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Notes on C language
From: Prof Saroj KaushikCSE dept, IIT DelhiStructure of C program
#include <stdio.h>
/* Include files for input/output functions*/
#define const_name value
/* constant declaration if required */
main() /* Main function */
{ /* each declarations and statements are
separated by semi colon */
declarations
/* variables; arrays; records;
function declarations etc */
statements
}
function definitionsCompiler Directives
#include statements
– used to include the header file for
input/output stdio.h, the standard
mathematics library math.h etc.
– These files are enclosed within < >
#define
– helps in defining constant symbol. Example
#include <stdio.h>
#define i 6
main()
{ /* integer declaration */
int x, y;
/* Assignment statements */
x=7;
y= i + x;
/* output statement */
printf("%d\n", y);}Data Types
• Standard:
– int, float, char, double
• User defined datatypes:
– arrays,
– structures, pointers,
– enumerated datatype etc.Declaration
Form of Declaration:
type list of variables;
/* each separated by , and finally terminated by ; */
Examples:
• int x, y, z;
• float p, q[3][4];
array
• char name[20];
• char ch = ‘A’; /* character is enclosed within ‘ ’*/Arithmetic Expression
• An expression is a combination of variables,
constants and operators written according to the
syntax of C language.
• Every expression evaluates to a value of a
certain type that can be assigned to a variable.
Precedence in Arithmetic Operators
• An arithmetic expression without parenthesis will
be evaluated from left to right using the rules of
precedence of operators.
• There are two distinct priority levels of arithmetic
operators in C.
High priority * / %
Low priority + -Rules for evaluation of an expression
• When Parenthesis are used, the expressions
within parenthesis assume highest priority.
• Parenthesized sub expression left to right are
evaluated.
• If parenthesis are nested, the evaluation begins
with the innermost sub expression.
• The precedence rule is applied in determining
the order of application of operators in evaluating
sub expressions.
• The associability rule is applied when two or
more operators of the same precedence level
appear in the sub expression. Operator precedence and associativity
• Each operator in C has a precedence associated
with it.
• The precedence is used to determine how an
expression involving more than one operator is
evaluated.
• There are distinct levels of precedence and an
operator may belong to one of these levels.
• The operators of higher precedence are evaluated
first.
• The operators of same precedence are evaluated
from right to left or from left to right depending on
the level.
• This is known as associativity property of an
operator. Examples
x + y * z / 2 + p
x + (y * z) / 2 + p
x + ((y * z) / 2) + p
(x + ((y * z) / 2)) + p
((x + ((y * z) / 2)) + p)
x + y - z / 2 * p
(x + y) - z / 2 * p
(x + y) – (z / 2)* p
(x + y) – ((z / 2) * p)
((x + y) – ((z / 2) * p))Type conversions in expressions
Implicit type conversion
• C permits mixing of constants and variables of different
types in an expression.
• C automatically converts any intermediate values to the
proper type so that the expression can be evaluated
without loosing any significance.
• This automatic type conversion is know as implicit type
conversion
• During evaluation it adheres to very strict rules and type
conversion.
• If the operands are of different types the lower type is
automatically converted to the higher type before the
operation proceeds. The result is of higher type. Conversion rules
1. If one operand is long double, the other will be
converted to long double and result will be long
double.
2. If one operand is double, the other will be convertedto double and result will be double.
3. If one operand is float, the other will be converted to
float and result will be float.
4. If one of the operand is unsigned long int, the other
will be converted into unsigned long int and result
will be unsigned long int.
5. If one of the operand is long int, the other will be
converted to long int and the result will be long int.
6. If one operand is unsigned int the other will be
converted to unsigned int and the result will be
unsigned int. Explicit Conversion
• Many times there may arise a situation where
we want to force a type conversion in a way that
is different from automatic conversion.
• Consider for example the calculation of number
of female and male students in a class
female_students
Ratio = -------------------
male_students
• Since if female_students and male_students are
declared as integers, the decimal part will be
rounded off and its ratio will represent a wrong
figure. • This problem can be solved by converting locally
one of the variables to the floating point as
shown below.
