Variables and C++ Data Types

CSE1222: Lecture 2 The Ohio State University 1

Variables and C++ Data Types


mathExample2.cpp// math example

#include <iostream>#include <cmath>

using namespace std;

int main(){ cout << "The reciprocal of 10 is " << 1.0/10.0 << endl; cout << "The square root of 10 is " << sqrt(10.0) << endl; cout << "e^(10.0) = " << exp(10.0) << endl; cout << "The reciprocal of 15 is " << 1.0/15.0 << endl; cout << "The square root of 15 is " << sqrt(15.0) << endl; cout << "e^(15.0) = " << exp(15.0) << endl;

return 0; // exit program}


mathExample2.cpp

> g++ -o mathExample2.exe mathExample2.cpp

> mathExample2.exe

The reciprocal of 10 is 0.1

The square root of 10 is 3.16228

e^(10.0) = 22026.5



e^(15.0) = 3.26902e+06

>


Variables (1)

• Programs like mathExample2.cpp have little use in practice…

• There is no room for change. They simply print the same output onto the screen every time they are executed.

• A more interesting program might have different behavior under different circumstances.


Variables (2)

• For instance, a program that asks for a number and outputs its reciprocal and square root is much more useful than mathExample2.cpp.

– This program needs placeholders for the incoming values.

– These placeholders are called variables





int main(){ double x;

x = 10.0; cout << "The reciprocal of 10 is " << 1.0/x << endl; cout << "The square root of 10 is " << sqrt(x) << endl; cout << "e^(" << x << ") = " << exp(x) << endl;

x = 15.0; cout << "The reciprocal of 15 is " << 1.0/x << endl; cout << "The square root of 15 is " << sqrt(x) << endl; cout << "e^(" << x << ") = " << exp(x) << endl;



Declaration Statements

• Before you can use a variable, you must declare it. This allows the computer to set aside the memory that will be needed for that variable.

• Variables consist of a name and its data type. C++ variable declarations are of the form:dataType variableName;

• dataType can be: int, float, char, double, unsigned int, ...• variableName must be composed of alphanumeric

characters or underscore ‘_’.


Declaration Example

#include <iostream>using namespace std;

int main(){int age;float wage;char initial;double height;



Variable Names

• Variable names are composed of the characters:

a,b,c,..,z,A,B,C,…,Z,0,1,2,…,9 and _.• Variables names must begin with:

a,b,c,..,z,A,B,C,…,Z or _.• Capitalized and lower case letters are different.• Examples:

int age; int age1;

int Age; int age2;

int myAge; int age3B;

int Jacks_age; int _age;

Variable Names

Which of these are valid variable names?• me2• Me2• 2L8• R-Wenger• He_who_hesitates_is_lost• HeWhoHesitatesIsLost• Gordon.Gee



Variable Assignments

• Variables can be assigned values during or after declaration, but never before (why?)

• Assignment is done with the equals signheight = 67.34;initial = ‘E’; //initial = E; will not work as expecteddouble totalPay = salary + overtime;

• Variable assignment is NOT commutative! The variable must always be on the left side of an equals sign. The following is NOT valid:67 = x;






cout << "Enter x: "; // Note: no new line cin >> x; // Note: operator ">>“, not operator "<<"

cout << "The reciprocal of " << x << " is " << 1.0/x << endl; cout << "The square root of " << x << " is " << sqrt(x) << endl; cout << "e^(" << x << ") = " << exp(x) << endl;



Input Using cin (1)

• cin (Console INput) can be used to obtain user input .• Unlike cout, use >> with cin, and not <<• When the program is run, cin will wait indefinitely for

user input.• cin will input a single value into a variable when it

detects a new line from the input:• Remember that before using inputting values into

variables, the variables MUST have already been declared!

… double x;

cout << "Enter x: "; // Note: no new line cin >> x; // Note: operator ">>“, not operator "<<"…


> mathExample4.exe

Enter x: 10.0



e^(10) = 22026.5

>

…

int main()

{

double x;

cout << "Enter x: "; // Note: no new line

cin >> x; // Note: operator ">>“, not operator "<<"

cout << "The reciprocal of " << x << " is " << 1.0/x << endl;

cout << "The square root of " << x << " is " << sqrt(x) << endl;

cout << "e^(" << x << ") = " << exp(x) << endl;

…


mathExample4.cpp

Try inputs:

