Reverse lookup key by value for QHash<int,QByteArray>

fcarney

I am trying to get the keys from a QHash<int,QByteArray> object. Is there a predefined function that does this or is this just a search through list every time kind of thing? The list is not long, but if there is a function to do this I would rather use that. Like some kind of q<helper> function. I know this is done in views already, but I am not sure where.

JonB

@fcarney

I am trying to get the keys from a QHash

I must be misunderstanding? QList<Key> QHash::keys() const, https://doc.qt.io/qt-5/qhash.html#keys?

EDIT OIC, I need to look at title, sorry...!

Then do you want const Key QHash::key(const T &value) const, https://doc.qt.io/qt-5/qhash.html#key

Returns the first key mapped to value.
[...]
This function can be slow (linear time),

Kent-Dorfman

@fcarney said in Reverse lookup key by value for QHash<int,QByteArray>:

I am trying to get the keys from a QHash<int,QByteArray> object. Is there a predefined function that does this or is this just a search through list every time kind of thing?

Given the type of underlying data structure that hashes come from, it is a linear search every time you do the lookup (negating the benefits of the hash) when you use
QList<Key> QHash::keys(const T &value) const

fcarney

I don't think I explained very well. I have the "value" and am wondering if there was a predefined way to get the "key" from the "value". Otherwise I will just search the list for a match and return an error if not found. Qt has a lot of prebuilt stuff and I was wondering if they had something for this already.

Edit:
@JonB
Doh, I totally missed your explanation of the key function! Thanks! I looked at the class definition multiple times and did not see it either!