c string assignment

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std::string::assign() in C++

The member function assign() is used for the assignments, it assigns a new value to the string, replacing its current contents.  Syntax 1: Assign the value of string str. 

Output: 

Syntax 2: Assigns at most str_num characters of str starting with index str_idx. It throws out_of _range if str_idx > str. size(). 

Syntax 3: Assign the characters of the C-string cstr. It throws length_error if the resulting size exceeds the maximum number of characters. 

Syntax 4: Assigns chars_len characters of the character array chars. It throws length_error if the resulting size exceeds the maximum number of characters. 

Syntax 5: Assigns num occurrences of character c. It throws length_error if num is equal to string::npos 

Syntax 6: Assigns all characters of the range [beg, end). It throws length_error if range outruns the actual content of string. 

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Strings in C: How to Declare & Initialize a String Variables in C

What is String in C?

A String in C is nothing but a collection of characters in a linear sequence. ‘C’ always treats a string a single data even though it contains whitespaces. A single character is defined using single quote representation. A string is represented using double quote marks.

‘C’ provides standard library <string.h> that contains many functions which can be used to perform complicated operations easily on Strings in C.

How to Declare a String in C?

A C String is a simple array with char as a data type. ‘C’ language does not directly support string as a data type. Hence, to display a String in C, you need to make use of a character array.

The general syntax for declaring a variable as a String in C is as follows,

The classic Declaration of strings can be done as follow:

The size of an array must be defined while declaring a C String variable because it is used to calculate how many characters are going to be stored inside the string variable in C. Some valid examples of string declaration are as follows,

The above example represents string variables with an array size of 15. This means that the given C string array is capable of holding 15 characters at most. The indexing of array begins from 0 hence it will store characters from a 0-14 position. The C compiler automatically adds a NULL character ‘\0’ to the character array created.

How to Initialize a String in C?

Let’s study the String initialization in C. Following example demonstrates the initialization of Strings in C,

In string3, the NULL character must be added explicitly, and the characters are enclosed in single quotation marks.

‘C’ also allows us to initialize a string variable without defining the size of the character array. It can be done in the following way,

The name of Strings in C acts as a pointer because it is basically an array.

C String Input: C Program to Read String

When writing interactive programs which ask the user for input, C provides the scanf(), gets(), and fgets() functions to find a line of text entered from the user.

When we use scanf() to read, we use the “%s” format specifier without using the “&” to access the variable address because an array name acts as a pointer. For example:

The problem with the scanf function is that it never reads entire Strings in C. It will halt the reading process as soon as whitespace, form feed, vertical tab, newline or a carriage return occurs. Suppose we give input as “Guru99 Tutorials” then the scanf function will never read an entire string as a whitespace character occurs between the two names. The scanf function will only read Guru99 .

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In order to read a string contains spaces, we use the gets() function. Gets ignores the whitespaces. It stops

reading when a newline is reached (the Enter key is pressed).

For example:

Another safer alternative to gets() is fgets() function which reads a specified number of characters. For example:

The fgets() arguments are :

• the string name,
• the number of characters to read,
• stdin means to read from the standard input which is the keyboard.

C String Output: C program to Print a String

The standard printf function is used for printing or displaying Strings in C on an output device. The format specifier used is %s

String output is done with the fputs() and printf() functions.

fputs() function

The fputs() needs the name of the string and a pointer to where you want to display the text. We use stdout which refers to the standard output in order to print to the screen. For example:

puts function

The puts function is used to Print string in C on an output device and moving the cursor back to the first position. A puts function can be used in the following way,

The syntax of this function is comparatively simple than other functions.

The string library

The standard ‘C’ library provides various functions to manipulate the strings within a program. These functions are also called as string handlers. All these handlers are present inside <string.h> header file.

Lets consider the program below which demonstrates string library functions:

Other important library functions are:

• strncmp(str1, str2, n) :it returns 0 if the first n characters of str1 is equal to the first n characters of str2, less than 0 if str1 < str2, and greater than 0 if str1 > str2.
• strncpy(str1, str2, n) This function is used to copy a string from another string. Copies the first n characters of str2 to str1
• strchr(str1, c): it returns a pointer to the first occurrence of char c in str1, or NULL if character not found.
• strrchr(str1, c): it searches str1 in reverse and returns a pointer to the position of char c in str1, or NULL if character not found.
• strstr(str1, str2): it returns a pointer to the first occurrence of str2 in str1, or NULL if str2 not found.
• strncat(str1, str2, n) Appends (concatenates) first n characters of str2 to the end of str1 and returns a pointer to str1.
• strlwr() :to convert string to lower case
• strupr() :to convert string to upper case
• strrev() : to reverse string