Ratio = (float) female_students /
male_students
• The operator float converts the female_students
to floating point for the purpose of evaluation of
the expression.
• Then using the rule of automatic conversion, the
division is performed by floating point mode,
thus retaining the fractional part of the result.
• The process of such a local conversion is known
as explicit conversion or casting a value.
• The general form is (type_name) expression Arithmetic Expression
• x = x + 2 x += 2
• i = i +1 i++ or ++i
// the value of x is added with the value of i
after incrementing it by 1 then i is incremented by 1
x +(++i); x + (i++);
after decreasing it by 1 then i is decreased by 1.
x +(--i ); x = x +(i-- );Conditional Expression
exp ? exp1 : exp2
• An expression exp is evaluated and
– if the value is nonzero (or true -
represented by 1) then expression exp1
is the final value
– otherwise exp2 is the final value of
entire expression.Logical Operators
&& → AND
|| → OR
! → NOT
Relational Operators
== → equality
!= → Not equal to
< → less than
<= → less than equal to
> → greater than
>= → greater than equal to Bitwise operations
& → bitwise AND
| → bitwise inclusive OR
^ → bitwise exclusive OR
<< → left shift
>> → right shift
~ → One's complementBasic Statements
• Assignment statement
x = expression;
• Compound statement
{s1; s2;…. };
– Collection of statements, each separated
by semi colon and enclosed in brackets
• Multiple lines comments are enclosed within
/* comments */
• Single line comment can be preceded by //Conditional statements
• if (cond) statement;
• if (cond) s1 else s2;
– Here cond is a boolean condition
which can have non zero value
representing true and 0 representing
false.
– Statement may be simple or compund.For statement
for (i = m1; i <= m2; i+=m3)
{ body };
– Here m1 : initial value;
m2 : maximum value of I
m3 : increment (positive or negative)
• body Æ sequence of statements.Loop statements
• While statement
while (cond)
{ body };
• Do-while statement
do
{body }
while cond;Switch statement
switch (exp)
{ case v1 : s1 ; break;
case v2 : s2 ; break;
case vn : sn ; break;
default : s optional
}
– If the value of exp is vj then sj is executed and
switch statement is exited using break
statement.
– Execution always starts from 1 to last.Input/Output statement
/* reads single character and stores in
character variable x */
x = getchar();
/* prints single character stored in x*/
putchar(x);
/* the following functions are in standard file
named stdio.h */
scanf(control, v1, v2, ..);
printf(control, e1,e2,...);
• Control in input/output
control = "seq of format descriptor“Format descriptor
Description Meaning
%d a decimal integer
%o a octal integer
%x a hexadecimal integer
%c a single character
%s a character string
%f a decimal number (float
or double)
\n skip to new lineExamples:
• printf("%4d%7.2f\n%c\n", x, y, z)
• printf(“%c %d %f”, ch, i, x);
• scanf("%4d%8.2f \n", &x, &y)
• scanf(“%c %d %f”, &ch, &i, &x);
– Here & represents memory addressesArrays
• Single dimensional Array
– Arrays in C are defined as:
int numbers[50];
– In C Array subscripts start at 0 and end
one less than the array size whereas in
other languages like fortran, pascal it
starts from 1.
– For example, in the above case valid
subscripts range from 0 to 49.
– Elements can be accessed in the
following ways:-
numbers[2] = 100; x = numbers[2];• Multi-dimensional arrays can be
defined as follows:
int x[50][50]; // for two dimensions
• X is an array with 50 rows and 50
columns
• Elements can be accessed in the
following ways:
y=x[2][3];
• For further dimensions simply add
more [ ]:
int x[50][50][40][30]......[50];Strings
• In C, Strings are defined as arrays of
characters.
– For example, the following defines a
string of 50 characters: char name[50];
• C has no string handling facilities built
in and so the following assignments
are illegal:
char fn[10],ln[10],fulln[20];
fn= "Arnold";
ln= "Schwarznegger";
fulln= "Mr"+fn +ln;• However, there is a special library of
string handling routines <string.h>
which may be included in header file
and then various string operations can
be used.
