dart label control- not understanding the flow - dart

Question

Consider the following dart code:
void main(List<String> args) {
firstLoop:
for (var i = 0; i < 2; i++) {
print('${i}::I am in the first loop');
secondLoop:
for (var j = 0; j < 2; j++) {
print('${j}::I am in the second loop.');
thirdLoop:
for (var k = 0; k < 2; k++) {
print('${k}::I am in the third loop');
fourthLoop:
for (var l = 0; l < 2; l++) {
print('${l}::I am in the fourth loop');
break secondLoop;
}
}
}
}
}
The output was:
0::I am in the first loop
0::I am in the second loop.
0::I am in the third loop***
0::I am in the fourth loop
1::I am in the first loop
0::I am in the second loop.
0::I am in the third loop***
0::I am in the fourth loop
And for the following code:
void main(List<String> args) {
firstLoop:
for (var i = 0; i < 2; i++) {
print('${i}::I am in the first loop');
secondLoop:
for (var j = 0; j < 2; j++) {
print('${j}::I am in the second loop.');
thirdLoop:
for (var k = 0; k < 2; k++) {
print('${k}::I am in the third loop***');
fourthLoop:
for (var l = 0; l < 2; l++) {
print('${l}::I am in the fourth loop');
break thirdLoop;
}
}
}
}
}
The output was:
0::I am in the first loop
0::I am in the second loop.
0::I am in the third loop***
0::I am in the fourth loop
1::I am in the second loop.
0::I am in the third loop***
0::I am in the fourth loop
1::I am in the first loop
0::I am in the second loop.
0::I am in the third loop***
0::I am in the fourth loop
1::I am in the second loop.
0::I am in the third loop***
0::I am in the fourth loop
I can't really understand this flow.
The behaviour is like:
when I jump to a certain label, the statement or block above that label is executed?
It doesn't look like my understanding is clear. Please give me more insight/clearance to this.

Answer 1

When you break a labeled statement, whether it's a loop or not, it behaves as if that statement had just completed normally (not throwing, not returning, not any other control flow).
A labeled statement is something of the form label: statement. The statement doesn't have to be a loop, but it usually is.
So, if you write:
print("outside before");
label: {
print("inside before");
break label; // Breaks to *end of labeled statement*.
print("inside after");
} // End of labeled statement is here.
print("outside after");
it will print:
outside before
inside before
outside after
You can break any statement, as long as you can refer to it.
A break with no label automatically refers to the closest enclosing loop or switch statement. If you want to break to break any other statement, you need to give it a name with a label first, so you can do break label; to designate the statement to break.
(You can also have labels on the cases of a switch. You cannot break those because they are not statements, you have to break the switch instead.)
In your example, you are breaking a loop which is also the last statement of an outer loop. When you break the inner loop, you reach the end of the body of the outer loop, and it goes on to the next iteration (if it has one). That's why it looks like it is going backwards, it's hitting the end of the loop and looping backwards, just as loops normally do.
So, break does not jump to a label, it exits the labeled statement.
If you want to continue with that label, you may want to use continue label; instead. Continuing a loop means going back to the loop header and (maybe) take another iteration of the loop. It works precisely as breaking out of the body of the loop. You end the body, then the normal loop behavior takes over.
Example:
for (var i = 0; i < 5; i++) {
print("before: $i");
if (i.isEven) continue;
print("after: $i");
}
This prints:
before: 0
before: 1
after: 1
before: 2
before: 3
after: 3
before: 4
It works with all kinds of loops, even do-while:
var i = 0;
do {
print("before: $i");
if (i.isEven) continue;
print("after: $i");
} while (++i < 5);
prints the same thing.