R cannot be resolved to a variable

public class MainActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        LinearLayout layout = (LinearLayout) findViewById(R.id.layout);
            TextView textView = new TextView(this);
            textView.setText("Text View ");
            LinearLayout.LayoutParams p = new LinearLayout.LayoutParams(
                    LinearLayout.LayoutParams.FILL_PARENT,
                    LinearLayout.LayoutParams.WRAP_CONTENT
            );
            layout.addView(textView, p);

            Button buttonView = new Button(this);
            buttonView.setText("Button");
            buttonView.setOnClickListener(mThisButtonListener);
            layout.addView(buttonView, p);
        
    }
    private OnClickListener mThisButtonListener = new OnClickListener() {
        public void onClick(View v) {
            Toast.makeText(MainActivity.this, "Hello !",
                    Toast.LENGTH_LONG).show();
        }
    };
}

LinearLayout layout = (LinearLayout) findViewById(R.id.layout); // delete R.id.layout
LinearLayout layout = (LinearLayout) findViewById(); 
LinearLayout layout = (LinearLayout) findViewById(R.id.layout); // re-write this code R.id.layout

<include  layout="@layout/my_layout" android:id="@+id/idMyLayout" />

public void myFunction(){
 for (int i = 0; i < 1; i++) {
	// some code
       int c = 10;
       if(c==10)
         break; // goto stop; in C++
 } // end of for
 // stop: // break moved process to end of for
 


 // next code

}

		// declaration and assignment of value type long
		long n = 22337203685477580L;
		// print formated value
		System.out.printf("The value of x is %d%n", n); // 22337203685477580
		System.out.format("%+,8d%n%n", n); // +22 337 203 685 477 580

		// declaring more variables in single statement
		long lo1 = 12L, lo2 = 56, lo3 = 1455555555589L;

		// long range of value
		System.out.println(Long.MAX_VALUE); // 9223372036854775807
		System.out.println(Long.MIN_VALUE); // -9223372036854775808

		// check if a string is a valid number in Java example
		// convert string to long Java example
		String sLong = "1288888888888888";
		long longParse = Long.parseLong(sLong);

		// convert strings to numbers
		long longFromString = (Long.valueOf(sLong)).longValue();

		// long to string in Java example code
		Long longObj = new Long(229999999999L);
		String str = longObj.toString();
		// else
		Long longS = 888888888888L;
		String strLong = longS.toString();

		// compare two long variables
		Long longComp1 = 5555L;
		if (longComp1.equals(55555555L))
			System.out.print("true");

		// compares the two specified long values in Java example

		int i = longS.compareTo(444444L); // -1 first < second
		// 0 first == second
		// 1 first > second
		System.out.print(i);