String Class in C++

There is no native “string” data type in C++ but its <string> library class provides a string object that emulates a string data type. To make this available to a program, the library must be added with an #include <string> directive at the start of the program.

#include<iostream>
#include<string.h>using namespace std;
int main()
{
    char str[50];
    int len;
    cout << "Enter an array or string : ":
    gets(str);
    len = strlen(str);
    cout << "Length of the string is : " << len;
    return 0;
}

Like the <iostream> class library, the <string> library is part of the std namespace that is used by the C++ standard library classes. That means that a string object can be referred to as std::string, or more simply as string when using namespace std; again the directive must be at the start of the program.

Initializing Strings

A string “variable” can be declared in the same way as other variables. The declaration may optionally initialized the variable using the = assignment operator, or it may be initialized later in the program. Additionally a string variable may be initialized by including a text string between parentheses after the variable name.

Text strings in C++ must always be enclosed with double quotes(“”).
Single quotes (‘’) are only used for character values of the char data type.

Any numeric values that are assigned to a string variable, are no longer a numeric data type, so attempting to add string values of “4” and “5” with the addition operator(+) would add up to “45” instead of 9.

Converting Strings to other Data Types

Arithmetic cannot be performed on numeric values assigned to string variables until they are converted to a numeric data type. Luckily, there is a C++ <sstream> library provides a “stringstream” object that acts as an intermediary to convert strings to other data types.

Other features of a string variable can be revealed by calling its size(), capacity(), and empty() functions. Written below is short summary of other features.

• a string variable can be emptied by assigning it an empty string (=“”) or by calling its clear() function.
• Multiple string values can be concatenated by the + operator
• A string can be can be appended to another string by the += operator or by calling its append() function.
• A string can be compared to another string by the == operator or by calling its append() function.
• A string can be assigned to a string variable using the = operator or by calling its assign() function.
• The swap() function swaps the values of two string variables.
• Substrings of a string can be sought with the find() function, or specialized functions such as find_first_of(), and a character retrieved from a specified index position by the at() function.

Next we will be focusing more on control structure of the flow such as while loops, do-while loops, and for loops in addition to using the switch case for complex conditional tests.

What’s in an Array?

An array is a variable that can store multiple items of data unlike a regular variable that stores one pierce of data. Just like any variable, arrays must be declared before they can be accessed.

Initializing Arrays

You can initialize a simple array of built-in types, like integers (int) or characters (char), when you first declare the array. After the array name, put an equal sign and a list of comma separated values enclosed in the braces.

int nums[10] = { 0, 10, 20, 30, 40, 50, 60 ,70, 80, 90 }; 

This declared nums to be an array of 10 integers. Remember that the data are stored sequentially in array are elements that are numbered starting at zero. So nums[0] equals to 0, nums[1] equals to 10, and so on.

Arrays can be created for any data type but each element may only contain data of the same data type.

Inserting and Printing Elements

int mike[5] = {19, 10, 8, 17, 9}// change 4th element to 9
mike[3] = 9;// take input from the user and insert in third element
cin >> mike[2];// take input from the user and insert in (i+1)th element
cin >> mike[i];// print first element of the array
cout << mike[0];// print ith element of the array
cout >> mike[i-1];

Character Arrays

An array of characters can be used to store a string of text if the final elements contains the special \0 null character. For example:

char name[5] = {'p', 'n', 'o', 'p', '\0'};

Because this character-by-character approach is difficult to type and admits too many opportunities for error, C++ enables a shorthand form of string initialization using a literal:

char name[] = "pnop";

This form of initialization doesn’t require the null character; the compiler adds it automatically. The string “pnop” is 5 bytes including null.

Multidimensial Arrays

Collectively elements in an array is known as an index. Arrays can have more than one index. Arrays are suitable for data that consists of a known number of elements like a chessboard or coordinates which would be good examples in need of a two dimensional array.

For example:

int coordinates[2][3] = {{1,2,3} , {4,5,6}};

A three-dimensional array has three subscripts:

int cube[5,13,8];

Run the program below to see the output.

https://repl.it/join/hxlyjcst-thanhnguyen91

C-Style Character String

C++ supports a wide range of functions that can manipulate null-terminated strings. The header file <cstring> defines several functions to manipulate C strings and arrays.

