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Variables, Data Types, I/O and Operators


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Variables, Data Types, I/O and Operators

In order to be useful a program must be able to represent real life quantities or data e.g. a person's name, age, height, bank balance, etc. This data will be stored in memory locations called variables that we will name ourselves. However so that the data may be represented as aptly as possible the variables will have to be of different types to suit their data. For example while an integer can represent the age of a person reasonably well it won't be able to represent the pounds and pence in a bank balance or the name of an individual quite so well.

1 Basic Data Types

There are five basic data types char, int, float, double, and void. All other data types in C are based on these. Note that the size of an int depends on the standard size of an integer on a particular operating system.


1 byte ( 8 bits ) with range -128 to 127


16-bit OS : 2 bytes with range -32768 to 32767

32-bit OS : 4 bytes with range -2,147,483,648 to 2,147,483,647


4 bytes with range 10-38 to 1038 with 7 digits of precision


8 bytes with range 10-308 to 10308 with 15 digits of precision


generic pointer, used to indicate no function parameters etc.

Modifying Basic Types

Except for type void the meaning of the above basic types may be altered when combined with the following keywords.





The signed and unsigned modifiers may be applied to types char and int and will simply change the range of possible values. For example an unsigned char has a range of 0 to 255, all positive, as opposed to a signed char which has a range of -128 to 127. An unsigned integer on a 16-bit system has a range of 0 to 65535 as opposed to a signed int which has a range of -32768 to 32767.

Note however that the default for type int or char is signed so that the type signed char is always equivalent to type char and the type signed int is always equivalent to int.

The long modifier may be applied to type int and double only. A long int will require 4 bytes of storage no matter what operating system is in use and has a range of -2,147,483,648 to 2,147,483,647. A long double will require 10 bytes of storage and will be able to maintain up to 19 digits of precision.

The short modifier may be applied only to type int and will give a 2 byte integer independent of the operating system in use.

NB : Note that the keyword int may be omitted without error so that the type unsigned is the same as type unsigned int, the type long is equivalent to the type long int and the type short is equivalent to the type short int

2 Variables

A variable is a named piece of memory which is used to hold a value which may be modified by the program. A variable thus has three attributes that are of interest to us : its type, its value and its address.

The variable's type informs us what type and range of values it can represent and how much memory is used to store that value. The variable's address informs us where in memory the variable is located (which will become increasingly important when we discuss pointers later on).

All C variables must be declared as follows :-

type variable-list ;

For Example :-

int i ;

char a, b, ch ;

Variables are declared in three general areas in a C program.

When declared inside functions as follows they are termed local variables and are visible (or accessible) within the function ( or code block ) only.

void main()

A local variable is created i.e. allocated memory for storage upon entry into the code block in which it is declared and is destroyed i.e. its memory is released on exit. This means that values cannot be stored in these variables for use in any subsequent calls to the function .

When declared outside functions they are termed global variables and are visible throughout the file or have file scope. These variables are created at program start-up and can be used for the lifetime of the program.

int i ;

void main()

When declared within the braces of a function they are termed the formal parameters of the function as we will see later on.

int func1( int a, char b ) ;

Variable Names

Names of variables and functions in C are called identifiers and are case sensitive. The first character of an identifier must be either a letter or an underscore while the remaining characters may be letters, numbers, or underscores. Identifiers in C can be up to 31 characters in length.

Initialising Variables

When variables are declared in a program it just means that an appropriate amount of memory is allocated to them for their exclusive use. This memory however is not initialised to zero or to any other value automatically and so will contain random values unless specifically initialised before use.

Syntax :- type var-name = constant ;

For Example :-

char ch = 'a' ;

double d = 12323 ;

int i, j = 20 ; /* note in this case i is not initialised */

Storage Classes

There are four storage class modifiers used in C which determine an identifier's storage duration and scope.





An identifier's storage duration is the period during which that identifier exists in memory. Some identifiers exist for a short time only, some are repeatedly created and destroyed and some exist for the entire duration of the program. An identifier's scope specifies what sections of code it is accessible from.

The auto storage class is implicitly the default storage class used and simply specifies a normal local variable which is visible within its own code block only and which is created and destroyed automatically upon entry and exit respectively from the code block.

