String in C

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STRINGS
LECTURE 7 - CSE 1102
STRUCTURED PROGRAMMING LANGUAGE
IN C
Instructor: Salim Shadman
Ankur
Date: 25 April 2025

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WHAT IS A STRING?
 In C programming, a string is not a distinct data type like in
Python or Java.
 It is an array of characters that ends with a null character '0'
which marks the end of the string.
 In C, a string is an array of characters ending with '0'
 No dedicated string data type in C
 Used with character arrays like: char str[] = "Hello";

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Why is '0' important?
 It tells C functions like printf() or strlen() where the string
ends.
 Without it, C would keep reading memory past the
string, leading to errors.
 Example:
char greeting[] = "Hello";
 This is stored in memory as:
Index 0 1 2 3 4 5
Value H e l l o 0

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DECLARING STRINGS
Example 1: Fixed size without initialization
char str[20]; Declares a string with max length 19.
→
This reserves space for 20 characters, enough for 19 characters + '0’.
Example 2: With Initialization
char name[] = "John"; Initializes and auto-adds '0'.
→
The compiler automatically adds the null character.
⚠️Important: If you write:
char str[] = {'H', 'i'}; Not valid string (no '0').
→
There is no null terminator, so it’s not a valid string for C string functions.

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STRING INPUT/OUTPUT
⚠️scanf() Limitation
scanf("%s", str); Stops at space.
→
• It cannot read strings with spaces.
• For example, typing Open AI will store only Open.
gets(str); Reads spaces but unsafe (deprecated).
→
fgets(str, sizeof(str), stdin); Safe, reads full line
→
✅ Better: Use fgets()
fgets(str, sizeof(str), stdin);
• Reads spaces.
• Automatically stops at newline or max size.
Best practice for string input = fgets()

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STRING HANDLING FUNCTIONS
 strlen(s): Returns length of string (excluding '0’).
char msg[] = "Hello";
int len = strlen(msg); // 5 (not 6!)
 strcpy(dest, src): Copies string.
char a[20], b[] = "World";
strcpy(a, b); // a = "World“
 strcmp(s1, s2): Compares strings.
strcmp("abc", "abc") 0
→
strcmp("abc", "abd") -1
→
strcmp("abd", "abc") 1
→
🔹 Returns 0 if equal, negative if a < b, positive if a > b.

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STRING HANDLING FUNCTIONS
 strcat(dest, src): Appends strings.
char a[20] = "Hello";
char b[] = "World";
strcat(a, b); // a = "HelloWorld"
 strstr(haystack, needle): Finds substring.
char text[] = "Welcome to C programming";
char word[] = "C";
char *result = strstr(text, word);
if (result != NULL)
printf("Found at position: %ldn", result - text);
else
printf("Not foundn");

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STRING LENGTH (STRLEN())
strlen() – Get the Length of a String
Syntax:
size_t strlen(const char *str);
 Counts the number of characters before the null terminator 0.
 Does not include 0 in the length.
 Returns a value of type size_t (usually an unsigned int).
✅ Example:
#include <stdio.h>
#include <string.h>
int main()
{
char name[] = "Programming";
int len = strlen(name);
printf("Length of the string is: %dn", len);
return 0;
}
Output: Length of the string is: 11

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STRING COPY (STRCPY())
strcpy() – Copy One String to Another
Syntax:
char *strcpy(char *dest, const char *src);
 Copies the content of src (including 0) into dest.
 dest must have enough space to hold the copied string.
 The return value is dest
✅ Example:
#include <stdio.h>
#include <string.h>
int main()
{
char source[] = "Learn C";
char destination[20]; // Make sure it's big enough
strcpy(destination, source);
printf("Copied string: %sn", destination);
return 0;
}
Output: Copied string: Learn
•.

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STRING CONCATENATION (STRCAT())
strcat() – Concatenate Two Strings
Syntax:
char *strcat(char *dest, const char *src);
 Appends the source string (src) to the destination string (dest).
 The resulting string is stored in dest.
 dest must have enough space to hold the result.
✅ Example:
#include <stdio.h>
#include <string.h>
int main()
{
char str1[50] = "Hello, ";
char str2[] = "world!";
strcat(str1, str2);
printf("Concatenated string: %sn", str1);
return 0;
}
Output: Concatenated string: Hello, world!
•.