╔═════════════════╦════════════════════════════════════════════╗
║ Keyword         ║ Functions and Purpose                      ║
╠═════════════════╬════════════════════════════════════════════╣
║ strcpy(s1,s2)   ║ copies string s2 into string s1.           ║        ╠═════════════════╬════════════════════════════════════════════╣
║ strcat(s1,s2)   ║ concatenates string s2 onto the end of     ║
║                 ║ string s1.                                 ║
╠═════════════════╬════════════════════════════════════════════╣
║ strlen(s1)      ║ Returns the length of string s1;           ║             ╠═════════════════╬════════════════════════════════════════════╣               
║ strcmp(s1,s2)   ║ Returns 0 if s1 and s2 are the same;       ║
║                 ║ less than 0 if s1<s2; greater than 0 if    ║                                       ║                 ║ if s1>s2.                                  ║
╠═════════════════╬════════════════════════════════════════════╣
║ strchr(s1,ch)   ║ Returns a pointer to the first occurrence  ║            ║                 ║ of character ch in string s1.              ║
╠═════════════════╬════════════════════════════════════════════╣
║ strstr(s1,s2)   ║ Returns a pointer to the first string s2   ║         
║                 ║ in string s1.                              ║
╚═════════════════╩════════════════════════════════════════════╝

The header file <cstring> defines several functions to manipulate C strings and arrays.

Operators are the foundation of any programming language. Thus the functionality of C/C++ programming language is incomplete without the use of operators. We can define operators as symbols that help us to perform specific mathematical and logical computations on operands. In other words, we can say that an operator operates the operands.
For example, consider the below statement:

c = a + b;

Here, ‘+’ is the operator known as addition operator and ‘a’ and ‘b’ are operands. The addition operator tells the compiler to add both of the operands ‘a’ and ‘b’.

C/C++ has many built-in operator types and they are classified as follows:

