For-loop type - can't recognise it

Question

In Robert Sedgewick's book i found the following code:

public class  QuickF{
 public static void main(String[] args){

    int N = 20;
    int id[] = new int[N];
    for (int i=0; i<N; i++){
        id[i] = i;
    }
    for( In.init(); !In.empty(); ){
        int p = In.getInt(), q = In.getInt();
        int t = id[p];
        if (t==id[q]) continue;
        for (int i =0; i<N; i++){
            if (id[i] == t) id[i] = id[q];
        }
        Out.println(" " +p+""+q);
    }
  }
}

My question is about this line: for( In.init(); !In.empty(); ) . What type of forr loop is this? Is this syntacticaly legal? If yes, what is the documentation of this for loop type?

Answer 1

It's a basic for loop.

It is documented in the Java Language Specification , section 14.14.1. The basic for Statement :

BasicForStatement:
  for ( [ForInit] ; [Expression] ; [ForUpdate] ) Statement

ForInit:
  StatementExpressionList
  LocalVariableDeclaration

ForUpdate:
  StatementExpressionList

StatementExpressionList:
  StatementExpression {, StatementExpression} 

Normally, you'll see it like this:

ForInit:      int i = 0   // LocalVariableDeclaration
Expression:   i < 10
ForUpdate:    i++         // StatementExpression

for ( int i = 0 ; i < 10 ; i++ ) {
    // code
}

// mostly the same as:
int i = 0;
while ( i < 10 ) {
    // code
    i++;
}

^{Mostly the same, since the scope of i is limited to the loop, and the continue statement will go to the i++ statement, not directly back to the loop.}

But as you can see, all 3 are optional, where ForInit can be a statement list instead of a variable declaration, and Expression defaults to true .

ForInit:      In.init()   // StatementExpression
Expression:   !In.empty()
ForUpdate:                // not present

for ( In.init() ; !In.empty() ;  ) {
    // code
}

// same as:
In.init();
while ( !In.empty() ) {
    // code
}

The Expression defaulting to true means that the following are the same. I personally prefer the first, since for (;;) almost reads like "forever".

for (;;) { // loop forever
    // code
}

while (true) { // loop forever
    // code
}

Answer 2

It's just a regular for-loop .

The general form of the for statement can be expressed as follows:

 for (initialization; termination; increment) { statement(s) }

It is no different from for (int i = 0; i < N; i++) , except with a different initialization and termination , and without an increment .

You are allowed to omit the increment statement because

The three expressions of the for loop are optional