Converting a String to a Number

In C programming, we can convert a string of numeric characters to a numeric value to prevent a run-time error. The stdio.h library contains the following functions for converting a string to a number:

• int atoi(str) Stands for ASCII to integer; it converts str to the equivalent int value. 0 is returned if the first character is not a number or no numbers are encountered.
• double atof(str) Stands for ASCII to float, it converts str to the equivalent double value. 0.0 is returned if the first character is not a number or no numbers are encountered.
• long int atol(str) Stands for ASCII to long int, Converts str to the equivalent long integer value. 0 is returned if the first character is not a number or no numbers are encountered.

The following program demonstrates atoi() function:

• A string pointer declaration such as char *string = “language” is a constant and cannot be modified.
• A string is a sequence of characters stored in a character array.
• A string is a text enclosed in double quotation marks.
• A character such as ‘d’ is not a string and it is indicated by single quotation marks.
• ‘C’ provides standard library functions to manipulate strings in a program. String manipulators are stored in <string.h> header file.
• A string must be declared or initialized before using into a program.
• There are different input and output string functions, each one among them has its features.
• Don’t forget to include the string library to work with its functions
• We can convert string to number through the atoi(), atof() and atol() which are very useful for coding and decoding processes.
• We can manipulate different strings by defining a array of strings in C.

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• Find the Frequency of Characters in a String
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In C programming, a string is a sequence of characters terminated with a null character \0 . For example:

When the compiler encounters a sequence of characters enclosed in the double quotation marks, it appends a null character \0 at the end by default.

• How to declare a string?

Here's how you can declare strings:

Here, we have declared a string of 5 characters.

• How to initialize strings?

You can initialize strings in a number of ways.

Let's take another example:

Here, we are trying to assign 6 characters (the last character is '\0' ) to a char array having 5 characters. This is bad and you should never do this.

Assigning Values to Strings

Arrays and strings are second-class citizens in C; they do not support the assignment operator once it is declared. For example,

Note: Use the strcpy() function to copy the string instead.

Read String from the user

You can use the scanf() function to read a string.

The scanf() function reads the sequence of characters until it encounters whitespace (space, newline, tab, etc.).

Example 1: scanf() to read a string

Even though Dennis Ritchie was entered in the above program, only "Dennis" was stored in the name string. It's because there was a space after Dennis .

Also notice that we have used the code name instead of &name with scanf() .

This is because name is a char array, and we know that array names decay to pointers in C.

Thus, the  name  in  scanf() already points to the address of the first element in the string, which is why we don't need to use & .

How to read a line of text?

You can use the fgets() function to read a line of string. And, you can use puts() to display the string.

Example 2: fgets() and puts()

Here, we have used fgets() function to read a string from the user.

fgets(name, sizeof(name), stdlin); // read string

The sizeof(name) results to 30. Hence, we can take a maximum of 30 characters as input which is the size of the  name string.

To print the string, we have used puts(name); .

Note: The gets() function can also be to take input from the user. However, it is removed from the C standard. It's because gets() allows you to input any length of characters. Hence, there might be a buffer overflow.

Passing Strings to Functions

Strings can be passed to a function in a similar way as arrays. Learn more about passing arrays to a function .

Example 3: Passing string to a Function

Strings and pointers.

Similar like arrays, string names are "decayed" to pointers. Hence, you can use pointers to manipulate elements of the string. We recommended you to check C Arrays and Pointers before you check this example.

Example 4: Strings and Pointers

Commonly used string functions.

• strlen() - calculates the length of a string
• strcpy() - copies a string to another
• strcmp() - compares two strings
• strcat() - concatenates two strings

Table of Contents

• Read and Write String: gets() and puts()
• Passing strings to a function
• Strings and pointers
• Commonly used string functions

Before we wrap up, let’s put your knowledge of C Programming Strings to the test! Can you solve the following challenge?

Write a function to check if a given string is empty or not.

• Return 1 if the string is empty, otherwise, return 0 .
• For example, if str = "Hello" , the expected output is 0

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22.5 — std::string assignment and swapping

String assignment

The easiest way to assign a value to a string is to use the overloaded operator= function. There is also an assign() member function that duplicates some of this functionality.

The assign() member function also comes in a few other flavors:

If you have two strings and want to swap their values, there are two functions both named swap() that you can use.

C Functions

C structures, c reference.

