child

The child filter returns the primary child of the current position. Thus,

  x=child

will set the variable x to the primary child of the current position (if the current position is a terminal position, then the assignment will not match and x will not be modified). Recall that if variations is not set in the CQL header, then the primary child of the current position is the same as the child, as any position has at most one child.

If the child filter is followed by a numeric filter enclosed in parentheses, then the the child with index i is returned, where i is the value of that numeric filter:

  x=child(1)

If no such child exists, the filter will not match. Thus child without an argument is the same as child(0).

Getting the number of children

You can use the following function to compute the number of children of the current position:

  function nchildren(){
   nChildrenVal=0
   if loop{
    child(nChildrenVal)
    nChildrenVal+=1
    } then nChildrenVal else 0}

After inserting the above into a CQL file, the expression nchildren() has as value the number of children in the current position.

The loop here will loop endlessly until its body fails, which can only happen when child(nChildrenVal) fails. At that point, nChildrenVal has value of the number of children of the current position. The if/then/else circumlocution is necessary because loop will not match if its body never matched.

Examples

The child filter is used in the examples: knightpawnforkecho.cql, parallelpaths.cql, and queenpawnpinecho.cql.

In parallelpaths.cql for instance, the variable current1 holds a position. The expression

  current1=current:child

sets current1 to its child, if it exists (and fails to match if the child does not exist, that is, if current1 is terminal)