Using 'using namespace std;'? But why?

Consider a simple code :

#include <iostream>
using namespace std;

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main() {

    int a;
    int b;
    a = 7;
    b = 5;
    swap(a, b);
    cout << a << b;

    return 0;
}

Now when you will try to run this code you will find that the compiler doesn't give any error. Again question is why is the compiler unable to find the error?

So this is all due to the 'using namespace std;' basically our own defined method doesn't gets called in the whole program. The std::swap method gets called and the found these values swapped.

Now this is just a small example. When we write big codes then this can be a big pain in our ass. And sorting it our will be difficult unless we are not much experienced.

Don't forget String is immutable in Java

Generally we try to manipulate the String (in Java) as we manipulate std::string in C++. But String is immutable in Java.

Now consider a program where you may need to change all the space with some string literal as "#45", then to solve this problem in place we need to use the char array to store the string. Using the String will result creation of more that one copies of String ( or one copy of String and other char array).

Consider this program :

public class Four {
    
    public static void main(String []args)
    {
        String str = new String();
        java.util.Scanner scanner = new java.util.Scanner(System.in);
        str = scanner.nextLine();
        Four obj = new Four();
        int len = str.length();
        str = obj.changedString(str, len);
        System.out.println("New String Formed : "+str);
        scanner.close();
    }
    
    public String changedString(String str, int len)
    {
        int sCount = 0;
        for(int i = 0; i < len; ++i)
        {
            if (str.charAt(i) == ' ')
                sCount++;
        }
        int newLen = (2*sCount) + len;
        
        char str2[] = new char[newLen+1];
        str2[newLen] = '\0';
        
        for (int i = len-1; i >= 0; i--) 
        {
            if (str.charAt(i) == ' ')
            {
                str2[newLen -1] = '0';
                str2[newLen -2] = '2';
                str2[newLen -3] = '%';
                newLen = newLen-3;
            }
            else
            {
                str2[newLen-1] = str.charAt(i);
                newLen = newLen - 1;
            }
        }
        return new String(str2);
    }
}

So this is not a in place algo. We need to use the char Array to make it in place.

Consider this program written in C++:

/*
program to replace all spaces in a string with '%20'. use inplace algo.
*/
#include<iostream>
#include<cstring>
int countSpaceLength(char *str, int len)
{
    int sCount = 0;
    for(int i = 0; i < len; ++i)
    {
        if(str[i] == ' ')
            sCount++;
    }
    return sCount;
}
int main()
{
    char str[500];
    std::cin.getline(str, sizeof(str));
    std::cout<<std::endl;
    int sLen = strlen(str);
    int sCount = countSpaceLength(str, sLen);
    int newStringLen = sLen + (2*sCount);
    str[newStringLen] = '\0';
    for(int i = sLen-1; i >= 0; --i)
    {
         if(str[i] == ' ')
         {
            str[newStringLen - 1] = '0';
            str[newStringLen - 2] = '2';
            str[newStringLen - 3] = '%';
            newStringLen -= 3;
         }
         else
         {
            str[newStringLen - 1] = str[i];
            newStringLen -= 1;
         }
    }
    std::cout<<"New String Formed "<<std::endl<<str<<std::endl;
    return 0;
}

We can some time use

String myName = "domanokz";
String newName = myName.substring(0,4)+'x'+myName.substring(5);

Or we can use a StringBuilder:

StringBuilder myName = new StringBuilder("domanokz");
myName.setCharAt(4, 'x');

System.out.println(myName);

Thanks