The register storage class also specifies a normal local variable but it also requests that the compiler store a variable so that it may be accessed as quickly as possible, possibly from a CPU register.

The static storage class causes a local variable to become permanent within its own code block i.e. it retains its memory space and hence its value between function calls.

When applied to global variables the static modifier causes them to be visible only within the physical source file that contains them i.e. to have file scope. Whereas the extern modifier which is the implicit default for global variables enables them to be accessed in more than one source file.

For example in the case where there are two C source code files to be compiled together to give one executable and where one specific global variable needs to be used by both the extern class allows the programmer to inform the compiler of the existence of this global variable in both files.


Constants are fixed values that cannot be altered by the program and can be numbers, characters or strings.

Some Examples :-

char : 'a', '$', '7'

int : 10, 100, -100

unsigned : 0, 255

float : 123456, -1.573765e10, 1.347654E-13

double : 1433.34534545454, 1.35456456456456E-200

long : 65536, 2222222

string : "Hello Worldn"

NB : Floating point constants default to type double. For example the following code segment will cause the compiler to issue a warning pertaining to floating point conversion in the case of f_val but not in the case of d_val

float f_val ;

double d_val ;

f_val = 123.345 ;

d_val = 123.345 ;

However the value may be coerced to type float by the use of a modifier as follows :-

f = 123.345F ;

Integer constants may also be forced to be a certain type as follows :-

100U --- unsigned

100L --- long

Integer constants may be represented as either decimal which is the default, as hexadecimal when preceded by '0x', e.g. 0x2A, or as octal when preceded by 'O', e.g. O27.

Character constants are normally represented between single quotes, e.g. 'a', 'b', etc. However they may also be represented using their ASCII (or decimal) values e.g. 97 is the ASCII value for the letter 'a', and so the following two statements are equivalent. (See Appendix A for a listing of the first 128 ASCII codes.)

char ch = 97 ;

char ch = 'a' ;

There are also a number of special character constants sometimes called Escape Sequences, which are preceded by the backslash character '', and have special meanings in C.







single quote

double quote

null character


represent as hexadecimal constant

3 Console Input / Output

This section introduces some of the more common input and output functions provided in the C standard library.


The printf() function is used for formatted output and uses a control string which is made up of a series of format specifiers to govern how it prints out the values of the variables or constants required. The more common format specifiers are given below

%c character

%f floating point

%d signed integer

%lf double floating point

%i  signed integer

%e exponential notation

%u unsigned integer

%s string

%ld signed long

%x unsigned hexadecimal

%lu unsigned long

%o unsigned octal

%% prints a % sign

For Example :-

int i ;

printf( '%d', i ) ;

The printf() function takes a variable number of arguments. In the above example two arguments are required, the format string and the variable i. The value of i is substituted for the format specifier %d which simply specifies how the value is to be displayed, in this case as a signed integer.

Some further examples :-

int i = 10, j = 20 ;

char ch = 'a' ;

double f = 23421.2345 ;

printf( '%d + %d', i, j ) ; /* values are substituted from

the variable list in order as required */

printf( '%c', ch ) ;

printf( '%s', 'Hello Worldn' ) ;

printf( 'The value of f is : %lf', f ) ;/*Output as : 23421.2345 */

printf( 'f in exponential form : %e', f ) ; /* Output as : 34212345e+4

Field Width Specifiers

Field width specifiers are used in the control string to format the numbers or characters output appropriately .

Syntax :- %[total width printed][.decimal places printed]format specifier

where square braces indicate optional arguments.

For Example :-

int i = 15 ;

float f = 13.3576 ;

printf( '%3d', i ) ; /* prints '_15 ' where _ indicates a space

character */

printf( '%6.2f', f ) ; /* prints '_13.36' which has a total width

of 6 and displays 2 decimal places */

printf( "%*.*f", 6,2,f ) ; /* prints '_13.36' as above. Here * is used as replacement character for field widths */

There are also a number of flags that can be used in conjunction with field width specifiers to modify the output format. These are placed directly after the % sign. A - (minus sign) causes the output to be left-justified within the specified field, a + (plus sign) displays a plus sign preceding positive values and a minus preceding negative values, and a 0 (zero) causes a field to be padded using zeros rather than space characters.


This function is similar to the printf function except that it is used for formatted input. The format specifiers have the same meaning as for printf() and the space character or the newline character are normally used as delimiters between different inputs.

