C Strings

Strings

Strings are used for storing text/characters.

For example, "Hello World" is a string of characters.

Unlike many other programming languages, C does not have a String type to easily create string variables. Instead, you must use the char type and create an array of characters to make a string in C:

char greetings[] = "Hello World!";

Note that you have to use double quotes ("").

To output the string, you can use the printf() function together with the format specifier %s to tell C that we are now working with strings:

Example

char greetings[] = "Hello World!";
printf("%s", greetings);

Access Strings

Since strings are actually arrays in C, you can access a string by referring to its index number inside square brackets [].

This example prints the first character (0) in greetings:

Example

char greetings[] = "Hello World!";
printf("%c", greetings[0]);

Note that we have to use the %c format specifier to print a single character.

Modify Strings

To change the value of a specific character in a string, refer to the index number, and use single quotes:

Example

char greetings[] = "Hello World!";
greetings[0] = 'J';
printf("%s", greetings);
// Outputs Jello World! instead of Hello World!

Loop Through a String

You can also loop through the characters of a string, using a for loop:

Example

char carName[] = "Volvo";
int i;

for (i = 0; i < 5; ++i) {
  printf("%c\n", carName[i]);
}

And like we specified in the arrays chapter, you can also use the sizeof formula (instead of manually write the size of the array in the loop condition (i < 5)) to make the loop more sustainable:

Example

char carName[] = "Volvo";
int length = sizeof(carName) / sizeof(carName[0]);
int i;

for (i = 0; i < length; ++i) {
  printf("%c\n", carName[i]);
}

Another Way Of Creating Strings

In the examples above, we used a "string literal" to create a string variable. This is the easiest way to create a string in C.

You should also note that you can create a string with a set of characters. This example will produce the same result as the example in the beginning of this page:

Example

char greetings[] = {'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!', '\0'};
printf("%s", greetings);

Why do we include the \0 character at the end? This is known as the "null terminating character", and must be included when creating strings using this method. It tells C that this is the end of the string.

Differences

The difference between the two ways of creating strings, is that the first method is easier to write, and you do not have to include the \0 character, as C will do it for you.

You should note that the size of both arrays is the same: They both have 13 characters (space also counts as a character by the way), including the \0 character:

Example

char greetings[] = {'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!', '\0'};
char greetings2[] = "Hello World!";

printf("%lu\n", sizeof(greetings));   // Outputs 13
printf("%lu\n", sizeof(greetings2));  // Outputs 13

Real-Life Example

Use strings to create a simple welcome message:

Example

char message[] = "Good to see you,";
char fname[] = "John";

printf("%s %s!", message, fname);

Strings - Special Characters

Because strings must be written within quotes, C will misunderstand this string, and generate an error:

char txt[] = "We are the so-called "Vikings" from the north.";

The solution to avoid this problem, is to use the backslash escape character.

The backslash (\) escape character turns special characters into string characters:

Escape character Result Description
\' ' Single quote
\" " Double quote
\\ \ Backslash

The sequence \"  inserts a double quote in a string:

 

Example

char txt[] = "We are the so-called \"Vikings\" from the north.";

The sequence \'  inserts a single quote in a string:

Example

char txt[] = "It\'s alright.";

The sequence \\  inserts a single backslash in a string:

Example

char txt[] = "The character \\ is called backslash.";

Other popular escape characters in C are:

Escape Character Result
\n New Line
\t Tab
\0 Null

String Functions

C also has many useful string functions, which can be used to perform certain operations on strings.

To use them, you must include the <string.h> header file in your program:

#include <string.h>

String Length

For example, to get the length of a string, you can use the strlen() function:

Example

char alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
printf("%d", strlen(alphabet));

In the Strings chapter, we used sizeof to get the size of a string/array. Note that sizeof and strlen behaves differently, as sizeof also includes the \0 character when counting:

Example

char alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
printf("%d", strlen(alphabet));   // 26
printf("%d", sizeof(alphabet));   // 27

It is also important that you know that sizeof will always return the memory size (in bytes), and not the actual string length:

Example

char alphabet[50] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
printf("%d", strlen(alphabet));   // 26
printf("%d", sizeof(alphabet));   // 50

Concatenate Strings

To concatenate (combine) two strings, you can use the strcat() function:

Example

char str1[20] = "Hello ";
char str2[] = "World!";

// Concatenate str2 to str1 (result is stored in str1)
strcat(str1, str2);

// Print str1
printf("%s", str1);

Note that the size of str1 should be large enough to store the result of the two strings combined (20 in our example).

Copy Strings

To copy the value of one string to another, you can use the strcpy() function:

Example

char str1[20] = "Hello World!";
char str2[20];

// Copy str1 to str2
strcpy(str2, str1);

// Print str2
printf("%s", str2);

Note that the size of str2 should be large enough to store the copied string (20 in our example).

Compare Strings

To compare two strings, you can use the strcmp() function.

It returns 0 if the two strings are equal, otherwise a value that is not 0:

Example

char str1[] = "Hello";
char str2[] = "Hello";
char str3[] = "Hi";

// Compare str1 and str2, and print the result
printf("%d\n", strcmp(str1, str2));  // Returns 0 (the strings are equal)

// Compare str1 and str3, and print the result
printf("%d\n", strcmp(str1, str3));  // Returns -4 (the strings are not equal)