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STRING COMPARISON (STRCMP())
strcmp() – Compare Two Strings
Syntax:
int strcmp(const char *str1, const char *str2);
 Compares str1 and str2 character by character.
 Returns:
0 if both strings are equal.
Negative value if str1 < str2.
Positive value if str1 > str2.
✅ Example:
#include <stdio.h>
#include <string.h>
int main()
{
char a[] = "apple";
char b[] = "banana";
int result = strcmp(a, b);
if (result == 0)
printf("Strings are equaln");
else if (result < 0)
printf("'%s' comes before '%s'n", a, b);
else
printf("'%s' comes after '%s'n", a, b);
return 0;
}
Output: 'apple' comes before 'banana'
•.

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STRING COMPARISON (STRCMP())
•.
⚠️Common Mistake:
if (a == b) // ❌ Wrong
 This compares addresses, not content.
 Always use strcmp() for string comparison.

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FINDING SUBSTRING (STRSTR())
strstr() – Find a Substring in a String
Syntax:
char *strstr(const char *haystack, const char *needle);
 Searches for the first occurrence of needle in haystack.
 Returns a pointer to the first match in haystack.
 Returns NULL if not found
✅ Example:
#include <stdio.h>
#include <string.h>
int main()
{
char text[] = "Welcome to C programming";
char word[] = "C";
char *result = strstr(text, word);
if (result != NULL)
printf("Found at position: %ldn", result - text);
else
printf("Not foundn"); return 0;
}
Output: Found at position: 11
•.
•.

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MANUAL STRING OPERATIONS
strlen: loop until '0' to count characters.
size_t my_strlen(const char s[]) {
size_t i = 0;
while (s[i] != '0') {
i++;
}
return i;
}

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MANUAL STRING OPERATIONS
strcpy: copy one char at a time until '0'.
char *my_strcpy(char dest[], const char src[]) {
size_t i = 0;
// copy each character, including the terminating null
do {
dest[i] = src[i];
} while (src[i++] != '0');
return dest;
}

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MANUAL STRING OPERATIONS
strcmp: compare character by character.
//lexicographically compare two strings. returns 0 if equal, <0 if s1<s2, >0 if s1>s2
int my_strcmp(const char s1[], const char s2[]) {
size_t i = 0;
// advance until chars differ or one hits '0'
while (s1[i] != '0' && s1[i] == s2[i]) {
i++;
}
// at this point either s1[i] or s2[i] is '0' or they differ
return (unsigned char)s1[i] - (unsigned char)s2[i];
}

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MANUAL STRING OPERATIONS
strcat: append src to the end of dest
char *my_strcat(char dest[], const char src[]) {
size_t i = 0, j = 0;
// find end of dest (i.e. its length)
while (dest[i] != '0') {
i++;
}
// copy src into dest starting at i
do {
dest[i + j] = src[j];
} while (src[j++] != '0');
return dest;
}

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MANUAL STRING OPERATIONS
strstr: Find the first occurrence of needle in haystack. Returns a
pointer into haystack, or NULL if not found
char *my_strstr(char haystack[], const char needle[]) {
size_t hlen = my_strlen(haystack);
size_t nlen = my_strlen(needle);
if (nlen == 0) {
return haystack; // empty needle matches at start
}
for (size_t i = 0; i + nlen <= hlen; i++) {
// check match at position i
size_t j = 0;
while (j < nlen && haystack[i + j] == needle[j]) {
j++;
}
if (j == nlen) {
return &haystack[i];
}
}
return NULL;
}

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EXAMPLE PROBLEM: REVERSE A STRING
 Use two pointers: start and end.
 Swap characters until pointers meet.
 Null terminator stays untouched.
void reverse(char str[]) {
int len = strlen(str);
for (int i = 0; i < len / 2; i++) {
char temp = str[i];
str[i] = str[len - i - 1];
str[len - i - 1] = temp;
}
}

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EXAMPLE PROBLEM: CHECK PALINDROME
int is_palindrome(char str[]) {
int i = 0, j = strlen(str) - 1;
while (i < j) {
if (str[i] != str[j])
return 0;
i++; j--;
}
return 1;
}

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WARNINGS & BEST PRACTICES
Mistake Why It's Bad Fix
Using == to compare
strings
Compares addresses,
not content
Use strcmp()
Forgetting '0'
Causes undefined
behavior
Always terminate
manually
Not checking buffer
overflow
Can crash the program
Use fgets() instead of
gets()

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PRACTICE LAB TASK
Take two strings as input.
Check if second string is substring of first.
Use strstr() or manual logic.
Print position or 'Not Found'.

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SUMMARY
 Strings are null-terminated character arrays.
 Use string.h functions for ease.
 Manual string manipulation helps in learning C internals.
 Always consider safety in input/output.