For Example :-

int i, d ;

char c ;

float f ;

scanf( '%d', &i ) ;

scanf( '%d %c %f', &d, &c, &f ) ; /* e.g. type '10_x_1.234ret' */

scanf( '%d:%c', &i, &c ) ; /* e.g. type '10:xret' */

The & character is the address of operator in C, it returns the address in memory of the variable it acts on. (Aside : This is because C functions are nominally call--by--value. Thus in order to change the value of a calling parameter we must tell the function exactly where the variable resides in memory and so allow the function to alter it directly rather than to uselessly alter a copy of it. )

Note that while the space and newline characters are normally used as delimiters between input fields the actual delimiters specified in the format string of the scanf statement must be reproduced at the keyboard faithfully as in the case of the last sample call. If this is not done the program can produce somewhat erratic results!

The scanf function has a return value which represents the number of fields it was able to convert successfully.

For Example :-

num = scanf( "%c %d", &ch, &i );

This scanf call requires two fields, a character and an integer, to be read in so the value placed in num after the call should be 2 if this was successful. However if the input was "a bc" then the first character field will be read correctly as 'a' but the integer field will not be converted correctly as the function cannot reconcile "bc" as an integer. Thus the function will return 1 indicating that one field was successfully converted. Thus to be safe the return value of the scanf function should be checked always and some appropriate action taken if the value is incorrect.

getchar() and putchar()

These functions are used to input and output single characters. The getchar() function reads the ASCII value of a character input at the keyboard and displays the character while putchar() displays a character on the standard output device i.e. the screen.

For Example :-

char ch1, ch2 ;

ch1 = getchar() ;

ch2 = 'a' ;

putchar( ch2 ) ;

NB : The input functions described above, scanf() and getchar() are termed buffered input functions. This means that whatever the user types at the keyboard is first stored in a data buffer and is not actually read into the program until either the buffer fills up and has to be flushed or until the user flushes the buffer by hitting ret whereupon the required data is read into the program. The important thing to remember with buffered input is that no matter how much data is taken into the buffer when it is flushed the program just reads as much data as it needs from the start of the buffer allowing whatever else that may be in the buffer to be discarded.

For Example :-

char ch1, ch2;

printf( 'Enter two characters : ' ) ;

ch1 = getchar() ;

ch2 = getchar() ;

printf( 'n The characters are %c and %cn', ch1, ch2 ) ;

In the above code segment if the input is 'abcdefret' the first two characters are read into the variables all the others being discarded, but control does not return to the program until the ret is hit and the buffer flushed. If the input was 'aret' then a would be placed in ch1 and ret in ch


The _flushall function writes the contents of all output buffers to the screen and clears the contents of all input buffers. The next input operation (if there is one) then reads new data from the input device into the buffers.

This function should be used always in conjunction with the buffered input functions to clear out unwanted characters from the buffer after each input call.

getch() and getche()

These functions perform the same operation as getchar() except that they are unbuffered input functions i.e. it is not necessary to type ret to cause the values to be read into the program they are read in immediately the key is pressed. getche() echoes the character hit to the screen while getch() does not.

For example :-

char ch ;

ch = getch() ;

4 Operators

One of the most important features of C is that it has a very rich set of built in operators including arithmetic, relational, logical, and bitwise operators.

Assignment Operator

int x ;

x = 20 ;

Some common notation :-   lvalue -- left hand side of an assignment operation

rvalue -- right hand side of an assignment operation

Type Conversions :- the value of the right hand side of an assignment is converted to the type of the lvalue. This may sometimes yield compiler warnings if information is lost in the conversion.

For Example :-

int x ;

char ch ;

float f ;

ch = x ;  /* ch is assigned lower 8 bits of x, the remaining bits are discarded so we have a possible information loss */

x = f ;  /* x is assigned non fractional part of f only within int range, information loss possible */

f = x ;   /* value of x is converted to floating point */

Multiple assignments are possible to any degree in C, the assignment operator has right to left associativity which means that the rightmost expression is evaluated first.

For Example :-

x = y = z = 100 ;

In this case the expression z = 100 is carried out first. This causes the value 100 to be placed in z with the value of the whole expression being 100 also. This expression value is then taken and assigned by the next assignment operator on the left i.e. x = y = ( z = 100 )

Arithmetic Operators

+ - * / --- same rules as mathematics with * and / being evaluated before + and -.

