Understanding Strings in C Programming

In this article, we will explore the fundamental aspects of strings in C, including declaration, initialization, and various operations such as input, length calculation, and manipulation using string.h library functions. We’ll also delve into the differences between character arrays and string literals, and discuss strings in the context of pointers. Along the way, we’ll include numerous code snippets to illustrate key points.

What Is String In C Language?

In C, a string is a sequence of characters terminated by a null character '\0'. This null character signifies the end of the string and is essential for various string manipulation operations. Strings are stored in arrays of char type, making them closely related to character arrays.

String Syntax In C : String Declaration

Declaring a string in C can be done by specifying an array of characters. The syntax for declaration is as follows:

char str[size];

Where size is the number of characters the string can hold, including the null terminator. It’s important to ensure the array is large enough to store all characters of the string plus the null terminator.

String Syntax In C : String Initialization

Strings can be initialized in several ways at the time of declaration. Here are a few examples:

Assigning a String Literal without Size

char str[] = "Hello, World!";

In this case, the compiler automatically calculates the size of the array to fit the string literal plus the null terminator.

Assigning a String Literal with a Predefined Size

char str[50] = "Hello, World!";

Here, the array size is explicitly specified. If the string is shorter than the array, the rest of the array remains uninitialized.

Assigning Character by Character with Size

char str[14] = {'H', 'e', 'l', 'l', 'o', ',', ' ', 'W', 'o', 'r', 'l', 'd', '!', '\0'};

This method explicitly lists all characters, including the null terminator, with a predefined size.

Assigning Character by Character without Size

In C programming, strings can be assigned character by character without pre-declaring a fixed size. The characters are inserted into a character array one by one. A null terminating character ‘\0’ is added at the end to mark the end of the string. The compiler automatically determines the size of the array based on the number of characters assigned to it.

char str[] = {'H','e','l','l','o',' ','W','o','r','l','d','!','\0'};

It’s not recommended to assign characters one by one without specifying the size because it can lead to errors if the null terminator is omitted accidentally. Always ensure the size is accounted for or use string literals for initialization.

#include <stdio.h>

int main() {
  char name[50];

  printf("Enter your name: ");
  scanf("%s", name);

  printf("Your name is: %s", name);

  return 0;
}

fgets(name, 50, stdin); // reads string with spaces

#include <stdio.h>

int main() {

  char str[50] = "Hello World"; 
  
  printf("%s", str);
  
  return 0;
}

puts(str); // prints string and newline

How To Read a String By Taking Input from the User?

To read a string from user input, scanf and gets functions can be used, but gets is unsafe and deprecated due to the risk of buffer overflow.

Here’s a safe way using scanf:

char str[100];
printf("Enter a string: ");
scanf("%99s", str); // Reads a string until a whitespace is encountered

Using gets:

char name[50];

  // Using gets (unsafe, causes buffer overflow)
  printf("Enter name again: "); 
  gets(name);
  printf("Name: %s\n", name);

How to Read a String Separated by Whitespaces in C?

To read a string that includes whitespaces, fgets is a safer alternative:

char str[100];
printf("Enter a string: ");
fgets(str, sizeof(str), stdin); // Reads until a newline or EOF is encountered

String Operations In C

Let’s look into some of the important string operations in C.

How to find length in string in C?

The strlen function from the string.h library is used to find the length of a string (excluding the null terminator):

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "Hello, World!";
    printf("Length: %lu\n", strlen(str));
    return 0;
}

// Output: Length: 13

Passing Strings to Functions

Strings can be passed to functions by simply passing the array:

void printString(char str[]) {
    printf("%s\n", str);
}

Converting String to Number In C

Converting a string to a number is a common task in programming. In C, there are a few ways to perform this conversion:

Using atoi() function

The atoi() function takes a string as an argument and returns its value converted to an integer.

#include <stdio.h>
#include <stdlib.h>

int main() {

  char str[10] = "12345";
  int x = atoi(str);

  printf("String value = %s\n", str); 
  printf("Integer value = %d", x);

  return 0;
}

Using strtol() function

The strtol() function converts a string to a long integer. An advantage over atoi() is that it can handle valid numbers that overflow the int type.

long y = strtol(str, NULL, 10); 

Manually parsing the string

We can also manually parse the string and accumulate the integer value character by character:

int value = 0;

for(int i = 0; str[i] != '\0'; i++) {
  value = value*10 + (str[i] - '0'); 
}

This loops through each character, converts it to its integer equivalent by subtracting ASCII value of ‘0’, and keeps accumulating the number.

Strings can also be manipulated using pointers. For instance, a string can be declared as a pointer to a character:

char *str = "Hello, World!";

This declaration makes str a pointer to the first character of the string literal.

List of Standard C Library – String.h Functions

The string.h library provides numerous functions for string manipulation, including:

Difference Between Character Array and String Literal

Here is the difference between character array and string literal in C provided in a tabular format: