vector interview questions
Top vector frequently asked interview questions
Should I use
std::sort(numbers.begin(), numbers.end(), std::greater<int>());
or
std::sort(numbers.rbegin(), numbers.rend()); // note: reverse iterators
to sort a vector in descending order? Are there any benefits/drawbacks with one approach or the other?
Source: (StackOverflow)
I need to take a C++ vector with potentially a lot of elements, erase duplicates, and sort it.
I currently have the below code, but it doesn't work.
vec.erase(
std::unique(vec.begin(), vec.end()),
vec.end());
std::sort(vec.begin(), vec.end());
How can I correctly do this?
Additionally, is it faster to erase the duplicates first (similar to coded above) or perform the sort first? If I do perform the sort first, is it guaranteed to remain sorted after std::unique is executed?
Or is there another (perhaps more efficient) way to do all this?
Source: (StackOverflow)
vector<int> v;
v.push_back(1);
v.push_back(v[0]);
If the second push_back causes a reallocation, the reference to the first integer in the vector will no longer be valid. So this isn't safe?
vector<int> v;
v.push_back(1);
v.reserve(v.size() + 1);
v.push_back(v[0]);
This makes it safe?
Source: (StackOverflow)
So, I wrote a bunch of code that accesses elements in an stl vector by index[], but now I need to copy just a chunk of the vector. It looks like vector.insert(pos, first, last) is the function I want... except I only have first and last as ints. Is there any nice way I can get an iterator to these values?
Source: (StackOverflow)
Suppose I have a line segment going from (x1,y1) to (x2,y2). How do I calculate the normal vector perpendicular to the line?
I can find lots of stuff about doing this for planes in 3D, but no 2D stuff.
Please go easy on the maths (links to worked examples, diagrams or algorithms are welcome), I'm a programmer more than I'm a mathematician ;)
Source: (StackOverflow)
Possible Duplicate:
How to find an item in a std::vector?
Is there something in algorithm.h which allows you to check if a std:: container contains something? Or a way to make one ex:
if(a.x == b.x && a.y == b.y)
return true;
return false;
can this only be done with std::map since it uses keys?
Thanks
Source: (StackOverflow)
I want to examine the contents of a std::vector in GDB, how do I do it? Let's say it's a std::vector<int> for the sake of simplicity.
Source: (StackOverflow)
When I do this:
std::vector<int> hello;
Everything works great. However, when I make it a vector of references instead:
std::vector<int &> hello;
I get horrible errors like "error C2528: 'pointer' : pointer to reference is illegal".
I want to put a bunch of references to structs into a vector, so that I don't have to meddle with pointers. Why is vector throwing a tantrum about this? Is my only option to use a vector of pointers instead?
Source: (StackOverflow)
Why is Java Vector considered a legacy class, obsolete or deprecated?
Isn't its use valid when working with concurrency?
And if I don't want to manually synchronize objects and just want to use a thread-safe collection without needing to make fresh copies of the underlying array (as CopyOnWriteArrayList does), then is it fine to use Vector?
What about Stack, which is a subclass of Vector, what should I use instead of it?
Source: (StackOverflow)
I can create an array and initialize it like this:
int a[] = {10, 20, 30};
How do I create a std::vector and initialize it similarly elegant?
The best way I know is:
std::vector<int> ints;
ints.push_back(10);
ints.push_back(20);
ints.push_back(30);
Is there a better way?
Source: (StackOverflow)
All I wanna do is to check whether an element exists in the vector or not, so I can deal with each case.
if ( item_present )
do_this();
else
do that();
Source: (StackOverflow)
I have found an interesting performance regression in a small C++ snippet, when I enable C++11:
#include <vector>
struct Item
{
int a;
int b;
};
int main()
{
const std::size_t num_items = 10000000;
std::vector<Item> container;
container.reserve(num_items);
for (std::size_t i = 0; i < num_items; ++i) {
container.push_back(Item());
}
return 0;
}
With g++ (GCC) 4.8.2 20131219 (prerelease) and C++03 I get:
milian:/tmp$ g++ -O3 main.cpp && perf stat -r 10 ./a.out
Performance counter stats for './a.out' (10 runs):
35.206824 task-clock # 0.988 CPUs utilized ( +- 1.23% )
4 context-switches # 0.116 K/sec ( +- 4.38% )
0 cpu-migrations # 0.006 K/sec ( +- 66.67% )
849 page-faults # 0.024 M/sec ( +- 6.02% )
95,693,808 cycles # 2.718 GHz ( +- 1.14% ) [49.72%]
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
95,282,359 instructions # 1.00 insns per cycle ( +- 0.65% ) [75.27%]
30,104,021 branches # 855.062 M/sec ( +- 0.87% ) [77.46%]
6,038 branch-misses # 0.02% of all branches ( +- 25.73% ) [75.53%]
0.035648729 seconds time elapsed ( +- 1.22% )
With C++11 enabled on the other hand, the performance degrades significantly:
milian:/tmp$ g++ -std=c++11 -O3 main.cpp && perf stat -r 10 ./a.out
Performance counter stats for './a.out' (10 runs):
86.485313 task-clock # 0.994 CPUs utilized ( +- 0.50% )
9 context-switches # 0.104 K/sec ( +- 1.66% )
2 cpu-migrations # 0.017 K/sec ( +- 26.76% )
798 page-faults # 0.009 M/sec ( +- 8.54% )
237,982,690 cycles # 2.752 GHz ( +- 0.41% ) [51.32%]
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
135,730,319 instructions # 0.57 insns per cycle ( +- 0.32% ) [75.77%]
30,880,156 branches # 357.057 M/sec ( +- 0.25% ) [75.76%]
4,188 branch-misses # 0.01% of all branches ( +- 7.59% ) [74.08%]
0.087016724 seconds time elapsed ( +- 0.50% )
Can someone explain this? So far my experience was that the STL gets faster by enabling C++11, esp. thanks to move semantics.
EDIT: As suggested, using container.emplace_back(); instead the performance gets on par with the C++03 version. How can the C++03 version achieve the same for push_back?
milian:/tmp$ g++ -std=c++11 -O3 main.cpp && perf stat -r 10 ./a.out
Performance counter stats for './a.out' (10 runs):
36.229348 task-clock # 0.988 CPUs utilized ( +- 0.81% )
4 context-switches # 0.116 K/sec ( +- 3.17% )
1 cpu-migrations # 0.017 K/sec ( +- 36.85% )
798 page-faults # 0.022 M/sec ( +- 8.54% )
94,488,818 cycles # 2.608 GHz ( +- 1.11% ) [50.44%]
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
94,851,411 instructions # 1.00 insns per cycle ( +- 0.98% ) [75.22%]
30,468,562 branches # 840.991 M/sec ( +- 1.07% ) [76.71%]
2,723 branch-misses # 0.01% of all branches ( +- 9.84% ) [74.81%]
0.036678068 seconds time elapsed ( +- 0.80% )
Source: (StackOverflow)