-- modulus / remainder operator

For Example :-

int a = 5, b = 2, x ;

float c = 5.0, d = 0, f ;

x = a / b ; // integer division, x =

f = c / d ; // floating point division, f = 5.

x = 5 % 2 ; // remainder operator, x = 1.

x = 7 + 3 * 6 / 2 - 1 ;// x=15,* and / evaluated ahead of + and -

Note that parentheses may be used to clarify or modify the evaluation of expressions of any type in C in the same way as in normal arithmetic.

x = 7 + ( 3 * 6 / 2 ) - 1 ; // clarifies order of evaluation without penalty

x = ( 7 + 3 ) * 6 / ( 2 - 1 ) ; // changes order of evaluation, x = 60 now.

Increment and Decrement Operators

There are two special unary operators in C, Increment ++, and Decrement -- , which cause the variable they act on to be incremented or decremented by 1 respectively.

For Example :-

x++ ;  /* equivalent to x = x + 1 ; */

++ and -- can be used in prefix or postfix notation. In prefix notation the value of the variable is either incremented or decremented and is then read while in postfix notation the value of the variable is read first and is then incremented or decremented.

For Example :-

int i, j = 2 ;

i = ++ j ; /* prefix :- i has value 3, j has value 3 */

i = j++ ; /* postfix :- i has value 3, j has value 4 */

Special Assignment Operators

Many C operators can be combined with the assignment operator as shorthand notation

For Example :-

x = x + 10 ;

can be replaced by

x += 10 ;

Similarly for -=, *=, /=, %=, etc.

These shorthand operators improve the speed of execution as they require the expression, the variable x in the above example, to be evaluated once rather than twice.

Relational Operators

The full set of relational operators are provided in shorthand notation

> >= < <= == !=

For Example :-

if ( x == 2 )

printf( "x is equal to 2n" ) ;

Logical Operators

&& -- Logical AND

| | -- Logical OR

! -- Logical NOT

For Example :-

if ( x >= 0 && x < 10 )

printf( " x is greater than or equal to zero and less than ten.n" ) ;

NB : There is no Boolean type in C so TRUE and FALSE are deemed to have the following meanings.

FALSE -- value zero

TRUE -- any non-zero value but 1 in the case of in-built relational operations

For Example :-

2 > 1 -- TRUE so expression has value 1

2 > 3 -- FALSE so expression has value 0

i = 2 > 1 ; -- relation is TRUE -- has value 1, i is assigned value 1

NB : Every C expression has a value. Typically we regard expressions like 2 + 3 as the only expressions with actual numeric values. However the relation 2 > 1 is an expression which evaluates to TRUE so it has a value 1 in C. Likewise if we have an expression x = 10 this has a value which in this case is 10 the value actually assigned.

NB : Beware of the following common source of error. If we want to test if a variable has a particular value we would write for example

if ( x == 10 ) .

But if this is inadvertently written as

if ( x = 10 ) .

this will give no compilation error to warn us but will compile and assign a value 10 to x when the condition is tested. As this value is non-zero the if condition is deemed true no matter what value x had originally. Obviously this is possibly a serious logical flaw in a program.

Bitwise Operators

These are special operators that act on char or int arguments only. They allow the programmer to get closer to the machine level by operating at bit-level in their arguments.

& Bitwise AND | Bitwise OR

^ Bitwise XOR ~ Ones Complement

>> Shift Right << Shift left

Recall that type char is one byte in size. This means it is made up of 8 distinct bits or binary digits normally designated as illustrated below with Bit 0 being the Least Significant Bit (LSB) and Bit 7 being the Most Significant Bit (MSB). The value represented below is 13 in decimal.

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0


An integer on a 16 bit OS is two bytes in size and so Bit 15 will be the MSB while on a 32 bit system the integer is four bytes in size with Bit 31 as the MSB.

Bitwise AND, &

RULE : If any two bits in the same bit position are set then the resultant bit in that position is set otherwise it is zero.

For Example :-

1011 0010 (178)

& 0011 1111 (63)

= 0011 0010 (50)

Bitwise OR, |

RULE : If either bit in corresponding positions are set the resultant bit in that position is set.

