C Programming : Pointers and Strings

C Programming : Pointer and Strings
By
Mr.S.Selvaraj
Asst. Professor (SRG) / CSE
Kongu Engineering College
Perundurai, Erode, Tamilnadu, India
Thanks to and Resource from : Sumitabha Das, “Computer Fundamentals and C Programming”, 1st Edition, McGraw Hill, 2018.

20CST11 – Problem Solving and Programming
3/2/2021 4.1 Pointers 2
Syllabus

Contents
1. Pointers
– Memory Access and Pointers
– Pointer basics
– Declaring, Initializing and Dereferencing a pointer
– Parameter Passing Mechanisms
– Operations on Pointers
2. Strings
– String Basics
– Declaring and Initializing a String
– Pointers for String Manipulation
– String handling functions
– Standard and user defined functions
– character oriented functions
– Two dimensional array of strings
3/2/2021 3
4.1 Pointers

Memory Access and Pointers
• A program evaluates a variable by accessing its
memory location and retrieving the value stored
there.
• C permits every legal area of memory to be directly
accessed.
• “legal” because not every memory location is
accessible by our programs.
• The language allows us to obtain the memory location
of every variable, array or structure used in a program.
• We can then use the address of this location to
access—and even change—the value stored there.
• Two values are associated with any variable:
– the address of its memory location and
– the value stored at the location
3/2/2021 4.1 Pointers 4

Pointer Basics
• A pointer is a variable with a difference.
• It contains, not a simple value, but the address of
another variable or object.
• For example,
• if a pointer p is assigned the address of an int
variable x which is set to 5, then
• where
– &x evaluates to the memory address of x.
3/2/2021 4.1 Pointers 5

Pointer Basics
• The pointer p must have been declared
previously as
• C offers two unary operators, the & and *, for
handling pointers.
3/2/2021 4.1 Pointers 6

Pointer Basics
• The & is used to obtain the address of a variable.
• The * is used for two purposes.
• One, to declare the pointer.
– When p is assigned the address of x, p is said to point to x,
which means you can use p to access and change x.
• Second, to obtain the value at the address pointed.
– If p is visible at some location of a program but x is not,
you can still access x at that location by dereferencing p to
obtain the value at the address pointed to, i.e. x.
– The expression *p (second use of *) evaluates to x:
3/2/2021 4.1 Pointers 7

Pointer Basics
• While *p represents the value of x, it can also be used
on the left-hand side of an assignment to change the
value of x:
• The power of a pointer lies in the preceding statement.
The assignment *p = 10 is the same as x = 10.
• A pointer thus provides access to two values—
– the address it stores and
– the value stored at the address
3/2/2021 4.1 Pointers 8

Pointer Basics
• The type of this pointer is not int, but pointer to int,
which we also refer to as int *.
• Even though a pointer has an integer value, it supports
a limited form of arithmetic.
– Adding 1 to a pointer value of 100 yields 101 when the
pointed-to variable is of type char,
– and 104 when the pointed-to variable is of type int.
• Using pointer arithmetic, we can navigate memory
and update the contents at any legal memory location.
• You can change the value stored in the pointer or the
value pointed to:
3/2/2021 4.1 Pointers 9

Pointer Basics
• Like arrays, pointers belong to the category of derived
data types.
• Since every variable has an address,
– we can define pointers for every data type, including
arrays and structures.
– We can also create a pointer to another pointer.
• Arrays and strings can also be treated as pointers for
most purposes.
– The name of an array evaluates to the address of the first
element.
– Double-quoted string evaluates to the address of the first
character of the string.
3/2/2021 4.1 Pointers 10

Pointer Basics
3/2/2021 4.1 Pointers 11

Declaring, Initializing and Dereferencing a pointer
• A pointer variable is declared (and automatically
defined) before use.
• Declaration specifies the type and allocates the
necessary memory for the pointer.
• The following statement declares a pointer variable
named p that can store the address of an int variable.
• By convention, the * is prefixed to the variable, but
you can place it anywhere between int and p
– Int *p
– int * p
– int* p
3/2/2021 4.1 Pointers 13