1. Arithmetic Operators: These are the operators used to perform arithmetic/mathematical operations on operands. Examples: (+, -, *, /, %,++,–). Arithmetic operator are of two types:
   1. Unary Operators: Operators that operates or works with a single operand are unary operators. For example: (++ , –)
   2. Binary Operators: Operators that operates or works with two operands are binary operators. For example: (+ , – , * , /)
   To learn Arithmetic Operators in details visit this link.
2. Relational Operators: These are used for comparison of the values of two operands. For example, checking if one operand is equal to the other operand or not, an operand is greater than the other operand or not etc. Some of the relational operators are (==, >= , <= ). To learn about each of these operators in details go to this link.
3. Logical Operators:  Logical Operators are used to combine two or more conditions/constraints or to complement the evaluation of the original condition in consideration. The result of the operation of a logical operator is a boolean value either true or false. For example, the logical AND represented as ‘&&’ operator in C or C++ returns true when both the conditions under consideration are satisfied. Otherwise it returns false. Therfore, a && b returns true when both a and b are true (i.e. non-zero). To learn about different logical operators in details please visit this link.
4. Bitwise Operators: The Bitwise operators is used to perform bit-level operations on the operands. The operators are first converted to bit-level and then the calculation is performed on the operands. The mathematical operations such as addition, subtraction, multiplication etc. can be performed at bit-level for faster processing. For example, the bitwise AND represented as & operator in C or C++ takes two numbers as operands and does AND on every bit of two numbers. The result of AND is 1 only if both bits are 1. To learn bitwise operators in details, visit this link.
5. Assignment Operators: Assignment operators are used to assign value to a variable. The left side operand of the assignment operator is a variable and right side operand of the assignment operator is a value. The value on the right side must be of the same data-type of variable on the left side otherwise the compiler will raise an error.
   Different types of assignment operators are shown below:
   1. “=”: This is the simplest assignment operator. This operator is used to assign the value on the right to the variable on the left.
      For example:a = 10; b = 20; ch = ‘y’;
   2. “+=”: This operator is combination of ‘+’ and ‘=’ operators. This operator first adds the current value of the variable on left to the value on right and then assigns the result to the variable on the left.
      Example:(a += b) can be written as (a = a + b) If initially value stored in a is 5. Then (a += 6) = 11.
   3. “-=”: This operator is combination of ‘-‘ and ‘=’ operators. This operator first subtracts the value on right from the current value of the variable on left and then assigns the result to the variable on the left.
      Example:(a -= b) can be written as (a = a – b) If initially value stored in a is 8. Then (a -= 6) = 2.
   4. “*=”: This operator is combination of ‘*’ and ‘=’ operators. This operator first multiplies the current value of the variable on left to the value on right and then assigns the result to the variable on the left.
      Example:(a *= b) can be written as (a = a * b) If initially value stored in a is 5. Then (a *= 6) = 30.
   5. “/=”: This operator is combination of ‘/’ and ‘=’ operators. This operator first divides the current value of the variable on left by the value on right and then assigns the result to the variable on the left.
      Example:(a /= b) can be written as (a = a / b) If initially value stored in a is 6. Then (a /= 2) = 3.
6. Other Operators: Apart from the above operators there are some other operators available in C or C++ used to perform some specific task. Some of them are discussed here:
   1. sizeof operator: sizeof is a much used in the C/C++ programming language. It is a compile time unary operator which can be used to compute the size of its operand. The result of sizeof is of unsigned integral type which is usually denoted by size_t. Basically, sizeof operator is used to compute the size of the variable. To learn about sizeof operator in details you may visit this link.
   2. Comma Operator: The comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, it then evaluates the second operand and returns this value (and type). The comma operator has the lowest precedence of any C operator. Comma acts as both operator and separator. To learn about comma in details visit this link.
   3. Conditional Operator: Conditional operator is of the form Expression1 ? Expression2 : Expression3 . Here, Expression1 is the condition to be evaluated. If the condition(Expression1) is True then we will execute and return the result of Expression2 otherwise if the condition(Expression1) is false then we will execute and return the result of Expression3. We may replace the use of if..else statements by conditional operators. To learn about conditional operators in details, visit this link.

Operator precedence chart

The below table describes the precedence order and associativity of operators in C / C++ . Precedence of operator decreases from top to bottom.

In C, we could use the function freopen() to redirect an existing FILE pointer to another stream. The prototype for freopen() is given as

FILE * freopen ( const char * filename, const char * mode, FILE * stream );

For Example to redirect the stdout to say a textfile, we could write

freopen ("text_file.txt", "w", stdout);

While this method is still supported in C++, this article discusses another way to redirect I/O streams.

C++ being an object-oriented programming language gives us the ability to not only define our own streams but also redirect standard streams. Thus in C++, a stream is an object whose behavior is defined by a class. Thus anything that behaves like a stream is also a stream.

Streams Objects in C++ are mainly of three types :

• istream : Stream object of this type can only perform input operations from the stream
• ostream : These objects can only be used for output operations.
• iostream : Can be used for both input and output operations

All these classes, as well as file stream classes, derive from the classes: ios and streambuf. Thus filestream and IO stream objects behave similarly.

All stream objects also have an associated data member of class streambuf. Simply put streambuf object is the buffer for the stream. When we read data from a stream, we don’t read it directly from the source, but instead, we read it from the buffer which is linked to the source. Similarly, output operations are first performed on the buffer, and then the buffer is flushed (written to the physical device) when needed.

C++ allows us to set the stream buffer for any stream. So the task of redirecting the stream simply reduces to changing the stream buffer associated with the stream. Thus the to redirect a Stream A to Stream B we need to do

1. Get the stream buffer of A and store it somewhere
2. Set the stream buffer of A to the stream buffer of B
3. If needed reset the stream buffer of A to its previous stream buffer

We can use the function ios::rdbuf() to perform two opeations.

1) stream_object.rdbuf(): Returns pointer to the stream buffer of stream_object
2) stream_object.rdbuf(streambuf * p): Sets the stream buffer to the object pointed by p