Strings(2007)

Introduction
 A string is a sequence of characters that
is treated as a single data item.
 Any group of characters(except double
quote sign) defined between double
quotation marks is a string constant.
Example:
“Man is obviously made to think.”
 Normally, string is useful for storing data
like name, address, city etc.
Copyright ©2013 Yash Agarwal. All rights reserved.

Strings
 C implements the stringstring data structure using arrays of
type char.
 You have already used the string extensively.
 printf(“This program is terminated!n”);
 Since stringstring is an array, the declaration of a string is
the same as declaring a char array.
 char string_var[30];
 char string_var[20] = “Initial value”;

Memory Storage for a
String
 ASCII code is internally used to represent string in
memory.
 In ‘C’ each string is always ended with a nullnull
charactercharacter ‘0’‘0’.
 The characters after the null character are ignored.
 e.g., char str[20] = “Initial value”;
n i t i a l v a l u e ? ? …I 00
[0] [13]

Common operations
performed on strings
 Reading and writing strings
 Combining strings together
 Copying one string to another
 Comparing strings for equality
 Extracting a portion of a string

Input/Output of a
String
 The %s%s format specifier is used to represent string arguments in
printf and scanf.
 printf(“Topic: %sn”, string_var);
 The string can be right-justified by placing a positive number in
the format specifier .
 printf(“%8s%8s”, str);
 The string can be left-justified by placing a negative number in
the format specifier .
 Printf(“%-8s%-8s”, str);

An Example of
Manipulating String
with scanf and printf
Copyright ©2004 Pearson Addison-Wesley. All rights reserved.
The dept is the initial memory
address of the string argument. Thus
we don’t apply the & operator on it.

Execution of scanf
("%s", dept);
• Whenever encountering a white space, the scanning stops
and scanf places the null character at the end of the string.
• e.g., if the user types “MATH 1234 TR 1800,” the string
“MATH” along with ‘0’ is stored into dept.

String Library
Functions
 The string can not be copied by the
assignment operator ‘=’.
 e.g., “str = “Test String”” is not
valid.
 C provides string manipulating or built-in
functions in the “string.h” library.
 We require to include string.h header file
for using built-in string functions.

Some String Functions from
String.h
Function Purpose Example
strcpy Makes a copy of a
string
strcpy(s1, “Hi”);
strcat Appends a string to the
end of another string
strcat(s1, “more”);
strcmp Compare two strings
alphabetically
strcmp(s1, “Hu”);
strlen Returns the number of
characters in a string
strlen(“Hi”)
returns 2.
strtok Breaks a string into
tokens by delimiters.
strtok(“Hi, Chao”,
“ ,”);
Copyright ©2013 Yash Agarwal. All rights reserved. 9-10

Function Purpose Example
strrev Reverses the string s strrev(“Jay”);
Gives: yaJ
strupr Convert string to
uppercase
strupr(“Jay”);
Gives: JAY
strlwr Convert string to lower
case
strlwr(“Jay”);
Gives: jay
strncmp Compares first n
characters in a string
strncmp(s1,“Hi”)
returns 2.

Functions strcpy
and strncpy Function strcpy copies the string in the second
argument into the first argument.
 e.g., strcpy(dest , “test string”);
 The null characternull character is appended at the end
automatically.
 If source string is longer than the destination string, the
overflow characters may occupy the memory space
used by other variables.
 Function strncpy copies the string by specifying
the number of characters to copy.
 You have to place the null character manually.
 e.g., strncpy(dest , “test string”, 6); dest[6] = ‘0’;dest[6] = ‘0’;
 If source string is longer than the destination string, the
overflow characters are discarded automatically.Copyright ©2013 Yash Agarwal. All rights reserved.

Extracting Substring of
a String (1/2)
• We can use strncpy to extract substring of one string.
– e.g., strncpy(result, s1, 9);

Extracting Substring of
a String (2/2)
• e.g., strncpy(result, &s1[5]&s1[5], 2);

Functions strcat and
strlen
 Functions strcat and strncat concatenate
the fist string argument with the second string
argument.
 strcat(dest, “more..”);
 strncat(dest, “more..”, 3);
 Function strlen is often used to check the
length of a string (i.e., the number of
characters before the fist null character).
 e.g., dest[6] = “Hello”;
strncat(dest, “more”, 5-strlen(dest));
dest[5] = ‘0’;Copyright ©2013 Yash Agarwal. All rights reserved.

Distinction Between
Characters and
Strings The representation of a char (e.g., ‘Q’) and a string
(e.g., “Q”) is essentially different.
 A string is an array of characters ended with the
null character.
Q
Character ‘Q’
Q 00
String “Q”

String Comparison
(1/2) Suppose there are two strings, str1 and str2.
 The condition str1 < str2 compare the initialinitial
memory addressmemory address of str1 and of str2.
 The comparison between two strings is done by
comparing each corresponding character in
them.
 The characters are compared against the ASCII table.
 “thrill” < “throw” since ‘i’ < ‘o’;
 “joy” < joyous“;
 The standard string comparison uses the strcmp
and strncmp functions.

String Comparison (2/2)
Relationship Returned Value Example
str1 < str2 Negative “Hello”< “Hi”
str1 = str2 0 “Hi” = “Hi”
str1 > str2 Positive “Hi” > “Hello”
• e.g., we can check if two strings are the same by
if(strcmp(str1, str2) != 0)
printf(“The two strings are
different!”);

Input/Output of
Characters and
Strings The stdio library provides getchar function which
gets the next character from the standard input.
 “ch = getchar();” is the same as “scanf(“%c”,
&ch);”
 Similar functions are putchar, gets, puts.
 For IO from/to the file, the stdio library also provides
corresponding functions.
 getc: reads a character from a file.
 Similar functions are putc, fgets, fputs.

Character Analysis and
Conversion
 The <ctype.h><ctype.h> library defines facilities for
character analysis and conversion.
Functions Description
isalpha Check if the argument is a letter
isdigit Check if the argument is one of the ten digits
isspace Check if argument is a space, newline or tab.
tolower Converts the lowercase letters in the
argument to upper case letters.Copyright ©2013 Yash Agarwal. All rights reserved. 9-20

Presentation By:
 F.Y. MECH-2 STUDENTS :
Name Id no.
Viraj Patel 100
Karan Parekh 103
Yash Agarwal 104
Pritesh Vaghela 105

Strings(2007)

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