• The pointer p, which now contains an unpredictable
value, can be used only after it is made to point to a
legal memory location.
• This can be the address of an int variable (say, x), which
is obtained by using the & operator:
• The & operator evaluates its operand (x) as an address.
• We can combine the declaration and assignment in this
manner:
3/2/2021 4.1 Pointers 14

• p has the data type int *, or pointer to int, which simply means that
the value pointed to has the type int.
• Similarly, a float variable is pointed to by a pointer of type float * or
pointer to float.
• However, the size of the pointer itself has no relation to the size of
the variable pointed to.
• A pointer has a fixed size and its value is printed with the %p
format specifier of printf.
• After a pointer has been assigned a valid address with p = &x;
• Use the concept of indirection to obtain the value of x from p.
• The expression *p evaluates to x.
3/2/2021 4.1 Pointers 15

• The *, used as a unary operator in dereferencing or
indirection, operates in the following sequence:
– It evaluates its operand (p) to obtain the stored address
(&x)
– It then fetches the value (x) stored at that address
• Hence, *p is synonymous with x and can be used
wherever x is used.
• Thus, *p changes x when used in the following way:
3/2/2021 4.1 Pointers 16

• As long as p points to x, any change made to x can be
seen by p, and vice versa.
• This is the most important feature of a pointer.
• p thus has two values—
– a direct (&x) and
– an indirect one (*p).
• The dereferenced value of p can also be assigned to
another variable or used in an arithmetic expression:
3/2/2021 4.1 Pointers 17

• The relationship between a pointer and the
variable it points to is depicted in Figure.
3/2/2021 4.1 Pointers 18

Parameter Passing Mechanism
• There are different ways in which parameter data
can be passed into and out of methods and
functions.
• Let us assume that a function B() is called from
another function A().
• In this case A is called the “caller
function” and B is called the “called function or
callee function”.
• Also, the arguments which A sends to B are
called actual arguments and the parameters
of B are called formal arguments.
3/2/2021 4.1 Pointers 20

Parameter Passing Mechanism
• Formal Parameter : A variable and its type as they
appear in the prototype of the function or method.
• Actual Parameter : The variable or expression
corresponding to a formal parameter that appears in
the function or method call in the calling environment.
• Modes:
– IN: Passes info from caller to calle.
– OUT: Callee writes values in caller.
– IN/OUT: Caller tells callee value of variable, which may be
updated by callee.
3/2/2021 4.1 Pointers 21

Pass By Value / Call By Value
• This method uses in-mode semantics.
• Changes made to formal parameter do not get
transmitted back to the caller.
• Any modifications to the formal parameter
variable inside the called function or method
affect only the separate storage location and
will not be reflected in the actual parameter in
the calling environment.
• This method is also called as call by value.
3/2/2021 4.1 Pointers 22

Call By Value
3/2/2021 4.1 Pointers 23

Example
3/2/2021 4.1 Pointers 24

• Shortcomings:
– Inefficiency in storage allocation
– For objects and arrays, the copy semantics are
costly
3/2/2021 4.1 Pointers 25

Pass by reference / Call by reference
• This technique uses in/out-mode semantics.
• Changes made to formal parameter do get
transmitted back to the caller through parameter
passing.
• Any changes to the formal parameter are
reflected in the actual parameter in the calling
environment as formal parameter receives a
reference (or pointer) to the actual data.
• This method is also called as call by reference.
• This method is efficient in both time and space.
3/2/2021 4.1 Pointers 26

Call by reference
3/2/2021 4.1 Pointers 27

Example 1
3/2/2021 4.1 Pointers 28

Example 2
3/2/2021 4.1 Pointers 29

• Shortcomings:
– Many potential scenarios can occur
– Programs are difficult to understand sometimes
3/2/2021 4.1 Pointers 31

