Join list returned from QtConcurrent::map into a single list

Defohin

I was wondering how to join multiple lists (QStringList) returned by QtConcurrent::map into a single list?

QStringList magic(const QString &name)
{
    return { name.toLower(), name.toUpper() };
}

QStringList names = { "john", "jane" };
QFuture<QStringList> example = QtConcurrent::mapped(names, magic);

In example.results() I will get (("john", "JOHN"), ("jane", "JANE")), I want to reduce it to ("john", "JOHN", "jane", "JANE") in a single QStringList.

VRonin

you can either merge them afterwards:

QStringList finalRes;
const auto interRes = example.results();
for(auto i=interRes.constBegin();i!=interRes.constEnd();++i)
finalRes.append(*i);

or use mappedReduced

Defohin

I tried to use mappedReduced as it seems to be the more appropriated way, but it doesn't work as I expected.

The first thing I noticed is that the reduce method can't be a lambda, and...

void reduce(QStringList &result, const QStringList &names)
{
    // ...
}

And the second thing is that mappedReduced will call reduce twice and I don't know how to merge from here...

I don't know where to go now...

VRonin

@Defohin said in Join list returned from QtConcurrent::map into a single list:

The first thing I noticed is that the reduce method can't be a lambda

Not really, you just need to explicitly set the template parameter.

@Defohin said in Join list returned from QtConcurrent::map into a single list:

And the second thing is that mappedReduced will call reduce twice

yes, 2 elements in the list = 2 calls.

using your example and a lambda:

QFuture<QStringList> example = QtConcurrent::mappedReduced<QStringList /*this has to be explicitly set to the same type as the first argument in the reduce lambda*/>(
names
, magic
, [](QStringList& redu,const QStringList& val)->void{redu.append(val);}
, QtConcurrent::OrderedReduce // ensures the order is the same as the input
);

Defohin

Oh gosh, you are amazing, thank you.