Code:
std::map<CString, S_DISCUSSION_HIST_ITEM> mapHistory;
// History list is in ascending date order
for (auto& sHistItem : m_listDiscussionItemHist)
{
if (m_bFullHistoryMode)
mapHistory.emplace(sHistItem.strName, sHistItem);
else if (sHistItem.eSchool == m_eActiveSchool)
mapHistory.emplace(sHistItem.strName, sHistItem);
}
// The map is sorted by Name (so reset by date later)
// The map has the latest assignment info for each Name now
Observation:
I now understand that std::emplace behaves like this:
The insertion only takes place if no other element in the container has a key equivalent to the one being emplaced (keys in a map container are unique).
Therefore my code is flawed. What I was hoping to acheive (in pseudo code) is:
For Each History Item
Is the name in the map?
No, so add to map with sHitItem
Yes, so replace the sHistItem with this one
End Loop
By the end of this loop iteration I want to have the most recent sHitItem, for each person. But as it is, it is only adding an entry into the map if the name does not exist.
What is the simplest way to get around this?
Use insert_or_assign method if the item is assignable. It will be assigned if it already exists. Or use [] operator followed by assignment, it will default-construct item if it does not exist.
For non-assignable types I'm afraid there's no convenient way.
Related
Let's say I have a list of variables similar to the following:
"first" = {
action = "trigger"
env = "prod"
}
I know that I can get the values of action and env variables in the foreach loop using ${each.value["action"]} and ${each.value["env"]}.
How would I go around getting the "first" field (name of the element in a list)?
I would appreciate any help on this!
Please understand first how for_each works and what each object [1] is. The for_each meta-argument can work with sets or maps. You are using maps. A map is represented by keys and values. Since you already know how to use values, the way to get a key in a map is with each.key. If your map has more than one key, you can get all of the keys with the built-in keys function [2].
[1] https://developer.hashicorp.com/terraform/language/meta-arguments/for_each
[2] https://developer.hashicorp.com/terraform/language/functions/keys
There is no "first" in a map. The only "first" you can get is a first key using keys after sorting. This is because:
The keys are returned in lexicographical order,
This question already has an answer here:
How can I replace a key:value pair by its value wherever the chosen key occurs in a deeply nested dictionary?
(1 answer)
Closed 1 year ago.
About
This question is about the most basic problem of deleting a key:value pair at a found key, iterating over a whole dictionary.
Other questions
Deleting a key:value pair should happen much more often than the special problem of replacing the key:value pair by its value at How can I replace a key:value pair by its value whereever the chosen key occurs in a deeply nested dictionary?. Saying that these two problems are different enough may not sound so plausible at first since the wording seems almost the same, but then, please check the code solutions and test it. There is a reason why it took some hour to find it out.
The 2011 question Modifying a Python dict while iterating over it (85k views) does not even seem to have found a working answer, though it is also outdated, admittedly.
Before:
I have a dictionary that is nested many times.
{
"key0": {
"key1a": {
"sub_key2a": "sub_value2a",
"sub_key2b": "sub_value2b"
},
"key1b": {
"key_XYZ": {
"sub_key2a": "sub_value2a",
"sub_key2b": "sub_value2b"
}
}
}
}
After:
The result should look like this, deleting all "sub_key2a" keys with their values:
{
"key0": {
"key1a": {
"sub_key2b": "sub_value2b"
},
"key1b": {
"key_XYZ": {
"sub_key2b": "sub_value2b"
}
}
}
}
Modifying a Python dict while iterating over it
When I looped through the items of the dictionary to delete, I got the error
RuntimeError: dictionary changed size during iteration
which needs somehow to be avoided.
How can I remove the "sub_key2a": SOME_VALUE key-value pair each time the key "sub_key2a" occurs somewhere in the dictionary?
Trick
The trick is to find out in advance whether a target_key is among the next children (= this_dict[key] = the values of the current dict iteration) before you reach the child level recursively. Only then you can still delete a key:value pair of the child level while iterating over a dictionary. Once you have reached the same level as the key to be deleted and then try to delete it from there, you get the error:
RuntimeError: dictionary changed size during iteration
Code
Thus, the code looks as follows:
import copy
def find_remove(this_dict, target_key, bln_overwrite_dict=False):
if not bln_overwrite_dict:
this_dict = copy.deepcopy(this_dict)
for key in this_dict:
# if the current value is a dict, dive into it
if isinstance(this_dict[key], dict):
if target_key in this_dict[key]:
this_dict[key].pop(target_key)
this_dict[key] = find_remove(this_dict[key], target_key)
return this_dict
dict_nested_new = find_remove(nested_dict, "sub_key2a")
Credits
This is almost a copy of the spin-off How can I replace a key:value pair by its value wherever the chosen key occurs in a deeply nested dictionary?. But it took me a while to change that answer so that it would delete a key:value by its key. That is why I am sharing this, please mind that 95% of the credits go to the link!
The main added value over the "spun-off answer" is that you search for the target_key in the values in advance of entering the next recursion level by checking if target_key in this_dict[key]:.
