Difference Between #include ‹filename› and #include 'filename'

In C++, the #include directive is used to include header files in a program. There are two common ways to include header files: using angle brackets (< >) and using double quotes (" "). The difference between #include <filename> and #include "filename" lies in the way the compiler searches for the header file.

#include <filename>

When you use angle brackets, the compiler searches for the header file in the standard system directories. These directories contain the standard library headers and other system-specific headers. Typically, these headers are not part of your program's source code but are provided by the compiler or the system.

Example Code:

#include <iostream>

int main() {
    std::cout << "Hello, world!" << std::endl;
    return 0;
}

In this example, #include <iostream> is used to include the standard input/output stream header file. The compiler knows where to find this file in the standard system directories.

#include "filename"

When you use double quotes, the compiler searches for the header file in the current directory or the directory specified by the -I flag (include directory) during compilation. This method is typically used for including user-defined header files or headers located within the project directory.

Example:

Suppose you are working on a C++ project that involves creating a game. You have a file called "game.h" that contains the declarations of classes, functions, and variables specific to your game. In order to use these declarations in your main program file, you would include the "game.h" header file using double quotes.

How does #include ‹bits/stdc++.h› work in C++?

The #include <bits/stdc++.h> is a preprocessor directive commonly used in C++ programming to include a large set of standard library headers in a single line. It is not a standard header provided by the C++ Standard Library, but rather a non-standard header specific to certain compilers, such as the GNU C++ compiler (g++).

When you include <bits/stdc++.h>, it includes a collection of commonly used standard library headers, such as <iostream>, <vector>, <algorithm>, <string>, and more. It saves you from manually including multiple individual headers that you might need for your program.

Here's an example of how to use #include <bits/stdc++.h> in a C++ program:

//C++ code Implementation
#include <bits/stdc++.h>

int main() {
    std::vector<int> numbers = {3, 1, 4, 1, 5, 9, 2, 6, 5};
    //sorting the array
    std::sort(numbers.begin(), numbers.end());

    for (const auto& num : numbers) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

Output

1 1 2 3 4 5 5 6 9

In this example, we use #include <bits/stdc++.h> at the beginning of the program. This single line effectively includes all the necessary standard library headers for our program. We then proceed to use various standard library features, such as std::vector, std::sort, and std::cout, without explicitly including their individual headers.

Advantages of using <bits/stdc++.h>:

Including <bits/stdc++.h> allows you to include a large set of commonly used standard library headers in a single line. It saves you from the hassle of including multiple individual headers manually. (alert-success)

With <bits/stdc++.h>, you don't need to spend time figuring out and including the specific headers you need for your program. It provides a quick way to get started with coding and prototyping. (alert-success)

<bits/stdc++.h> is supported by some popular compilers, such as the GNU C++ compiler (g++). If you are using a compiler that supports it, you can rely on the convenience of including a comprehensive set of headers without worrying about portability issues. (alert-success)

 

Disadvantages of using <bits/stdc++.h>:

<bits/stdc++.h> is not a standard C++ header. It is a non-standard header specific to certain compilers. This means that it may not be available or supported by all compilers. Using non-standard headers can make your code less portable and dependent on specific compiler implementations. (alert-error)

Including <bits/stdc++.h> pulls in a large number of headers, many of which you may not need for your specific program. This can increase the compilation time of your code unnecessarily, as the compiler needs to process and parse all the included headers. (alert-error) 

<bits/stdc++.h> includes a wide range of headers, potentially introducing a lot of names into the global namespace. This can lead to naming conflicts and make the code less readable. It is generally considered good practice to explicitly include only the headers you need to avoid polluting the namespace. (alert-error)

It is worth noting that the use of #includes <bits/stdc++.h> is not recommended in professional or production code because it is not a standardized header and may not be supported by all compilers. It is more common to include the specific headers you need for your program, as it provides better control over the dependencies and improves code portability.

Difference Between Pointer and Reference Variable in C++

In C++, both pointer variables and reference variables are used to indirectly access and manipulate data. However, there are some important differences between the two. Here's an explanation of the difference between pointer variables and reference variables, along with examples:

Pointer Variables:

• A pointer variable is a variable that stores the memory address of another variable.

• It is declared using the * symbol before the variable name.

• Pointers can be reassigned to point to different memory addresses.

• Pointers can be assigned a nullptr value to indicate that they are not currently pointing to any valid memory address.

• Pointers can perform pointer arithmetic.

