Serial Monitor in Arduino || using LED || simple to complex

code for opening serial monitor:

void setup(){
}
void loop(){
delay(500);
}

Code No. 2:

int yellow=9;
void setup(){
pinMode(yellow,OUTPUT);
}
void loop(){
for(int i=1; ;i++){
digitalWrite(yellow,HIGH);
delay(500);
digitalWrite(yellow,LOW);
delay(500);
}
}

Code No. 3:

int yellow=9;
void setup(){
pinMode(yellow,OUTPUT);
}
void loop(){

for(int i=1;i=a;i++){
digitalWrite(yellow,HIGH);
delay(500);
digitalWrite(yellow,LOW);
delay(500);
}

}

Code No. 4:

int yellow=9;

void setup(){
pinMode(yellow,OUTPUT);
}

void loop(){
switch(a){

case 1:
analogWrite(yellow,20);
break;

case 2:
analogWrite(yellow,40);
break;
case 3:
analogWrite(yellow,60);
break;

case 4:
analogWrite(yellow,80);
break;
case 5:
analogWrite(yellow,100);
break;

case 6:
analogWrite(yellow,120);
break;
case 7:
analogWrite(yellow,140);
break;

case 8:
analogWrite(yellow,180);
break;
case 9:
analogWrite(yellow,200);
break;
case 10:
analogWrite(yellow,220);
break;
case 11:
analogWrite(yellow,255);
break;
}
}