20

-20

1000

-1000

0

-0

// math example




cout << "Enter x: "; // Note: no new line cin >> x; // Note: operator ">>“, not operator "<<“

cout << "The reciprocal of " << x << " is " << 1.0/x << endl; cout << "The square root of " << x << " is " << sqrt(x) << endl; cout << "e^(" << x << ") = " << exp(x) << endl;



xyz1.cpp// multiple declarations example


int main(){ double x; double y; double z;

cout << "Enter x, y and z: "; cin >> x; cin >> y; cin >> z;

cout << "x = " << x << endl; cout << "y = " << y << endl; cout << "z = " << z << endl;



> xyz1.exeEnter x, y and z: 1 2 3x = 1y = 2z = 3

>

…

cout << "Enter x, y and z: ";

cin >> x;

cin >> y;

cin >> z;

cout << "x = " << x << endl;

cout << "y = " << y << endl;

cout << "z = " << z << endl;

…



>

…


cin >> x;

cin >> y;

cin >> z;

cout << "x = " << x << endl;

cout << "y = " << y << endl;

cout << "z = " << z << endl;

…


Multiple Declarations

• In the previous example we haddouble x;

double y;

double z;

• This can be done in one statement like this:double x, y, z;

• This is very useful when creating several variables of the same type.




int main(){ double x, y, z; // multiple declarations

cout << "Enter x, y and z: "; cin >> x; cin >> y; cin >> z;






int main(){ double x, y, z;

cout << "Enter x, y and z: "; cin >> x >> y >> z; // read x, then y, then z




Multiple Inputs Using cin

• cin can be used to obtain multiple inputs.

• cin knows when to delimit (i.e. start looking for the next input) upon reaching a “whitespace.”

• Whitespaces are: tabs, spaces, new lines



>

…


cin >> x >> y >> z;

cout << "x = " << x << endl;

cout << "y = " << y << endl;

cout << "z = " << z << endl;

…



>

…


cin >> x >> y >> z;

cout << "x = " << x << endl;

cout << "y = " << y << endl;

cout << "z = " << z << endl;

…


Breaking up Multiple Inputs

• Sometimes it makes more sense to break up multiple inputs into single inputs, even if they are correlated:

int x, y, z;

cout << “Enter x: “;cin >> x;

cout << “Enter y: “;cin >> y;

cout << “Enter z: “;cin >> z;

cin and cout

Which of the following C++ statements have syntax errors and what are the errors?

• cout >> “Answer = ” >> x+y >> endl;

• cin << x;

• cout << “Yes, ” << “ or ” << “ no “ << “ or ” << “ maybe.” << endl;

• cin >> yes >> no >> maybe;

• cout << “x + y = ” (x + y) << endl;



Variables (3)

• Variables are used to hold data within a program (the data is held in your computer’s main memory). A program can read-from and write-to variables. That is, their values can vary.

• Every variable consists of two parts:1. The name of a variable is tied to a location in

memory

2. Its data type (discussed next...)


Data Type: Integers (1)

• An integer value is any number that has no decimal point.

123 -45000 +1432431 0 are all valid integers

• 1,244 is not a valid integer in C++; no commas are used. Neither is $12 because $ is not a valid part of an integer.


Data Type: Integers (2)

• The largest and smallest integers supported is dependent of the computer on which the program is compiled. Most of today’s computers use 32-bits to represent an integer, so 232 values can be represented.

• Integers can be signed or unsigned.– What is the maximum value of an unsigned 32 bit

integer?– What is the maximum value of a signed 32 bit

integer?


intExample1.cpp// example using type int


int main(){ int x, y;

cout << "Enter x and y: "; cin >> x >> y; // Read in x and then y

cout << "x = " << x << endl; cout << "y = " << y << endl; cout << "x+y = " << x+y << endl; cout << "x/y = " << x/y << endl; cout << "Done." << endl;


Try inputs:

17 3

3 17

0 17

17 0

2000000000 2000000000


Integer Division

• In C++, (15 / 2) is 7. Why?• Remember that integers have no fractional parts!• There is no rounding in integer division. If your program

contained: cout << (15 / 16); The output would be 0.

• The fractional part of an integer division is truncated so the result can be an integer. If you need the remainder, use %. If you need a decimal answer, make your operands floating-point numbers:cout << (15.0 / 16.0);


Data Types: Floating Point Numbers (1)

• Floating-point numbers have a decimal point, and they can also be signed or unsigned.

• There are three basic types:

float

double

long double– The differences between these are their

supported range and precision.