• To access the value at the memory address stored in a pointer variable, we need to use the dereference operator *.

• Pointers can be made to point to dynamically allocated memory using the new keyword.

C++ Example code for the pointer:

#include<iostream>
using namespace std;

int main() {
    int num = 10;
    int* ptr = &num; // pointer variable

    cout << *ptr << endl; // Output: 10 (dereferencing the pointer)

    *ptr = 20; // changing the value using the pointer
    cout << num << endl; // Output: 20 (value updated)

    return 0;
}

Output:

10
20

Reference Variables:

• A reference variable is an alias or alternative name for an existing variable.

• It is declared using the & symbol after the variable name.

• References must be initialized when declared and cannot be reassigned to refer to a different variable.

• References cannot be nullptr and must always refer to a valid object.

• References do not have a separate memory address; they are simply another name for the original variable.

• To access the value of a reference variable, we use the reference variable itself.

C++ Example Code for Reference Variable.

#include<iostream>
using namespace std;

int main() {
    int num = 10;
    int& ref = num; // reference variable

    cout << ref << endl; // Output: 10 (directly accessing the reference)

    ref = 20; // changing the value using the reference
    cout << num << endl; // Output: 20 (value updated)

    return 0;
}

Output:

10
20

Pointer Vs Preference Variable in C++.

Pointer Variable	Reference Variable
The pointer is Declared using * symbol.	Reference is Declared using & symbol.
Can be initialized to nullptr or memory address.	Must be initialized upon declaration to refer to an existing variable.
Can be reassigned to point to different memory locations.	Cannot be reassigned once initialized; it always refers to the same object.
Requires explicit memory allocation and deallocation (e.g., new and delete).	Does not require explicit memory allocation or deallocation.
Can point to nullptr, which means it doesn't point to any valid memory location.	Cannot be null; must always refer to a valid object.
Syntax: int* ptr;	Syntax: int value = 10; int& ref = value;

When to use what:

Use pointers when you need to deal with dynamic memory allocation, create data structures, or perform pointer arithmetic. Also, use pointers when you want to point to multiple different objects during the program's execution or when you need a nullable reference. (alert-success)

Use reference variables when you want to avoid the overhead of copying large objects in function calls, want to modify the value of an existing variable directly, or when want a more convenient and safer alternative to pointers for passing parameters to functions. 

Conclusion:

The main difference between pointer variables and reference variables in C++ is that pointers store memory addresses, can be reassigned, and require dereferencing to access the value, while references are aliases for existing variables, cannot be reassigned, and allow direct access to the value without dereferencing.

What is --> Operator in C++?

In C++, the --> operator is not a valid operator. The --> operator is often mistakenly used in place of the decrement operator -- followed by the greater than operator >.

The decrement operator -- reduces the value of a variable by 1, while the greater than operator > is used for comparison between two values. When combined, the -- followed by > operator sequence forms the compound operator -->.

C++ example code snippet that demonstrates the usage of the --> operator sequence:

//C++ code for explaning --> operator
#include <iostream>
using namespace std;

int main() {
    int a = 10;
    int b = 5;

    if (a-- > b) {
        cout << "a is greater than b" << endl;
    } else {
        cout << "b is greater than a" << endl;
    }
    cout<<"Value of a: " << a;

    return 0;
}

Output:

a is greater than b
Value of a: 9

In this code, we are using the --> operator sequence as a-- > b in the if condition to compare the values of a and b. The postfix decrement operator -- reduces the value of a by 1 after the comparison is performed, and then the greater than operator > compares the new value of a with the value of b.

The output of this program is a is greater than b because the value of a is initially 10 and the value of b is 5. After the comparison, the value of a becomes 9 and is greater than the value of b.

It is important to note that the --> operator is not a standard operator in C++, and using it may lead to unexpected results. It is always better to use standard operators in your code to ensure proper functioning and readability.

How To Compare Strings in Java

In Java, a String is a sequence of characters. It is a reference type, meaning it is an object rather than a primitive data type. Strings are immutable, which means that their values cannot be changed after they are created.

There are several ways to compare two strings in Java and we are going to discuss three methods out of them.

1. Using the equals() method: This method compares the contents of the strings and returns a boolean value. For strings, the equals() method is used to compare the characters of two strings to check if they are equal or not.