For Example :-

1011 0010 (178)

| 0000 1000 (63)

= 1011 1010 (186)

Bitwise XOR, ^

RULE : If the bits in corresponding positions are different then the resultant bit is set.

For Example :-

1011 0010 (178)

0011 1100 (63)

= 1000 1110 (142)

Shift Operators, << and >>

RULE : These move all bits in the operand left or right by a specified number of places.

Syntax : variable << number of places

variable >> number of places

For Example :-

2 << 2 = 8



NB : shift left by one place multiplies by 2

shift right by one place divides by 2

Ones Complement

RULE : Reverses the state of each bit.

For Example :-


NB : With all of the above bitwise operators we must work with decimal, octal, or hexadecimal values as binary is not supported directly in C.

The bitwise operators are most commonly used in system level programming where individual bits of an integer will represent certain real life entities which are either on or off, one or zero. The programmer will need to be able to manipulate individual bits directly in these situations.

A mask variable which allows us to ignore certain bit positions and concentrate the operation only on those of specific interest to us is almost always used in these situations. The value given to the mask variable depends on the operator being used and the result required.

For Example :- To clear bit 7 of a char variable.

char ch = 89 ; // any value

char mask = 127 ; // 0111 1111

ch = ch & mask ; // or ch &= mask ;

For Example :- To set bit 1 of an integer variable.

int i = 234 ; // any value

int mask = 2 ; // a 1 in bit position 2

i |= mask ;

Implicit & Explicit Type Conversions

Normally in mixed type expressions all operands are converted temporarily up to the type of the largest operand in the expression.

Normally this automatic or implicit casting of operands follows the following guidelines in ascending order.

long double



unsigned long


unsigned int

signed int

For Example :-

int i ;

float f1, f2 ;

f1 = f2 + i ;

Since f2 is a floating point variable the value contained in the integer variable is temporarily converted or cast to a floating point variable also to standardise the addition operation in this case. However it is important to realise that no permanent modification is made to the integer variable.

Explicit casting coerces the expression to be of specific type and is carried out by means of the cast operator which has the following syntax.

Syntax : ( type ) expression

For Example if we have an integer x, and we wish to use floating point division in the expression x/2 we might do the following

( float ) x / 2

which causes x to be temporarily cast to a floating point value and then implicit casting causes the whole operation to be floating point division.

The same results could be achieved by stating the operation as

x / 0

which essentially does the same thing but the former is more obvious and descriptive of what is happening.

NB : It should be noted that all of these casting operations, both implicit and explicit, require processor time. Therefore for optimum efficiency the number of conversions should be kept to a minimum.

Sizeof Operator

The sizeof operator gives the amount of storage, in bytes, associated with a variable or a type (including aggregate types as we will see later on).

The expression is either an identifier or a type-cast expression (a type specifier enclosed in parentheses).

Syntax : sizeof ( expression )

For Example :-

int x , size ;

size = sizeof ( x ) ;

printf("The integer x requires %d bytes on this machine", size);

printf( "Doubles take up %d bytes on this machine", sizeof ( double ) ) ;

Precedence of Operators

When several operations are combined into one C expression the compiler has to rely on a strict set of precedence rules to decide which operation will take preference. The precedence of C operators is given below.





( ) [ ] -> .

left to right

! ~ ++ -- +(unary) -(unary) (type) * & sizeof

right to left

* / %

left to right


left to right

<< >>

left to right

< <= > >=

left to right


left to right


left to right

left to right

left to right


left to right

left to right

right to left

+= -= *= /= %= &= ^= |= <<= >>=

right to left


left to right

Operators at the top of the table have highest precedence and when combined with other operators at the same expression level will be evaluated first.

For example take the expression

2 + 10 * 5 ;

Here * and + are being applied at the same level in the expression but which comes first ? The answer lies in the precedence table where the * is at a higher level than the + and so will be applied first.

When two operators with the same precedence level are applied at the same expression level the associativity of the operators comes into play.

For example in the expression

2 + 3 - 4 ;

the + and - operators are at the same precedence level but associate from left to right and so the addition will be performed first. However in the expression

x = y = 2 ;

as we have noted already the assignment operator associates from right to left and so the rightmost assignment is first performed.