• String is enclosed in “ “.
– Printf(“Well done”);’
• To print a string we use printf with a
special %s control character:
printf(``%s'',name);
– NOTE: We just need to give the name of the
string. • In order to allow variable length
strings the 0 character is used to
indicate the end of a string.
• So if we have a following declaration
char name[50];
• Initialization can be done at the
declaration time as follows:
char name[50] = “DAVE”;
• The contents will look like: String Handling Functions
• Include <string.h> as a header file. The
following functions are available for use.
• Concatenate two strings: strcat(s1, s2)
• Compare two strings : strcmp(s1, s2)
• Length of string : strlen(s)
• Copy one string over other: strcpy(s1, s2)
– Here contents of s2 are copied to s1
• Locating substring: strstr(s1,s2)
– Gives the position of s1 in s2Structure in C
• A structure in C is a collection of items
of different types.
• The main use of structures is to
conveniently treat such collection as a
unit.
• For example:
struct employee
{ char name[50];
char sex;
float salary;
};• The following declaration defines a
variable xyz of struct type.
struct empolyee xyz;
• Variables can also be declared
between } and ; of a struct
declaration, i.e.:
struct employee
{ char name[50];
char sex;
float salary;
} xyz;• struct variable can be pre-initialized at
declaration:
struct employee
{ char name[50];
char sex;
float salary;
} xyz = {"john", ’m’, 20000.50};
• To access a member (or field) of a
struct, C provides dot (.) operator.
• For example,
– xyz . sex ; xyz . salary; xyz . name User Defined Data Types
• Enumerated Types
– It contains a list of constants that can be
addressed in integer values.
• We can declare types as follows.
enum days {MONDAY, TUESDAY, ...,
SUNDAY};
• Variables of enumerated type are defined
as follows:
enum days week1, week2;
where week1 and week2 are variablesPossible uses of enumerated constants
• Enumerated constants can be assigned
to variable of that type
week1 = MONDAY;
• Conditional expression can be formed
If (week1 == week2) ….
if (week1 != TUESDAY) …
• Can be used in switch or for statement.• Similar to arrays, first enumerated name
has index value 0.
– So MONDAY has value 0,
– TUESDAY value1, and so on.
• We can also override the 0 start value as
follows:
enum months {JAN = 1, FEB, MAR, ..., DEC};
– Here it is implied that FEB = 2 and so on
enum colors {RED, BLUE, GREEN=5, WHITE,
PINK=9};
– Here RED=1, BLUE=2, GREEN=5, WHITE=6,
PINK=9#include <stdio.h>main()
{
enum Color {RED=5, YELLOW, GREEN=4,
BLUE};
printf("RED = %d\n", RED);
printf("YELLOW = %d\n", YELLOW);
printf("GREEN = %d\n", GREEN);
printf("BLUE = %d\n", BLUE);
}
Output:
RED = 5
YELLOW = 6
GREEN = 4
BLUE = 5 Type Definitions
• We can give a name to enum colors as COLOR
by using typedef as follows:
typedef enum colors COLOR;
COLOR x, y, z;
x = RED;
y = BLUE;
• Now, every time the compiler sees COLOR, it'll
know that you mean enum colors.
• We can also define user named data type for
even existing primitive types:
typedef int integer;
typedef bool boolean; • typedef can also be used with structures to
creates a new type.
• Example:
typedef struct employee
{ char name[50];
char sex;
float salary;
} emp_type xyz ={"john", ’m’, 2000.50};
• emp_type is new data type of struct employee
type and can be initialized as usual:
• It can be now used for declaring variables similar
to primitive data types are used.• Examples:
emp_type x, y, z
– Here x, y and z are variables of type
emp_type which are structures themselves.
emp_type emp[100];
– Here emp is an array of 100 elements with
each element of type emp_type.
• Both declarations given below are same.
struct employee x, y, z;
emp_type x, y, z;Unions
• A union is an object similar to a structure except
that all of its members start at the same location
in memory.
• A union variable can represent the value of only
one of its members at a time.
