vector的时间复杂度有什么区别<int> a {N, 0} 和 int arr a[N] = {0}

Question

What is the difference between the time complexities of vector <int> a {N, 0} and int arr a[N] = {0} , when initializing them with a fixed variable?

If they both are same then what techniques can we use to decrease time complexity in our program when initializing arrays?

Answer 1

The vector data is allocated on the heap and the array on the stack. Other than that hopefully you compiler will optimise both to a single memset(0) .

To know the exact timing you will need to benchmark this yourself on your compiler. I doubt that this will take a significant amount of time though, the rest of your program is likely to be slower.

Write your program first, see if its fast enough. If it isn't then profile it and concentrate on the slow bits. Making small optimisations like this before you even know whether they are needed is often fruitless.

Answer 2

Vector allocates memory in the heap hence it is little expensive to allocate memory for vector since heap data structure need to search for available block of memory in heap. In case of array, memory is allocated in stack which is equivalent reducing stack pointer. However both vector and array initializing almost takes same time, since both of the use memset and then mapped to stos hardware instruction. Main difference is in allocating memory. When I was developing a high performance software, I did find that allocating and initializing vector was taking significant time (depends on the size).

Time complexity of initialization is same for vector and array. Allocating is vector far more expensive than vector.