NB : As we have seen already parentheses can be used to supersede the precedence rules and force evaluation along the lines we require. For example to force the addition in 2 + 10 * 5 ; to be carried out first we would write it as (2 + 10) * 5;

5 Type Overflow & Underflow

When the value to be stored in a variable of a particular type is larger than the range of values that type can hold we have what is termed type overflow. Likewise when the value is smaller than the range of values the type can hold we have type underflow.

Overflow and underflow are only a problem when dealing with integer arithmetic. This is because C simply ignores the situation and continues on as if nothing had happened. With signed integer arithmetic adding two large positive numbers, the result of which will be larger than the largest positive signed int, will lead to a negative value being returned as the sign bit will be overwritten with data.

The situation is not quite so bad with unsigned integer arithmetic. Here all values are forced to be within range which will of course cause problems if you don't expect overflow to occur. Adding 1 to the largest unsigned integer will give 0.

The unfortunate aspect of the matter however is that we cannot check for overflow until it has occurred. There are a number of ways to do this. For example when performing integer addition you might check the result by subtracting one of the operands from the result to see if you get the other. On the other hand you might subtract one operand from the largest integer to see if the result is greater than the second operand. If it is you know your operation will succeed. However the major flaw with these methods is that we are reducing the overall efficiency of the program with extra operations.

In general the optimum method for dealing with situations where overflow or underflow is possible is to use type long over the other integer types and inspect the results. Operations using long operands are in general slower than those using int operands but if overflow is a problem it is still a better solution than those mentioned above.

Floating point overflow is not a problem as the system itself is informed when it occurs which causes your program to terminate with a run-time error. If this happens you need to promote the variables involved in the offending operation to the largest possible and try again.

NB : The C standard library includes a number of exception handling functions to allow you to intercept these situations in your program.

6 Exercises

Write a program to check the sizes of the main C data types on your machine.

Write a program to illustrate whether the printf() standard library function truncates or rounds when printing out a floating point number.

Write a program to check what the following code segment outputs and explain the results.

char c ;

printf('sizeof( c ) = %dn', sizeof( c ) ) ;

printf('sizeof( 'a' ) = %dn', sizeof( 'a' ) ) ;

printf('sizeof( c = 'a' ) = %dn', sizeof( c='a' ) ) ;

Write a program which reads a character from the keyboard and writes out its ASCII representation.

Now write a program which reads in an integer from the keyboard and print out its character representation. Make certain you carry out appropriate bounds / error checking.

Use the getchar() function to read in a single character and output it to the screen e.g.

puts('Enter a character');

c = getchar() ;

printf('The character was %cn',c);

Add another c = getchar() statement immediately after the existing one and explain what happens.

Repeat the above using the getch() function in place of getchar()

Describe the output from each of the following statements.

i. printf( "%-10dn", 10000 ) ;

ii. printf( "%8.3fn", 23.234 ) ;

iii.printf( "%+*.*lfn", 10, 3, 1234.234 ) ;

iv. printf( "%xn", 16 ) ;

v. printf( "%10.3E", 234.65343 ) ;

Write down appropriate C statements to do the following.

i.      Print a long int, 400000L, left justified in a 10 digit field padding it out with zeros if possible.

ii.    Read a time of the form hh:mm:ss storing the parts of the time in integer variables hour, minute, and second. Skip the colons in the input field.

iii.  Print out the following sequence of characters "%, and ' require special treatment" .

iv.  Read the value 123456789.012345456789e+5 into a floating point, a double and a long double variable and print all three out again with the maximum precision possible.

What value does x contain after each of the following where x is of type float.

i. x = 7 + 3 * 6 / 2 - 1 ;

ii. x = 2 % 2 + 2 * 2 - 2 / 2 ;

iii.x = ( 3 * 9 * ( 3 + ( 4 * 5 / 3 ) ) ) ;

iv. x = 10 + 2 / 5 * 10.0 ;

v. x = 2 / 5 + 10.0 * 3 - 5 ;

vi. x = 15 > 10 && 5 < 2 ;

Write a program to read Fahrenheit temperatures and print them in Celsius. The formula is

C = (5/9)( F - 32). Use variables of type double in your program.

Write a program that reads in the radius of a circle and prints the circle's diameter, circumference and area. Use the value 3.14159 for "pi".

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