Other methods of Parameter Passing
• These techniques are older and were used in earlier programming
languages like Pascal, Algol and Fortran.
• These techniques are not applicable in high level languages.
– Pass by Result : This method uses out-mode semantics. Just before
control is transferred back to the caller, the value of the formal
parameter is transmitted back to the actual parameter. This method is
sometimes called call by result. In general, pass by result technique is
implemented by copy.
– Pass by Value-Result : This method uses in/out-mode semantics. It is a
combination of Pass-by-Value and Pass-by-result. Just before the
control is transferred back to the caller, the value of the formal
parameter is transmitted back to the actual parameter. This method is
sometimes called as call by value-result.
– Pass by name : This technique is used in programming language such
as Algol. In this technique, symbolic “name” of a variable is passed,
which allows it both to be accessed and update.
3/2/2021 4.1 Pointers 32

Operations on Pointers
• Apart from dereferencing with the *, there
are a number of operations that can be
performed on pointers.
• We consider here three types of operations:
– Assignment
– Arithmetic and
– Comparison,
• using a pointer p that points to int.
3/2/2021 4.1 Pointers 33

Assignment
• A pointer can be assigned
– the address of a variable (p = &c;) or
– the name of an array (p = arr;)
• It can also be assigned the value of another
pointer variable of the same type.
3/2/2021 4.1 Pointers 34

Assignment
• It is possible to assign an integer, suitably cast to the
type of the pointer, to a pointer variable using.
• However, you can, and often will, assign the value 0
to a pointer. This value has a synonym in the
symbolic constant, NULL.
• You can assign either 0 or NULL to a pointer of any
type without using a cast
3/2/2021 4.1 Pointers 35

Pointer Arithmetic Using + and -
• The domain of pointer arithmetic is small and simple.
• Among the basic arithmetic operators, only the + and -
can be used with pointers.
• An integer may be added to or subtracted from a
pointer.
• The resultant increase or decrease occurs in terms of
storage units, so p + 2 advances the pointer by two
storage units (12.8.1).
• For an array, this means skipping two elements.
• The following operations on &x and an array element
are also permitted:
3/2/2021 4.1 Pointers 36

3/2/2021 4.1 Pointers 37

3/2/2021 4.1 Pointers 38

Pointer Arithmetic Using ++ and --
• A pointer variable can also be used with the
increment and decrement operators.
• Because ++ and -- have the side effect of
changing their operand, they can’t be used
with the name of the array which is a
constant pointer.
• The following expressions are not permitted
for the array arr:
3/2/2021 4.1 Pointers 39

• But if p points to arr, the expressions p++ or --
p are legal.
• p++ increases the pointer to point to the next
array element and p-- decrements it to point
to the previous element.
• If p points to an int variable, each ++ or --
operation changes the numerical value of the
pointer by 4 bytes.
3/2/2021 4.1 Pointers 40

• The expressions p++ and p-- are often
combined with the * for dereferencing the
pointer.
• Expressions like *p++ and ++*p are commonly
used in programs to evaluate the value
pointed to by p, before or after the
incrementing operation.
3/2/2021 4.1 Pointers 41

Example
3/2/2021 4.1 Pointers 42

Comparing Two Pointers
• Finally, two pointers can be compared only if
they point to members of the same array.
• There are two exceptions though.
– First, any pointer can be compared to the null
pointer (p == NULL) that is often returned by a
function when an error condition is encountered.
– Also, a pointer of any type can be compared to a
generic or void pointer
3/2/2021 4.1 Pointers 43

Thank you
3/2/2021 4.1 Pointers 44

20CST11 – Problem Solving and Programming
3/2/2021 4.2 Strings 46
Syllabus

Contents
1. Pointers
– Memory Access and Pointers
– Pointer basics
– Declaring, Initializing and Dereferencing a pointer
– Parameter Passing Mechanisms
– Operations on Pointers
2. Strings
– String Basics
– Declaring and Initializing a String
– Pointers for String Manipulation
– String handling functions
– Standard and user defined functions
– character oriented functions
– Two dimensional array of strings
3/2/2021 47
4.2 Strings

