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javafor-loop
keser
keser

Reputation: 2642

Java declaring iterator outside for loop

According to the documentation of Oracle for loop is formed as we know it:

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

E.g.,

class ForDemo {
    public static void main(String[] args){
         for(int i=1; i<11; i++){
              System.out.println("Count is: " + i);
         }
    }
}

Why can't we declare the initialization part outside the for loop like this? 

 class ForDemo {
    public static void main(String[] args){
         int i = 1;
         for(i; i<11; i++){
              System.out.println("Count is: " + i);
         }
    }
}

Upvotes: 8

Views: 3519

Answers (4)

keser
keser

Reputation: 2642

Quoting Java Docs

it's best to declare the variable in the initialization expression. The names i, j, and k are often used to control for loops; declaring them within the initialization expression limits their life span and reduces errors. The three expressions of the for loop are optional; an infinite loop can be created as follows:

// infinite loop
for ( ; ; ) {

    // your code goes here
}

So, I missed it. It is possible, however not favorable. Leaving initialization part blank is the solution

  for (  ;i <= 10 ; i++ ) {

        // i is defined outside already
    }

Upvotes: 0

Gatusko
Gatusko

Reputation: 2598

What is really happening in the for loop that 

BasicForStatement:
    for ( ForInit ; Expression; ForUpdate )

Initialization need a statment as the docs says

If the ForInit code is a list of statement expressions

From Java Docs

So in this code

 for(i; i<11; i++){ 
      System.out.println("Count is: " + i);
 }

i is not a statment, it is just a variable. So what is a statment?

Statements are roughly equivalent to sentences in natural languages. A statement forms a complete unit of execution. The following types of expressions can be made into a statement by terminating the expression with a semicolon (;).

Assignment expressions
Any use of ++ or --
Method invocations
Object creation expressions

Whit this knowlodge you can work any for loop if you know what is statemnt for example this for loop works 

int i = 1; // Initializated
for(i++; i<11; i++){ // Whit a statemnt
    System.out.println("Count is: " + i);
}

and the output will be :

Count is: 2
Count is: 3
Count is: 4
Count is: 5
Count is: 6
Count is: 7
Count is: 8
Count is: 9
Count is: 10

Upvotes: 4

GBlodgett
GBlodgett

Reputation: 12819

You can. However you would simply have a blank ; in where the initialization usually goes:

int i = 1;
for(; i<11; i++){
    System.out.println("Count is: " + i);
}

The difference of this is that the scope of i is now broadened to outside of the loop. Which may be what you want. Otherwise it is best to keep variables to the tightest scope possible. As the docs for the for loop say:

declaring them within the initialization expression limits their life span and reduces errors.

Output:

Count is: 1
Count is: 2
Count is: 3
Count is: 4
Count is: 5
Count is: 6
Count is: 7
Count is: 8
Count is: 9
Count is: 10

Upvotes: 5

JFPicard
JFPicard

Reputation: 5168

You can with:

 for(; i<11; i++){
      System.out.println("Count is: " + i);
 }

But the scope of i is different. i will now exist outside of the loop.

Upvotes: 9

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