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[RM2K3] C++ DATA STRUCTURES
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McBick
1339
McBick
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CloseIs there an easy way to update the size of a Vector? I am using a vector to store condition IDs of the actor, but when I erase an element and resize the vector, each time the condition is removed or expired, that vector's size does not update and I get an incorrect value for the number of elements in the vector.
Example:
Should I just be using a different structure to track conditions? If so, what would be a better method?
Example:
std::vector<int> conditions; int conditionId; bool removeCondition; //... if(removeCondition == true) { removeCondition = false; int oldSize = conditions.size(); int i = std::distance(conditions, std::find(conditions, conditions + conditions.size(), conditionId)); conditions.erase (i); int newSize = conditions.size(); //Why does this still equal the old size? ... }
Should I just be using a different structure to track conditions? If so, what would be a better method?
AubreyTheBard
1801
According to the vector::erase function documentation, the erase function should handle the resizing already, you shouldn't need to call resize. I'm puzzled by this line in your code:
From what I was able to find about std::find with a quick search, the first and second arguments should be iterators to the start and end of your search area, like this:
My guess is you're getting a different value for i than you're expecting, so when it's fed into the erase function, it doesn't find that position and so it doesn't get erased. Hence, your structure stays the same size.
int i = std::find(conditions, conditions + conditions.size, conditionId);
From what I was able to find about std::find with a quick search, the first and second arguments should be iterators to the start and end of your search area, like this:
int i = std::find(conditions.begin(), conditions.end(), conditionId);
My guess is you're getting a different value for i than you're expecting, so when it's fed into the erase function, it doesn't find that position and so it doesn't get erased. Hence, your structure stays the same size.
McBick
1339
author=AubreyTheBardThis will not compile which is why I wrote it the other way. Also I added the resize line because erase did not properly resize.int i = std::find(conditions.begin(), conditions.end(), conditionId);
I checked my code again and you're right int i is wrong. The correct line should have been this, but this still doesn't fix my resize issue. I will fix my example to reflect this too.
int i = std::distance(conditions, std::find(conditions, conditions + conditions.size, conditionId));
The example is not my actual code, just a demonstration of what I am trying to do. I don't have anything coded yet, until I can solve the issue. I figured out the issue, but I'm not sure how to fix it. The issue is that I can increase my vector size, but I can't decrease it, no matter which function I use. I think the memory for the vector needs to be reallocated for it to properly resize, but I am not sure how I would achieve this. I am having the same issue with the clear function as well.
Either this way (handles also multiple occurences): https://stackoverflow.com/a/3385251/1871033
No need to call `resize`.
Or this way: https://stackoverflow.com/a/3385249/1871033
#include <algorithm> // ... conditions.erase(std::remove(conditions.begin(), conditions.end(), conditionId), conditions.end());
No need to call `resize`.
Or this way: https://stackoverflow.com/a/3385249/1871033
#include <algorithm> // ... // Note this is an std::vector<int>::iterator and not an int std::vector<int>::iterator pos = std::find(conditions.begin(), conditions.end(), conditionId); if (pos != conditions.end()) // == conditions.end() means the element was not found conditions.erase(pos);
McBick
1339
author=CherryThis is much cleaner than what I wrote, but I still have the same issue. I still can't reduce the size of my vectors when using erase and clear. Every time I try to use something like:#include <algorithm> // ... // Note this is an std::vector<int>::iterator and not an int std::vector<int>::iterator pos = std::find(conditions.begin(), conditions.end(), conditionId); if (pos != conditions.end()) // == conditions.end() means the element was not found conditions.erase(pos);
int numConditions = conditions.size()
New Example
std::vector<int> conditions; int currentSize = conditions.size(); //Value of 0 conditions.push_back (50); conditions.push_back (51); currentSize = conditions.size(); //New value of 2 conditions.clear(); currentSize = conditions.size(); //This should be a value of 0, but the value is 2
Hm are you sure?
http://cpp.sh/8daae
As you can see here, the output is zero.
Maybe some other part of your code is the issue?
http://cpp.sh/8daae
As you can see here, the output is zero.
Maybe some other part of your code is the issue?
DNKpp
0
I might be mistaken, but it does sound like as you call erase on an implicitly copied vector but checking the size on the original one. Perhaps you can show a little more code, so I can provide more useful advises.
McBick
1339
I found the issue now. I checked what I was doing, and my pointer is in fact off which resulted in the size value to not be used. I have been coding on and off, so I had to refresh myself on my old code and I did not look carefully enough at the pointers.
Thanks for the feedback everyone.
On another note, should I be using a vector for storing condition Ids or an array? The reason I am using a vector now is because I can easily sort through the elements. This is necessary because I reorder the elements based on the priority in the database manager, which lets me control which condition is displayed in the status/battle menu.
Thanks for the feedback everyone.
On another note, should I be using a vector for storing condition Ids or an array? The reason I am using a vector now is because I can easily sort through the elements. This is necessary because I reorder the elements based on the priority in the database manager, which lets me control which condition is displayed in the status/battle menu.
DNKpp
0
In fact a std::vector is an array. It's just been dynamically allocated. If you need a resizable array, vector is the right one. Otherwise use std::array (no plain arrays like !).
int[]
DNKpp
0
If you need a vector, use a vector. Std::vector is in fact THE default Container if you need a dynamically amount of data.
If you simply need a fixed amount of any Kind of data, than you do not need a vector, indeed. But it doesn't change much. As I already said a vector is an array.
If you simply need a fixed amount of any Kind of data, than you do not need a vector, indeed. But it doesn't change much. As I already said a vector is an array.
DNKpp
0
As I already told you:
* you need a fixed amount of objects of equal type -> go for std::array
* you need a dynamically amount of objects of equal type -> go for std::vector
List is a complete different story. It has other advantages but also harsh tradeoffs. Only use list on contexts where you do not frequently loop over each element. Even if list offers a nice interface on the first look, it shouldn't be your default container.
* you need a fixed amount of objects of equal type -> go for std::array
* you need a dynamically amount of objects of equal type -> go for std::vector
List is a complete different story. It has other advantages but also harsh tradeoffs. Only use list on contexts where you do not frequently loop over each element. Even if list offers a nice interface on the first look, it shouldn't be your default container.
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