickels;

    double value;

    cin >> numOfQuarters >> numOfDimes
>> numOfNickels;

    value =
0.25*numOfQuarters+0.1*numOfDimes+0.05*numOfNickels;

    cout << value << endl;

    return 0;

}

Variables

a memory location to store data for a program
must declare all data before use
has

type (int, double, float etc etc)
identifier (the name of the variable)
value (e.g. 2.7)

Variable declaration

Each variable must be declared before it is used; otherwise, it can be declared anywhere.
Most people declare all the variables at the beginning of the program. (inside {... })
You must declare the type of the variable (e.g. int, float, double...)
e.g.
```
int numOfStudents;
double percentage;
```
This lets the computer know how to set aside memory and how to interpret it.

Format

Syntax

Type_name variable_name_1, variable_name_2,......;

Example

int numOfQuarters, numOfDimes, numOfNickels;

double speed;

Types

Refer to Savitch for more details.

Type Name	Meaning	Memory Used
`short`	small integer	2 bytes
`int`	integer	4 bytes
`long`	large integer	4 bytes
`float`	decimal number	4 bytes
`double`	decimal number (more precious)	8 bytes
`long double`	decimal number (most precious)	10 bytes
`char`	characters	1 byte
`bool`	true/false	1 byte

(The meaning of bytes will be discussed during the class.)

A program to find the size (won't be tested in the exams)

This program is used to find the sizes of different types.

#include<iostream>

using namespace std;

      

int main(){

    char ch = 'a';

    int i = 0;

    cout << "A char uses " << sizeof(ch)
<< " byte(s).\n";

    cout << "An int uses " << sizeof(i)
<< " byte(s).\n";

    return 0;

}

Modify the program to find the size of double.

Assignment Statements

Syntax

Variable = Expression;

Example

distance = rate * time;
count = count + 2;
value = 0.25*numOfQuarters+0.1*numOfDimes+0.05*numOfNickels;

Expressions may include variables and arithmetic with +,-.*, / (add, subtract, multiply, divide) and other stuff...
The right-hand side(expression) is evaluated. Then the variable of the left-hand side is set equal to this value.

Look at the following program. Is there any error? If not, what is the output of the program

#include<iostream>
using namespace std;
int main() {
    int a;
    cout << a;
    return 0;
}

no compiling error
in Visual C++, there is a warning
variable a has not been given a value
garbage is inside variable a
the output of the program is a strange number

Initializing Variables in declaration

You can initialize a variable(that is, give it a value) at the time you declare the variable.

Syntax

Type_Name Variable_Name_1 = Expression_for_Value_1, Variable_Name_2 = Expression_for_value_2, ....;

Examples

int count = 0, limit = 10;

double distance = 32.5;

Syntax

Type_Name Variable_Name_1(Expression_for_Value_1), Variable_Name_2 (Expression_for_value_2), ....;

Examples

int count(0), limit(10);

double distance(32.5);

Example

#include<iostream>

using namespace std;

      

int main() {

    double quarterValue = 0.25, dimeValue = 0.1,
nickelValue = 0.05;

    int numOfQuarters, numOfDimes, numOfNickels;

    double value;

    cout << "Input the number of quarters, dimes
and nickels.\n";

    cin >> numOfQuarters >> numOfDimes
>> numOfNickels;

    value = quarterValue*numOfQuarters

           +
dimeValue*numOfDimes 

           +
nickelValue*numOfNickels;

    cout << "The value is " << value
<<".\n";

 

    return 0;

}

Question

What happens if you write int a = 2.55 ?