Lesson in Strings for C Programming Lessons

1
What is a pointer?
 First of all, it is a variable, just like other
variables you studied
 So it has type, storage etc.
 Difference: it can only store the address
(rather than the value) of a data item
 Type of a pointer variable – pointer to the type
of the data whose address it will store
 Example: int pointer, float pointer,…
 Can be pointer to any user-defined types also like
structure types

2
Values vs Locations
 Variables name memory locations, which hold
values
32
x
1024:
address
name
value

3
Contd.
 Consider the statement
int xyz = 50;
This statement instructs the compiler to
allocate a location for the integer variable xyz,
and put the value 50 in that location
Suppose that the address location chosen is
1380 xyz  variable
50  value
1380  address

4
Contd.
 During execution of the program, the system always
associates the name xyz with the address 1380
 The value 50 can be accessed by using either the
name xyz or the address 1380
 Since memory addresses are simply numbers, they
can be assigned to some variables which can be
stored in memory
 Such variables that hold memory addresses are
called pointers
 Since a pointer is a variable, its value is also stored
in some memory location

5
Contd.
 Suppose we assign the address of xyz to a
variable p
 p is said to point to the variable xyz
Variable Value Address
xyz 50 1380
p 1380 2545
p = &xyz;
*p=xyz (50)

6
Pointers
 A pointer is just a C variable whose value can contain
the address of another variable
 Needs to be declared before use just like any other
variable
 General form:
data_type *pointer_name;
 Three things are specified in the above declaration:
 The asterisk (*) tells that the variable
pointer_name is a pointer variable
 pointer_name needs a memory location
 pointer_name points to a variable of type
data_type

7
Example
int *count;
float *speed;
char *c;
 Once a pointer variable has been declared, it can be made
to point to a variable using an assignment statement like
int *p, xyz;
:
p = &xyz;
 This is called pointer initialization

9
Strings
• 1-d arrays of type char
• By convention, a string in C is terminated by the
end-of-string sentinel ‘0’ (null character)
• char s[21] - can have variable length string
delimited with 0
• Max length of the string that can be stored is 20 as the
size must include storage needed for the ‘0’
• String constants : “hello”, “abc”
• “abc” is a character array of size 4

10
Character Arrays and Strings
char C[8] = { 'a', 'b', 'h', 'i', 'j', 'i', 't', '0' };
 C[0] gets the value 'a', C[1] the value 'b', and so on.
The last (7th) location receives the null character ‘0’
 Null-terminated (last character is ‘0’) character arrays
are also called strings
 Strings can be initialized in an alternative way. The
last declaration is equivalent to:
char C[8] = "abhijit";
 The trailing null character is missing here. C
automatically puts it at the end if you define it like this
 Note also that for individual characters, C uses single
quotes, whereas for strings, it uses double quotes

11
Reading strings: %s format
void main()
{
char name[25];
scanf("%s", name);
printf("Name = %s n", name);
}
%s reads a string into a character array
given the array name or start address.
It ends the string with ‘0’

12
An example
void main()
{
#define SIZE 25
int i, count=0;
char name[SIZE];
scanf("%s", name);
printf("Name = %s n", name);
for (i=0; name[i]!='0'; i++)
if (name[i] == 'a') count++;
printf("Total a's = %dn", count);
}
Satyanarayana
Name = Satyanarayana
Total a's = 6
Note that character strings read
in %s format end with ‘0’
Seen on screen
Typed as input
Printed by program

13
Differences : array & pointers
char *p = “abcde”;
The compiler allocates
space for p, puts the
string constant “abcde”
in memory somewhere
else, initializes p with
the base address of
the string constant
char s[ ] = “abcde”;
 char s[ ] = {‘a’,’b’,’c’,’d’,’e’.’0’};
The compiler allocates 6 bytes
of memory for the array s
which are initialized with the
6 characters
a b c d e 0
a b c d e 0
p
S

14
String Constant
• A string constant is treated as a pointer
• Its value is the base address of the string
char *p = “abc”;
printf (“%s %sn”,p,p+1); /* abc bc is printed */
a b c 0
p

15
Library Functions for String
Handling
 You can write your own C code to do different
operations on strings like finding the length of a
string, copying one string to another, appending
one string to the end of another etc.
 C library provides standard functions for these
that you can call, so no need to write your own
code
 To use them, you must do
#include <string.h>
At the beginning of your program (after #include
<stdio.h>)

16
String functions we will see
 strlen : finds the length of a string
 strcat : concatenates one string at the end
of another
 strcmp : compares two strings
lexicographically
 strcpy : copies one string to another

17
strlen()
int strlen(const char *s)
 Takes a null-terminated
strings (we routinely refer
to the char pointer that
points to a null-terminated
char array as a string)
 Returns the length of
the string, not counting
the null (0) character
int strlen (const char *s) {
int n;
for (n=0; *s!=‘0’; ++s)
++n;
return n;
}
You cannot change contents
of s in the function

18
strcat()
 char *strcat (char *s1,
const char *s2);
 Takes 2 strings as
arguments,
concatenates them,
and puts the result in
s1. Returns s1.
Programmer must
ensure that s1 points
to enough space to
hold the result.
char *strcat(char *s1, const char
*s2)
{
char *p = s1;
while (*p != ‘0’) /* go to end */
++p;
while(*s2 != ‘0’)
*p++ = *s2++; /* copy */
*p = ‘0’;
return s1;
}
You cannot change contents
of s2 in the function

19
strcmp()
int strcmp (const char
*s1, const char *s2);
Two strings are passed
as arguments. An
integer is returned
that is less than,
equal to, or greater
than 0, depending on
whether s1 is
lexicographically less
than, equal to, or
greater than s2.

20
strcmp()
int strcmp (const char
*s1, const char *s2);
Two strings are passed
as arguments. An
integer is returned
that is less than,
equal to, or greater
than 0, depending on
whether s1 is
lexicographically less
than, equal to, or
greater than s2.
int strcmp(char *s1, const char *s2)
{
for (;*s1!=‘0’&&*s2!=‘0’; s1++,s2++)
{
if (*s1>*s2) return 1;
if (*s2>*s1) return -1;
}
if (*s1 != ‘0’) return 1;
if (*s2 != ‘0’) return -1;
return 0;
}

21
char *strcpy (char *s1, char *s2);
The characters is the string s2 are copied into s1 until
0 is moved. Whatever exists in s1 is overwritten. It
is assumed that s1 has enough space to hold the
result. The pointer s1 is returned.
strcpy()

22
char *strcpy (char *s1, const char *s2);
The characters is the string s2 are copied into s1 until
‘0’ is moved. Whatever exists in s1 is overwritten. It
is assumed that s1 has enough space to hold the
result. The pointer s1 is returned.
char * strcpy (char *s1, const char *s2)
{
char *p = s1;
while (*p++ = *s2++) ;
return s1;
}
strcpy()

23
Example: Using string functions
25
9
-1
big sky country
beautiful brown cows!
int main()
{
char s1[ ] = "beautiful big sky country",
s2[ ] = "how now brown cow";
printf("%dn",strlen (s1));
printf("%dn",strlen (s2+8));
printf("%dn", strcmp(s1,s2));
printf("%sn",s1+10);
strcpy(s1+10,s2+8);
strcat(s1,"s!");
printf("%sn", s1);
return 0;
}
Output

Lesson in Strings for C Programming Lessons

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