Adding char to string

[surepic]

While it pains me to ignore the obvious mistake and the likely other mistakes in your snippet I will directly address your question...

a[_length+0]=id;
a[_length+1]='\0';
_length=strlen(a);

_length=strlen(a);
     Serial.println(_length); //prints 11 
     a[_length]=id;
     a[_length+1]='\0';
     _length=strlen(a);
     Serial.println(_length); //prints 11
      Serial.println(id);
    Serial.println(a);

byte id has value from atoi conversion done manually (x-48); id is holding numbers from 0 to 4.

Serial.print(a);
Serial.println(id); solves the problem but I cant understand how to add a variable to string.