Side note: Formatting the output
If you want to print or save the dictionary nicely, see How do I write JSON data to a file?.
delete a["key_"]["key0a"]["sub_key2a"]
I'm looking for an efficient way using linq.js to determine if a collection has more than one distinct value. I assume that the following approach is inefficient because it has to consider the entire collection.
if (Enumerable.From(collection).Distinct().Take(2).Count() > 1) {
//it's not unique, continue loop
}
My question is similar to one:
Efficient Linq Enumerable's 'Count() == 1' test
Is there a more efficient linq.js-based technique? Thanks!
If you're specifically testing to see if a collection has more than one item in it, the idiomatic way to write it (IMHO) is to use Skip in conjunction with Any. Skip the first item and if there are any others in the collection, it has more than one. If it was empty, the Skip would effectively do nothing and there still wouldn't be any other items in the collection.
In your case, your condition would be:
if (Enumerable.From(collection).Distinct().Skip(1).Any()) {
//it's not unique, continue loop
}
var test = collection[0];
if (Enumerable
.From(collection)
.Skip(1)
.Any(function (e) { return e != test; })
)
Let me explain it. At least 2 distinct items mean that for any item there is at least one item that is not equal to it. Let's pick first item, you could pick any other, just first is more convenient and let's see if there is any other number not equal to it (except itself).
I understand that the reduce function is supposed to somewhat combine the results of the map function but what exactly is passed to the reduce function?
function(keys, values){
// what's in keys?
// what's in values?
}
I tried to explore this in the Futon temporary view builder but all I got were reduce_overflow_errors. So I can't even print the keys or values arguments to try to understand what they look like.
Thanks for your help.
Edit:
My problem is the following. I'm using the temporary view builder of Futon.
I have a set of document representing text files (it's for a script I want to use to make translation of documents easier).
text_file:
id // the id of the text file is its path on the file system
I also have some documents that represent text fragments appearing in the said files, and their position in each file.
text_fragment:
id
file_id // correspond to a text_file document
position
I'd like to get for each text_file, a list of the text fragments that appear in the said file.
Update
Note on JavaScript API change: Prior to Tue, 20 May 2008 (Subversion revision r658405) the function to emit a row to the map index, was named "map". It has now been changed to "emit".
That's the reason why there is mapused instead of emitit was renamed. Sorry I corrected my code to be valid in the recent version of CouchDB.
Edit
I think what you are looking for is a has-many relationship or a join in sql db language. Here is a blog article by Christopher Lenz that describes exactly what your options are for this kind of scenario in CouchDB.
In the last part there is a technique described that you can use for the list you want.
You need a map function of the following format
function(doc) {
if (doc.type == "text_file") {
emit([doc._id, 0], doc);
} else if (doc.type == "text_fragment") {
emit([doc.file_id, 1], doc);
}
}
Now you can query the view in the following way:
my_view?startkey=["text_file_id"]&endkey;=["text_file_id", 2]
This gives you a list of the form
text_file
text_fragement_1
text_fragement_2
..
Old Answer
Directly from the CouchDB Wiki
function (key, values, rereduce) {
return sum(values);
}
Reduce functions are passed three arguments in the order key, values and rereduce
Reduce functions must handle two cases:
When rereduce is false:
key will be an array whose elements are arrays of the form [key,id], where key is a key emitted by the map function and id is that of the document from which the key was generated.
values will be an array of the values emitted for the respective elements in keys
i.e. reduce([ [key1,id1], [key2,id2], [key3,id3] ], [value1,value2,value3], false)
When rereduce is true:
key will be null
values will be an array of values returned by previous calls to the reduce function
i.e. reduce(null, [intermediate1,intermediate2,intermediate3], true)
Reduce functions should return a single value, suitable for both the value field of the final view and as a member of the values array passed to the reduce function.
I have a Couchdb database with documents of the form: { Name, Timestamp, Value }
I have a view that shows a summary grouped by name with the sum of the values. This is straight forward reduce function.
Now I want to filter the view to only take into account documents where the timestamp occured in a given range.
AFAIK this means I have to include the timestamp in the emitted key of the map function, eg. emit([doc.Timestamp, doc.Name], doc)
But as soon as I do that the reduce function no longer sees the rows grouped together to calculate the sum. If I put the name first I can group at level 1 only, but how to I filter at level 2?
Is there a way to do this?
I don't think this is possible with only one HTTP fetch and/or without additional logic in your own code.
If you emit([time, name]) you would be able to query startkey=[timeA]&endkey=[timeB]&group_level=2 to get items between timeA and timeB grouped where their timestamp and name were identical. You could then post-process this to add up whenever the names matched, but the initial result set might be larger than you want to handle.
An alternative would be to emit([name,time]). Then you could first query with group_level=1 to get a list of names [if your application doesn't already know what they'll be]. Then for each one of those you would query startkey=[nameN]&endkey=[nameN,{}]&group_level=2 to get the summary for each name.
(Note that in my query examples I've left the JSON start/end keys unencoded, so as to make them more human readable, but you'll need to apply your language's equivalent of JavaScript's encodeURIComponent on them in actual use.)
You can not make a view onto a view. You need to write another map-reduce view that has the filtering and makes the grouping in the end. Something like:
map:
function(doc) {
if (doc.timestamp > start and doc.timestamp < end ) {
emit(doc.name, doc.value);
}
}
reduce:
function(key, values, rereduce) {
return sum(values);
}
I suppose you can not store this view, and have to put it as an ad-hoc query in your application.