How the memory requirements of the following codes are different irrespective of the code-length?

Question

// An efficient program to randomly select a number from stream of numbers.
#include <iostream>
using namespace std;
#include <stdlib.h>
#include <time.h>

/* A program to randomly select a item from stream[0], stream[1], ..
stream[i-1]*/


int main()
{
    int stream[] = { 1, 2, 3, 4 };
    int n = sizeof(stream) / sizeof(stream[0]);

    // Use a different seed value for every run.
    srand(time(0));
    cout << "Random no. selected: " << stream[(rand() % n)] << "\n";
    return 0;
}

The above C++ code requires memory ranging from 2.9...(something) MB or greater, to 3.04..(something) MB or greater.

And consider the following C code:

/* An efficient program to randomly select a number from
stream of numbers.*/
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

/* A function to randomly select a item from stream[0], stream[1], .. 
stream[i - 1]*/
int selectRandom(int x)
{
    static int res;    // The resultant random number
    static int count = 0;  //Count of numbers visited so far in stream

    count++;  // increment count of numbers seen so far

              // If this is the first element from stream, return it
    if (count == 1)
        res = x;

    else
    {
        // Generate a random number from 0 to count - 1
        int i = rand() % count;

        // Replace the prev random number with new number with 1/count 
        //probability
        if (i == count - 1)
            res = x;
    }
    return res;
}

// Driver program to test above function.
int main()
{
    int stream[] = { 1, 2, 3, 4 };
    int n = sizeof(stream) / sizeof(stream[0]);

    // Use a different seed value for every run.
    srand(time(NULL));
    for (int i = 0; i < n; ++i)
        printf("Random number from first %d numbers is %d \n",
            i + 1, selectRandom(stream[i]));
    return 0;
}

The above code, requires memory in the range 1.28..(something) MB or greater but surely less than 2MB.

My question is, why does the first program takes more space than the second one, and how?

When you say memory are you talking about the memory the process needs or its size on the disk? — NathanOliver, Aug 04 '17 at 17:28
@NathanOliver and what is your OS, compiler, optimization settings? — SergeyA, Aug 04 '17 at 17:32
@NathanOliver I am not sure about that, but it should be related to space complexity of the program. — scoder, Aug 04 '17 at 17:34
I think for starters in your second example, your running a loop that's requiring more memory every time. — arias_JC, Aug 04 '17 at 17:37
@scoder which one, does it have share links you can post here? — Ryan Haining, Aug 04 '17 at 17:45
@Ryan Haining Yeah, http://ide.geeksforgeeks.org/index.php, this is the link — scoder, Aug 04 '17 at 17:48
Your first snippet requires the whole array in memory to select a number. The second one doesn't need to store the input values (but requires a computation by input value). — Jarod42, Aug 04 '17 at 17:51
This won't be a popular thing to say, but the difference probably comes down to the use of `` in the first example. Try changing it to use C I/O (i.e. `printf()`) and see what happens. — Tristan Brindle, Aug 04 '17 at 19:12
@Jarod42 Yes I feel that may be one of the reasons but when I tried Tristan's Solution, I found that to be correct, i.e., by using stdio.h header file, the memory usage is reduced ....to 1.3....(something). Thank you all for the answers .... — scoder, Aug 06 '17 at 05:28

Ryan Haining · Answer 1 · 2017-08-05T21:34:07.923

-1

Using your provided online editor the memory usage doesn't seem to correspond to actual allocations. If I write a program with malloc(1); the "memory" stat is the same as doing malloc(10000000);. I'm led to believe this stat is just the size of the executable.

edited Aug 05 '17 at 21:34

answered Aug 04 '17 at 17:56

Ryan Haining

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Thanks for the answer, Yes I checked the same,but found that the use of malloc shows reduced size sometimes than, when it wasn't used....couldn't understand why? – scoder Aug 06 '17 at 06:28

How the memory requirements of the following codes are different irrespective of the code-length?

1 Answers1