Strings Basics
• Unlike Java, C doesn’t support a primary data
type for a string.
• The language approaches strings through the
“backdoor”—by tweaking the char data type.
• A string belongs to the category of derived
data types and is interpreted as an array of
type char terminated by the NUL character.
• C offers no keywords or operators for
manipulating strings but its standard library
supports a limited set of string-handling
functions.
3/2/2021 4.2 Strings 48

Strings Basics
• Even though a string is a collection of characters, they are
interpreted differently.
• A character constant is a single-byte integer whose
equivalent symbol is enclosed in single quotes.
– Thus, ‘A’ and its ASCII value, 65, can be used interchangeably in
programs.
• But a string constant is a set of one or more characters
enclosed in double quotes.
– Thus, “A” signifies a string.
– ‘A’ uses one byte but “A” needs two bytes which include the
NUL character.
• Functions that operate on characters don’t work with
strings and vice versa.
3/2/2021 4.2 Strings 49

Strings Basics
• The pointer introduces an additional dimension
to a string.
• The string constant “Micromax” represents a
pointer to the address of its first character.
– i.e., 'M'.
• This string can thus be assigned to a pointer
variable of type char *.
• A string can be manipulated with both array and
pointer notation even though they are not
entirely equivalent.
3/2/2021 4.2 Strings 50

Strings Basics
• Because strings don’t have fixed sizes, every string-
handling function must know where a string begins
and where it ends.
– A function knows the beginning of a string from the
pointer associated with it.
– The end is signified by the NUL character.
• Functions of the standard library that create or modify
strings make sure that the NUL is always in place.
• However, when you create a string-handling function,
it’s your job to append the NUL at the end of the
string.
• NUL has the decimal value 0 and is represented by the
escape sequence ‘0’.
3/2/2021 4.2 Strings 51

Declaring and Initializing a String
• A string can be declared either as
– an array of type char or
– as a pointer of type char *.
• The two techniques of declaration are not
fully equivalent.
• You must understand how they differ and
when to use one technique in preference to
the other.
3/2/2021 4.2 Strings 52

Using an Array to Declare a String
• We have previously declared and initialized an
array with a set of values enclosed in curly
braces.
• The same technique applies to strings as well.
• The following declaration (and definition)
statements also include initialization with a
set of single-quoted characters:
3/2/2021 4.2 Strings 53

• The last element in each case is NUL.
• It has to be explicitly inserted when you
initialize a string like this.
• The first declaration wastes space because
the string uses only nine elements.
• But the compiler automatically sets the size
of stg2 by counting eight items in the list.
3/2/2021 4.2 Strings 54

• Keying in single-quoted characters for initialization is a
tedious job, so C also offers a convenient alternative.
• You can use a double-quoted string to assign an array
at the time of declaration.
• The NUL is automatically inserted when an array is
assigned in this way.
• So, even though sizeof stg1 evaluates to 20 (the
declared size), sizeof stg2 and sizeof stg3 evaluate to 8
and 9 respectively.
3/2/2021 4.2 Strings 55

When an Array is Declared But Not Initialized
3/2/2021 4.2 Strings 56

Using a Pointer to Declare a String
• C supports another way of creating a string.
• Declare a pointer to char.
• Assign a double-quoted string to the pointer
variable.
3/2/2021 4.2 Strings 57

• This string comprises multiple words which
– scanf reads with two %s specifiers, but
– printf needs a single %s to print.
• Like with a regular variable, you can combine
declaration and initialization in one
statement.
3/2/2021 4.2 Strings 58

• The previous declarations that used an array allocated
memory only for the array.
• But the preceding one creates space in memory for
two objects.
– for the pointer p and
– for the string constant, “Redmi Note”
• It then makes p point to the first character of the
string.
3/2/2021 4.2 Strings 59

When an Array of Characters Is Not a String
• An array of characters is not a string when it
doesn’t include the ‘0’ character as the
terminator.
• Usually, we don’t bother to include the NUL
because the compiler does this job for us,
• but negligence on our part can prevent the
compiler from doing so.
3/2/2021 4.2 Strings 61