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:

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:

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 :

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 :

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Loop Through a String

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

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:

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:

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:

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Use strings to create a simple welcome message:

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Std::basic_string<chart,traits,allocator>:: assign.

Replaces the contents of the string.

• If only t is provided, replaces the contents with all characters in sv .
• If count is npos , replaces the contents with all characters in sv starting from pos .
• Otherwise, replaces the contents with the std:: min ( count, sv. size ( ) - pos ) characters in sv starting from pos .
• std:: is_convertible_v < const SV & , std:: basic_string_view < CharT, Traits >> is true .
• std:: is_convertible_v < const SV & , const CharT * > is false .
• If count is npos , replaces the contents with all characters in str starting from pos .
• Otherwise, replaces the contents with the std:: min ( count, str. size ( ) - pos ) characters in str starting from pos .

[ edit ] Parameters

[ edit ] return value, [ edit ] exceptions.

              propagate_on_container_move_assignment :: value ||

If the operation would cause size() to exceed max_size() , throws std::length_error .

If an exception is thrown for any reason, this function has no effect ( strong exception safety guarantee ).

[ edit ] Example

[ edit ] defect reports.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[ edit ] See also

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• <cassert> (assert.h)
• <cctype> (ctype.h)
• <cerrno> (errno.h)
• C++11 <cfenv> (fenv.h)
• <cfloat> (float.h)
• C++11 <cinttypes> (inttypes.h)
• <ciso646> (iso646.h)
• <climits> (limits.h)
• <clocale> (locale.h)
• <cmath> (math.h)
• <csetjmp> (setjmp.h)
• <csignal> (signal.h)
• <cstdarg> (stdarg.h)
• C++11 <cstdbool> (stdbool.h)
• <cstddef> (stddef.h)
• C++11 <cstdint> (stdint.h)
• <cstdio> (stdio.h)
• <cstdlib> (stdlib.h)
• <cstring> (string.h)
• C++11 <ctgmath> (tgmath.h)
• <ctime> (time.h)
• C++11 <cuchar> (uchar.h)
• <cwchar> (wchar.h)
• <cwctype> (wctype.h)

Containers:

• C++11 <array>
• <deque>
• C++11 <forward_list>
• <list>
• <map>
• <queue>
• <set>
• <stack>
• C++11 <unordered_map>
• C++11 <unordered_set>
• <vector>

Input/Output:

• <fstream>
• <iomanip>
• <ios>
• <iosfwd>
• <iostream>
• <istream>
• <ostream>
• <sstream>
• <streambuf>

Multi-threading:

• C++11 <atomic>
• C++11 <condition_variable>
• C++11 <future>
• C++11 <mutex>
• C++11 <thread>
• <algorithm>
• <bitset>
• C++11 <chrono>
• C++11 <codecvt>
• <complex>
• <exception>
• <functional>
• C++11 <initializer_list>
• <iterator>
• <limits>
• <locale>
• <memory>
• <new>
• <numeric>
• C++11 <random>
• C++11 <ratio>
• C++11 <regex>
• <stdexcept>
• <string>
• C++11 <system_error>
• C++11 <tuple>
• C++11 <type_traits>
• C++11 <typeindex>
• <typeinfo>
• <utility>
• <valarray>

class templates

• basic_string
• char_traits
• C++11 u16string
• C++11 u32string
• C++11 stold
• C++11 stoll
• C++11 stoul
• C++11 stoull
• C++11 to_string
• C++11 to_wstring
• string::~string
• string::string

member functions

• string::append
• string::assign
• C++11 string::back
• string::begin
• string::c_str
• string::capacity
• C++11 string::cbegin
• C++11 string::cend
• string::clear
• string::compare
• string::copy
• C++11 string::crbegin
• C++11 string::crend
• string::data
• string::empty
• string::end
• string::erase
• string::find
• string::find_first_not_of
• string::find_first_of
• string::find_last_not_of
• string::find_last_of
• C++11 string::front
• string::get_allocator
• string::insert
• string::length
• string::max_size
• string::operator[]
• string::operator+=
• string::operator=
• C++11 string::pop_back
• string::push_back
• string::rbegin
• string::rend
• string::replace
• string::reserve
• string::resize
• string::rfind
• C++11 string::shrink_to_fit
• string::size
• string::substr
• string::swap

member constants

• string::npos

non-member overloads

• getline (string)
• operator+ (string)
• operator<< (string)
• >/" title="operator>> (string)"> operator>> (string)
• relational operators (string)
• swap (string)

std:: string ::assign

Return value, iterator validity, exception safety.