• To represent a floating point number:– float uses 32 bits (4 bytes)– double uses 64 bits (8 bytes)– long double uses 128 bits (16 bytes)

• The tradeoff is storage vs. precision and range

• What exactly is the precision and range, and how are floating point numbers represented in binary format? IEEE 754 Standard



• Always use “double” to represent floating point numbers.



• Floating-point numbers can be written in exponential notation:

134.56 or 1.3456e2

-0.00345 or -3.45e-3

• Here, e is short for “times ten to the power of”, just like in scientific notation.


rationalExample1.cpp// example using type double


int main(){ double x, y; cout << "Enter x and y: "; cin >> x >> y;

cout << "x = " << x << endl; cout << "y = " << y << endl; cout << "x+y = " << x+y << endl; cout << “x*y = " << x*y << endl; cout << “x/y = " << x/y << endl; cout << "Done." << endl;


Try inputs:

17 3

3 17

17 0

4000000000 4000000000

1e100 1e100

1e200 1e200


Data Type: Characters

• Characters include:– All letters of the alphabet (upper and lower case)– The symbolic representation of digits 0 – 9– All various symbols such as: + * & ^ % $ | , !

• A character value is simply one of these in single quotes, such as 'A' or '8' or ':' or ' ' (blank space).



• A character value is a character in single quotes, such as 'A' or '8' or ':' or ' ' (blank space).

• NOTE: '8' and 8 are different.– '8' is the symbolic representation of the

character, ‘8’;– 8 is the integer 8.



• Characters usually take up 8 bits (1 byte)

• That means there are 28 (or 256) different characters, and every number within the range of [0, 255] is mapped onto some character

• So a character really boils down to a numerical representation known as ASCII encoding.

ASCII Code


Code Char

32 Space

33 !

34 "

35 #

36 $

37 %

38 &

39 '

40 (

41 )

… …

Code Char

48 0

49 1

50 2

51 3

52 4

53 5

54 6

55 7

56 8

57 9

… …

Code Char

65 A

66 B

67 C

68 D

69 E

70 F

71 G

72 H

73 I

74 J

… …

Code Char

97 a

98 b

99 c

100 d

101 e

102 f

103 g

104 h

105 i

106 j

… …

ASCII Table



charExample1.cpp// example using type char


int main(){ char c1, c2, c3;

cout << "Enter first initial: "; cin >> c1; cout << "Enter second initial: "; cin >> c2;

c3 = 'X';

cout << "Created by: "; cout << c1 << c2 << c3 << endl;



> charExample1.exeEnter first initial: REnter second initial: W{What is the output?}

… char c1, c2, c3;

cout << "Enter first initial: "; cin >> c1; cout << "Enter second initial: "; cin >> c2;

c3 = 'X';

cout << "Created by: "; cout << c1 << c2 << c3 << endl;…


Characters Strings

• A character string is an array of characters.• Examples:

– "Hello"– "Hello World!" (Note: Blank space is part of the

string.) – "He who hesitates is lost.\nHaste makes waste.\n"– "" (The empty string.)


Character Strings

• NOTE: 'A' and "A" are different.– 'A' is the symbolic representation of the

character, ‘A’;– "A" is a string containing a single character.

• NOTE: '8' and "8" and 8 are different.– '8' is the symbolic representation of the

character, ‘8’;– "8" is a string containing a single character;– 8 is the integer 8.


Data Type: Boolean

• The Boolean data type is used for just two values: true and false

• Like characters, they only consume 1 byte of storage.

• It is worth noting that when dealing with Boolean values in C++, any number other than 0 is always interpreted as true.


Arithmetic Operations (1)

• The basic operations of +, - , *, and / are available, as is % which is used for modulus division (returns the remainder).

• A simple arithmetic expression is of the form:

operand operator operand

5 % 3


Arithmetic Operations (2)// Examples of arithmetic operations


int main(){ cout << "5 % 3 = " << (5 % 3) << endl; cout << "4 - 3 = " << (4 - 3) << endl; cout << "5.0 / 2.0 = " << (5.0 / 2.0) << endl; cout << "5 / 2 = " << (5 / 2) << endl;

// Is there any real difference in the last two statements?

return 0;}


>arithmetic.exe5 % 3 = 24 - 3 = 15.0 / 2.0 = 2.55 / 2 = 2

>

… cout << "5 % 3 = " << (5 % 3) << endl; cout << "4 - 3 = " << (4 - 3) << endl; cout << "5.0 / 2.0 = " << (5.0 / 2.0) << endl; cout << "5 / 2 = " << (5 / 2) << endl;

…

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Variables and C++ Data Types