When an Array of Characters Is Not a String
• Consider the following declarations
• In this scenario, stg5 has no space left to store the NUL character.
• When printf encounters this string, it continues printing beyond
the ‘t’ until it encounters a NUL somewhere on the way.
• This means looking at a region of memory not reserved for it.
• You may see extra characters printed at the end of the word float
or the program could even crash.
• The NUL character has not been explicitly inserted in stg6, so one
would be tempted to conclude that stg6 is not a string.
• However, stg6 is a partially initialized array, which by definition,
has the uninitialized elements set to NUL. Hence, stg6 is a string.
3/2/2021 4.2 Strings 62

Pointers for String Manipulation
• String manipulation is an important
programming activity.
– Navigating a string and
– Accessing each character
• a string can be treated as an array and a pointer.
• By using pointer and array notation in pointer
arithmetic we can manipulate strings.
3/2/2021 4.2 Strings 66

• Consider the following strings defined in two different
ways:
• Because stg is a constant, we can use limited pointer
arithmetic (like stg + 1) to access and update each
element, but we cannot use stg++ and stg--.
• The pointer p, being a variable, knows no such
restrictions. In addition to using expressions like p + 1,
we can also use p++ and p-- to change the current
pointer position in the string.
3/2/2021 4.2 Strings 67

• There is one more difference between stg and p.
• you can change the contents of stg
(“Nyasaland”).
• you can’t change the dereferenced value of p
(“Malawi”) because p is a pointer constant.
• p points to a region of memory that is marked
read-only.
• However, if p is made to point to stg, then you
can change any character in stg using p.
3/2/2021 4.2 Strings 68

3/2/2021 4.2 Strings 69

String Handling Functions
• There are five string-handling functions can be
perform basically.
– Function to Evaluate Length of a String
– Function to Reverse a String
– Function to Copy a String
– Function to Concatenate Two Strings
– Function to Extract a Substring
3/2/2021 4.2 Strings 72

Function to Evaluate Length of a String
3/2/2021 4.2 Strings 73

Function to Reverse a String
3/2/2021 4.2 Strings 74

Function to Copy a String
3/2/2021 4.2 Strings 75

Function to Concatenate Two Strings
3/2/2021 4.2 Strings 76

Standard String Handling Functions
3/2/2021 4.2 Strings 79

Scan Set %[...] / %[^...]
• A format specifier of the scanf function that uses
a set of characters enclosed by [ and ]
• Used for matching characters that either belong
or don’t belong to the set.
• The format specifier %[^n] represents a scan set
that matches an entire line not containing the
newline character.
3/2/2021 4.2 Strings 80

Strlen ()
3/2/2021 4.2 Strings 81

Strcpy()
3/2/2021 4.2 Strings 82

Strcat()
3/2/2021 4.2 Strings 83

strcmp()
3/2/2021 4.2 Strings 84

Strchr()
3/2/2021 4.2 Strings 85

Strstr()
3/2/2021 4.2 Strings 86

Character oriented functions
• C supports a number of character-handling functions.
• All character-oriented functions need the #include
<ctype.h> directive.
• Most of these functions test a specific attribute and
simply return true or false.
• For instance, the isdigit function can be used in a loop
to validate a string that must not have any digits.
3/2/2021 4.2 Strings 87

Two Dimensional Array of Strings
• A string can be stored in an array of type char.
• Multiple strings can be stored in a two-
dimensional array.
• The number of strings that can be stored is
determined by the number of rows (first
subscript).
• the maximum length of each string is set by
the number of columns (second subscript).
3/2/2021 4.2 Strings 90

• Here’s how we assign four strings to a 2D array of type
char:
• When using this technique, we need the inner braces
because all four strings are shorter than the number of
columns (12).
• This is not a convenient way of populating arrays with
strings.
3/2/2021 4.2 Strings 91

• A simpler method is to use a set of double-
quoted strings enclosed in a single set of
braces:
3/2/2021 4.2 Strings 92

Thank you
3/2/2021 4.2 Strings 93

C Programming : Pointers and Strings

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