• So an union is a variable which may hold (at
different times) objects of different sizes and
types.
• Example:
union number
{ short shortnumber;
long longnumber;
double floatnumber;
} anumber• It defines a union called number and an
instance of it called anumber.
• Members can be accessed in the following
way:
printf("%d\n",anumber.longnumber);
• This clearly displays the value of
longnumber.
• When C compiler is allocating memory for
unions, it will always reserve enough room
for the largest member
– (in the above example this is 8 bytes for the
double). • Example:
union u_t
{ char a;
short b;
int c;
};
union u_t x;
x.a = ‘B’;
printf("%c\n", x.a);
Output is: B • In order that the program can keep track of
the type of union variable being used, it is
embedded in a structure and a variable
which flags the union type.
• For example:
typedef struct { int maxpassengers; } jet;
typedef struct { int liftcapacity;} helicopter;
typedef struct { int maxpayload; } cargoplane;
typedef union
{ jet j; helicopter h; cargoplane c; } aircraft;
typedef struct
{ aircrafttype kind; int speed;
aircraft description; } an_aircraft;Function
• C provides functions which are again
similar in most languages.
• One difference is that C regards main()
as a function.
• The form of a C function is as follows:
type fun_name(parameter along with type)
{ local declarations;
body;
}
• type : is the type of value returned by the
function and can be basic type or user
defined. • return statement is used in the body of a
function to pass the result back to the
calling program.
• Example: Write function to find the
average of two integers:
float findaverage(float a, float b)
{ float average;
average=(a+b)/2;
return(average);
}
• We would call the function as follows:
result=findaverage(6,23);#include <stdio.h>
main()
{ int i, x;
int power (x, n); function declaration
for (i =0; i < 10; ++i)
{ x = power(2, i);
printf("%d%d\n", i, x); }
}
int power(int x, n) function definition
{ int i, p;
p = 1;
for (i =1; i <=n; i++) p = p*x;
return (p); }void functions
• The void function provides a way of not
returning any value through function name
• Here return statement is not used:
void squares()
{ int i;
for (i=1;i<10;i++);
printf("%d\n",i*i);
}
• In the main function we call it as follows:
main( )
{ squares( ); }Parameter Passing
• Default parameter passing is by value.
– The values of actual parameters are copied in
formal parameters.
– The change is not visible in the calling program.
main()
{ int i, x, y,s;
int sqsum (a,b);
x = 5; y = 7;
s = sqsum(x,y);
printf("%d%d%d\n“
, x,y,s); }
int sqsum(int a, b)
{ int sum;
a=a*a; b= b*b;
sum = a + b;
return(sum);
}• Another mechanism is to call by reference.
• It can be achieved by passing addresses of
actual parameters to formal parameters.
• For such case variables in formal parameter list
are represented as pointers.
• For writing functions where call by reference is
to be achieved then Void type is used.
• In this case the function will not returning any
value.
• Note that return statement is not.
• Changes to formal parameters will be visible in
actual parameters of calling program.Example:
void swap (int *p,*q) call by reference
{ int t;
t = *p;
*p = *q;
*q = t;
}
• Corresponding call statement
x = 4;
y = 5;
swap(&x, &y); addresses are passedFunctions and Arrays
• Single dimensional arrays can be passed to
functions as follows:
float findaverage(int size,float list[])
{ int i;
float sum=0.0;
for (i=0; i<size; i++) sum+=list[i];
return(sum/size);
}
• Here the declaration float list[] tells C compiler
that list is an array of float type.
• It should be noted that dimension of array is not
specified when it is a parameter of a function. • Multi-dimensional arrays can be passed to
functions as follows:
void printtable(int xsize,int ysize, float table[][5])
{ int x,y;
for (x=0; x<xsize; x++)
{ for (y=0; y<ysize;y++)
printf(“\t%f”,table[x][y]);
printf(“\n”);
}
}
• Here float table[][5] tells C compiler that table is
an array of dimension N X 5 of float.
• Note we must specify the second (and
subsequent) dimension of the array BUT not the
first dimension.