Java Example Code:

class StringComp {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "Hello";
        String str3 = "World";

        if(str1.equals(str2)) {
            System.out.println("str1 is equal to str2");
        }
        else {
            System.out.println("str1 is not equal to str2");
        }

        if(str1.equals(str3)) {
            System.out.println("str1 is equal to str3");
        }
        else {
           System.out.println("str1 is not equal to str3");
        }

    }
}

Output:

str1 is equal to str2
str1 is not equal to str3

2. Using the compareTo() method: This method compares the lexicographical ordering of the strings and returns an integer value. It returns 0 if the strings are equal, a negative value if the first string is lexicographically less than the second, and a positive value if the first string is lexicographically greater than the second.

Java Example Code:

//Java code to compare two strings
class StringComp {
    public static void main(String[] args) {
        String str1 = "Apple";
        String str2 = "Banana";
        int result = str1.compareTo(str2);
        if (result == 0) {
            System.out.println("Strings are equal");
        } else if (result < 0) {
            System.out.println("String 1 is less than String 2");
        } else {
            System.out.println("String 1 is greater than String 2");
        }
    }
}

Output:

String 1 is less than String 2

3. Using the equalsIgnoreCase() method: The equalsIgnoreCase() method in Java is used to compare two strings for equality, ignoring the case of the characters. In other words, this method compares two strings by ignoring their case and returns true if they are equal or false if they are not.

Java Example Code:

//Java code to compare two with case strings
class StringComp {
    public static void main(String[] args) {
        String str1 = "HELLO";
        String str2 = "hello";
        if (str1.equalsIgnoreCase(str2)) {
           System.out.println("Strings are equal");
    }
}

Output:

Strings are equal

Why we should not use == for String Comparison in Java?

In Java, the == operator compares the reference or memory address of two objects, not their actual values. When comparing strings, it is necessary to compare their actual values, not just their memory addresses. Therefore, using the == operator to compare two string variables may lead to unexpected results.

Java Example Code:

class StringCompare {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "Hello";
        String str3 = new String("Hello");

        System.out.println(str1 == str2); // true
        System.out.println(str1 == str3); // false
    }
}

Output:

true
false

In the above example, str1 and str2 both refer to the same memory address since they are assigned the same string literal value. Therefore, the == operator returns true for their comparison. However, str3 is a new String object with a different memory address, even though it contains the same value as str1. Hence, the == operator returns false for the comparison of str1 and str3.

How To Fix NullPointerException in Java

In Java, a NullPointerException is a runtime exception that occurs when you try to access or call a method on a null object reference. The null reference is a special reference that does not point to any object in the heap. 

If you try to access or call a method on a null reference, Java throws a NullPointerException. This can be a common error in Java programs, especially for beginners. In this article, we will discuss how to prevent and handle NullPointerException in Java.

Causes of NullPointerException

A NullPointerException occurs when you try to access or call a method on a null object reference. Here are some common scenarios where NullPointerException can occur:

• Calling methods on a null object reference.

• Accessing instance or static variables on a null object reference.

• Passing a null reference to a method that expects a non-null reference.

• Trying to access or modify an array element with a null reference.

Preventing NullPointerException

The best way to prevent NullPointerException is to make sure that you never access or call a method on a null object reference. Here are some tips to prevent NullPointerException:

• Always initialize object references before using them.

• Check for null before accessing or calling methods on object references.

• Use try-catch blocks to catch NullPointerException and handle it gracefully.

• Use the Objects.requireNonNull method to check for null references.

Handling NullPointerException

In some cases, it may be impossible to prevent NullPointerException. In such cases, you should handle the exception gracefully to avoid program termination. Here are some tips to handle NullPointerException:

1. Use try-catch blocks to catch NullPointerException and handle it gracefully. For example:

try {
    // code that may throw NullPointerException
} catch (NullPointerException e) {
    // handle NullPointerException
}

2. Use the Objects.requireNonNull method to check for null references and throw a custom exception. For example:

public void doSomething(Object obj) {
    Objects.requireNonNull(obj, "Object is null");
    // rest of the code
}

3. Use the Optional class to handle null values. The Optional class provides methods to check for null values and handle them gracefully. For example: 

Optional<String> optional = Optional.ofNullable(null);
if (optional.isPresent()) {
    String value = optional.get();
    // do something with value
} else {
    // handle null value
}

Conclusion.

NullPointerException is a common error in Java programs, but it can be prevented and handled gracefully. Always make sure to initialize object references before using them, check for null before accessing or calling methods on object references, and handle NullPointerException gracefully to avoid program termination.

