for loop without braces in java

问题

I wonder why this two following codes give different results;

for(i = 1, j = 0; i < 10; i++) {
    j += i;
    System.out.println(i);
}

This one gives the numbers between 1 included and 10 excluded.

for(i = 1, j = 0; i < 10; i++)
    j += i;
System.out.println(i);

However this one gives 10. I could not get the logic of this;

for()
do_something;
do_something_else;

回答1:

That is because the default scope of an iteration in your case is the only line following it, something equivalent to -

for(i=1, j=0;i<10;i++) {
    j += i;
}
System.out.println(i);

Hence the entire loop is iterated and since after that the value of i==10, that is your output in the second case.

---

In the first case, it's pretty obvious that the value is printed with each iteration and hence 1 to 9(less than 10) :

for(i=1, j=0;i<10;i++){
    j += i;
    System.out.println(i);
}

回答2:

The for loop, the while loop, and the if statement (ignoring else) all control a single Statement:

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

for ( {VariableModifier} UnannType VariableDeclaratorId : Expression ) Statement

while ( Expression ) Statement

if ( Expression ) Statement

That statement may be a Block, i.e. braces {} with multiple statements.

So, this code is all the same:

for(i=1, j=0;i<10;i++)
j += i;
System.out.println(i);

for(i=1, j=0;i<10;i++)
    j += i;
    System.out.println(i);

for(i=1, j=0;i<10;i++)
    j += i;
System.out.println(i);

for(i=1, j=0;i<10;i++) {
    j += i;
}
System.out.println(i);

The first two are very bad, because they hide (misrepresent) the code structure.

Many people advocate always using blocks, to prevent confusing and coding errors.

回答3:

In the second example, the output is outside the for loop. A for loop without curly brackets only includes the following statement.

回答4:

Loop without braces apply only to the next statement

So :

 for(i=1, j=0;i<10;i++)
    j += i;
System.out.println(i);

is equivalent to :

 for(i=1, j=0;i<10;i++){
        j += i;
 }
    System.out.println(i);

i is defined out of loop scope and is incremented after the last loop so it has value of 10

回答5:

The first statement contains a block of code. A block of code is handled as one unit.

To create a block of code you should insert statements between curly braces.

loop and conditional expressions are handling only the first statement under them, if no block of code specified.

Therefore:

// Both statements handled 10 times
for(i = 1, j = 0; i < 10; i++) {
    j += i;
    System.out.println(i);
}

// Only the first statement handled 10 times, the second one is out of scope of the loop
for(i = 1, j = 0; i < 10; i++)
    j += i;
System.out.println(i);

来源：https://stackoverflow.com/questions/44221915/for-loop-without-braces-in-java