Question

Lets say I have an array as:

int a[]={4,5,7,10,2,3,6}

when I access an element, such as a[3], what does actually happen behind the scenes?
Why do many algorithm books (such as the Cormen book…) say that it takes a constant time?

(I’m just a noob in low-level programming so I would like to learn more from you guys)

Answer 1

Just to be complete, “what structure is accessed in linear time?” A Linked List structure is accessed in linear time. To get the n element you have to travel through n-1 previous elements. You know, like a tape recorder or a VHS cassette, where to go to the end of the tape/VHS you had to wait a long time 🙂

An array is more similar to an hard disk: every point is accessible in “constant” time 🙂

This is the reason the RAM of a computer is called RAM: Random Access Memory. You can go to any location if you know its address without traversing all the memory before that location.

Some persons told me that HD access isn’t really in constant time (where by access I mean “time to position the head and read one sector of the HD”). I have to say that I’m not sure of it. I have googled around and I haven’t found anyone speaking of it. I DO know that the time isn’t linear, because it is still accessed randomly. In the end, if you think that HD access isn’t constant enough for you (but then, what is constant? the access of the RAM? considering Cache, Prefetching, Data Locality and Compiler optimizations?), feel free to consider the sentence as An array is more similar to an USB disk stick: every point is accessible in “constant” time 🙂