15 Git Commands Every Developer Should Know.

Git is a distributed version control system designed to manage software code changes and collaboration between developers. It is an essential tool for any software development project as it helps developers to track changes to their codebase, work on code collaboratively with other team members, and maintain different versions of their codebase.

In this article, I am going to share the list of 15 important git commands that every developer should know because you all are going to use them once in your coding journey.

• git init: Initializes a new Git repository in your local directory.

• git clone: Creates a copy of an existing Git repository on your local machine.

• git add: Adds changes to your working directory to the staging area for the next commit.

• git commit: Creates a new commit with the changes from the staging area and adds a commit message.

• git status: Shows the status of your working directory and staging area, including which files have been modified, added, or deleted.

• git log: Shows a list of all commits in the repository, including the commit message, author, and timestamp.

• git branch: Lists all branches in the repository or creates a new branch.

• git checkout: Switches to a different branch or restores files from a previous commit.

• git merge: Merges changes from one branch into another.

• git push: Uploads your local commits to a remote repository.

• git pull: Downloads changes from a remote repository to your local machine.

• git fetch: Downloads changes from a remote repository to your local machine, but does not merge them into your local branches.

• git reset: Resets changes in your working directory to a previous commit or unstages changes from the staging area.

• git revert: Reverses a commit and creates a new commit with the changes reversed.

• git tag: Creates a new tag for a specific commit or lists all existing tags.

These are just some of the many Git commands available, but they cover the most essential tasks you need to manage your Git repository effectively.

C++ Program for Concatenation of Array.

Given an integer array nums of length n, create a new array ans of length 2n such that ans is the concatenation of two nums arrays. 

• ans[i] = nums[i] and ans[i+n] = nums[i]
• 0 <= i < n

Example:
Input: nums = [3, 2, 4]
Output: ans = [3, 2, 4, 3, 2, 4]

Input: nums = [1, 3, 2, 4]
Output: ans = [1, 3, 2, 4, 1, 3, 2, 4]

Explaination: 

ans = [nums[0], nums[1], nums[2], nums[3], nums[0], nums[1], nums[2], nums[3]]

Algorithm 1:

• Declare two integer arrays nums and ans of size n and 2n respectively.

• Read the values of the array nums from the user.

• Use a loop to copy the elements of nums into the first half of ans, i.e., from ans[0] to ans[n-1].

• Use another loop to copy the elements of nums into the second half of ans, i.e., from ans[n] to ans[2n-1].

• Print the elements of ans to the console.

C++ Code Implementation:

//C++ Program to concatenation of array
#include<iostream>
using namespace std;

//Concatenation function
void getConcatenation(int n, int nums[], int ans[]){
     // copying elements of nums into the first half of ans
   for(int i = 0; i < n; i++) {
      ans[i] = nums[i];
   }

   // copying elements of nums into the second half of ans
   for(int i = 0; i < n; i++) {
      ans[i+n] = nums[i];
   }
}
int main(){

    int nums[] = {1, 3, 2, 8};
    //size of given array
    int size = sizeof(nums)/sizeof(nums[0]);
    int ans[size*2];

    getConcatenation(size, nums, ans);

    for(int i = 0; i < 2*size; i++){
        cout<<ans[i]<<" ";
    }
}

Output:

1 3 2 8 1 3 2 8

• Time Complexity: O(n) where is the size of the given array.
• Space Complexity: O(2n) where 2n is the size of the answer array.

Algorithm 2:

In this algorithm, we are iterating over the output array ans and assigning nums[i % n] to ans[i], where % is the modulo operator. This ensures that we repeat the elements of nums from the beginning once we reach the end of the array. 

C++ Code Implementation:

//C++ Program to concatenation of array
#include<iostream>
#include<vector>
using namespace std;

//Concatenation function
vector<int> concatenateArrays(int n, int nums[]) {
   
    vector<int> ans(2 * n);
    for (int i = 0; i < 2 * n; i++) {
        ans[i] = nums[i % n];
    }
    return ans;
}
 
int main(){

    int nums[] = {1, 3, 2, 8};
    //size of given array
    int size = sizeof(nums)/sizeof(nums[0]);

    vector<int> ans  = concatenateArrays(size, nums);

    for(int i = 0; i < 2*size; i++){
        cout<<ans[i]<<" ";
    }
}

Output:

1 3 2 8 1 3 2 8

• Time Complexity: O(n) where is the size of the given array.
• Space Complexity: O(2n) where 2n is the size of the answer array.