What is the lookahead symbol?

Question

In the grammars (for example LL(1)), 1 denotes the lookahed symbol. In practice i don't understand what is this symbol. To understand, i need a simple and practical example.

Answer 1

LL(1) grammars help you decide immediately which grammar rule you will use. This one lookahead token means that you only have to read the next one character from the current character you are reading.

LL(1) grammars help you to decrease complexity to O(n) and have no backtracking on parsing the input.

Wikipedia Example

Let % to be the character you are reading and the input string to be ( a + a )

A LL(1) grammar :

S -> F (Rule1)

S -> ( S + F ) (Rule2)

F -> a (Rule3)

Parsing Table is :

    (   )   a   +   $
S   2   -   1   -   -
F   -   -   3   -   -

Then you have:

%( a + a ) ( read the start of the string and lookahead is (, so decide to apply Rule2 according to the parsing table)

The abstract syntax tree is now :

            S
        / / | \ \
       (  S + F  )

Then you consume (. And you continue the same way.

Step 2:

            S
        / / | \ \
       (  S + F  )
          |
          F
          |
          a

Step 3:

            S
        / / | \ \
       (  S + F  )
          |   |
          F   a
          |
          a

You can see Wikipedia example that uses a stack, instead of an abstract syntax tree in the exactly same way.