CS 120 Variable Declaration

7/31/2019 CS 120 Variable Declaration

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Variable Declaration in C

Variables are simply names used to refer to some location inmemory-allocation that holds a value with which we areworking.

We must declare every variable before we use. Every variable has a type and a name. Variables should be declared in three basic places:

Inside functions local variables In the definition of function parameters formal parameters

Outside of all functions global variables

Declarations of types should always be together at the top ofmain program or a function.


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Variable Declaration in C (type)

l In standard C, there are four basic data types:

1. int : uses integer numbers (e.g. int a=3, b=-2;)

1. float : uses floating point numbers (e.g. float d=3.141159, e=-5.01;)

1. double : uses big floating numbers (e.g. double g=6.002e23;)

4. char: uses single character (e.g. char x=a; )

Other types are signed, unsigned, long, short and const.


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Variable Declaration in C (name)

l Variables in C can be given any name made from numbers, letters andunderscores which is not a keyword and does not begin with a number.

l int 2mark; char for; illegal

l A good name for your variables is important

start_time foo_bar

student_no _foo42

course_mark BAZ

Very descriptive valid but not descriptive

(fewer comments are needed) (lots of comments are needed)


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Variable Declaration in C

l Multiple variables can be declared with one statement.

int a, b, c, d;

l Initializing: declare and assign some content to a variable at the same time.int a=3;

l Assigning: after declaring variables, you can assign a value to a variable later on.

int a;

a=6;

You can also assign a variable the value of another variable.a=b;

You can assign multiple variables the same value with one statement.

a=b=c=d=7;


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Input / Output statements

#include

/* addition program */

int main ()

{

int integer1, integer2, sum; /* Variable Declaration */

printf( Enter first integer \n); /* prompt : tells the user to take a specific action */

scanf( %d, &integer1); /* read an integer : to obtain a value from the user*/

printf( Enter second integer \n);

scanf( %d, &integer2);

sum=integer1 + integer2;

printf( sum is %d \n, sum);

return 0;

}

Enter first integer

45

Enter second integer

72

Sum is 117


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Input / Output statements

scanf( %d, &integer1); /* scanf has 2 arquments */

%d : format control string which indicates the type of data that

should be input by the user. d stands for decimal integer %d : int

%f : float

%c : char

& integer1 : consists of :-

& : address operator integer1 : variable name

The combination tells scanf the location in memory in which thevariable integer1 is stored


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Note

The following two statements can be written in one statement:

sum=integer1 + integer2;

printf( sum is %d \n, sum);

as

printf( sum is %d \n,integer1 + integer2);

l Common programming errorinteger1 + integer2 = sum /* calculation on the left side */


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Examples : printf

#include

int main ()

{

int x = 6;float y = 5.127, b=5;

char z = 'A';

printf(" Welcome to C course ");

printf(" \n %d \n %3.2f \n %2.1f \n %c", x, y, b, z);

/* % 3.2f means floating point number with a minimum of 3 characters wide and 2 decimal places*/

return 0;

}

Welcome to C course65.135.0A


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Arithmetic in C

remainder division multiplication subtraction addition

% / * - + operator

X mod Y X

Y

5X X-5 X+5 Algebraic

expression

X % Y X/Y 5*X X-5 X+5 C expression


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Arithmetic in C

l () is the 1st precedence, if there are several pairs ofparentheses on same level and not nested, they are

evaluated left to right.(5+3) * (6/2)

(5+3*(6/2))

*, /, % are the 2nd precedence, if they are several,

they are evaluated left to right. +, - are the 3rd precedence, if they are several, they

are evaluated left to right.


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Arithmetic in C (examples)

Algebra : y= mx+b

C : y=m*x+b;

Algebra : z=pr mod q+w/x-y

C : z=p*r%q+w/x-y

int X, Y;

X=5;

Y=2;printf(%d, X/Y); /* will display 2 */

printf(%d, X%Y); /* will display 1 */


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(1) Arithmetic operators

l Increment operator (++)

l Decrement operator (--)

B++ or++B is equivalent to B=B+1;

B-- or--B is equivalent to B=B-1;

Example:

int x=8, y;

y=x--; /* x=7 & y=8*/

y=--x; /* x=7 & y=7*/

Arithmetic assignment statement

i+=10; is equivalent to i=i+10;

i-=10; is equivalent to i=i-10;

i*=10; is equivalent to i=i*10;


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Arithmetic operators (2)

#include

int main()

{int i = 7 , j = 5 ;

float x , y = 1.3 ;

x = --j + i / 2.0 ;

printf("j=%d x=%.2f \n", j, x) ;

y += 7 % j * 1.1 ;printf("y=%.2f \n", y) ;

return 0 ;

}


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#include

int main()

{int i = 3 , j = 5 ;

float x , y = 1.5 ;

x = --j + i / 2.0 ;

printf("j=%d x=%.2f \n", j, x) ;

y += i++ - i % 4 * 2 ;printf("y=%.2f \n", y) ;

return 0 ;

}


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#include

intmain() {

int i = 7 , j = 5;float x , y = 1.5;

x = --j/3.0 + i++ ;

printf("j=%d x=%.2f \n", j, x);

y += i-- % 4 + ++i * 2;printf("i=%d y=%.2f \n", i, y);

return 0;

}


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Relational operators(1)

less orequal

greater orequal

less greater not equal equal

XY XY XY XY X=Y Algebraexpression

X=Y XY X!=Y X==YC expression


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> greater than 5 > 4 is TRUE

< less than 4 < 5 is TRUE>= greater than or equal 4 >= 4 is TRUE


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l Relational operators are lower in precedence than the arithmetic operators.

example:

10> 1+12 evaluated as 10>(1+12)

Common programming errors:1- if the two symbols in any of the operators (==, !=, >=,, =

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CS 120 Variable Declaration