How to stop a nested for loop - c++

Question

I want both loops to stop as soon as I find an object that has the same position in x.
Here is my C++ code:
for(int i = 0; i < sizeArray; ++i){
for(int j = i; j > 0; --j){
if (s[i].positionX == s[j-1].positionX){
s[i].positionY = s[j-1].positionY;
}
}
}
If I use break; it only breaks out of the inner for loop. What is the best way to stop both?
Options:
Set the indexes of the loops to max value (or min value) to terminate the loop.
Put all this inside a function and use return.
Use a goto
Use a Lambda
Set a boolean stop code to true, break, then listen for break and break out of other loop?
?

Answer 1

To break out of the innermost for loop, use break.
To break out of the outermost one, either use goto or use a combination of break and a "should stop" flag.

Answer 2

I would suggest lambda:
auto do_work = [&] {
for(int i = 0; i < sizeArray; ++i){
for(int j = i; j > 0; --j){
if (s[i].positionX == s[i-1].positionX){
s[i].positionY = s[i-1].positionY;
return;
}
}
}
};
do_work(); //you can call this multiple times if you need to!
No break, no goto. :-)
It serves the same purpose as break and goto in this case, but the technique seems to be clean, at least to me. Also, once you have the lambda with a name (a good name), you can use it multiple times if you need to — so this technique increases the code readability and encourages code-reuse.
Of course, if you don't need to call this multiple times, then probably you don't need a name. You just could do this:
[&]
{
//your code with the added return statement.
}();
But as I said, name increases readability even if you don't call it multiple times.
If you cannot use lambda for some reason, then you could still avoid using an extra variable such as stop and the extra work involving it (as #ssantos's answer suggested):
for(int i = 0; i < sizeArray; ++i){
for(int j = i; j > 0; --j){
if (s[i].positionX == s[i-1].positionX){
s[i].positionY = s[i-1].positionY;
i = sizeArray; //it will break the outer loop!
break;
}
}
}
Hope that helps.

Answer 3

I'm guessing you ask for stopping a for loop. The keyword you're looking for is break. However, if you want to exit both for loops, you'll need to add a variable to your code.-
bool stop = false;
for(int i = 0; i < sizeArray; ++i){
for(int j = i; j > 0; --j){
if (s[i].positionX == s[i-1].positionX){
s[i].positionY = s[i-1].positionY;
stop = true;
break;
}
}
if (stop) {
break;
}
}

Answer 4

Put the code you posted in its own function and return out of it after the assignment.

Answer 5

You can stop a for loop with break.
With nested for loops, things are not so easy. You can achieve your goal
either with setting a flag (done = 1; and use it with for(int j = i; j > 0 && !done; --j))
or with goto. Although goto is frowned upon by some people, it can, if used correctly and with care, be a legitimate solution for some problems, such as error handling, or in general, "finished handling".

Answer 6

Try the following
bool match = false;
for(int i = 0; i < sizeArray && !match; ++i){
for(int j = i; j > 0 && !match; --j){
if ( match = ( s[i].positionX == s[i-1].positionX ) ){
s[i].positionY = s[i-1].positionY;
}
}
}
Another way is the following
for(int i = 0; i < sizeArray; ++i){
int j = i;
while ( j != 0 && s[i].positionX != s[i-1].positionX ) --j;
if ( j != 0 ) {
s[i].positionY = s[i-1].positionY;
break;
}
}