# Time taken by Loop unrolling vs Normal loop

• Difficulty Level : Medium
• Last Updated : 16 Sep, 2019

We have discussed loop unrolling. The idea is to increase performance by grouping loop statements so that there are less number of loop control instruction and loop test instructions

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## C++

 `// CPP program to compare normal loops and``// loops with unrolling technique``#include ``#include ``using` `namespace` `std;`` ` `int` `main() {`` ` `  ``// n is 8 lakhs``  ``int` `n = 800000;`` ` `  ``// t to note start time``  ``clock_t` `t = ``clock``();`` ` `  ``// to store the sum``  ``long` `long` `int` `sum = 0;`` ` `  ``// Normal loop``  ``for` `(``int` `i = 1; i <= n; i++)``    ``sum += i;`` ` `  ``// to mark end time``  ``t = ``clock``() - t;`` ` `  ``cout << ``"sum is: "` `<< sum << endl;``  ``cout << ``"time taken by normal loops:"``      ``"(float)t / CLOCKS_PER_SEC << "` `seconds"``       ``<< endl;`` ` `  ``// to mark start time of unrolling``  ``t = ``clock``();``   ` `  ``// Unrolling technique (assuming that``  ``// n is a multiple of 8).``  ``sum = 0;``  ``for` `(``int` `i = 1; i <= n; i += 8) {``    ``sum += i sum += (i + 1);``    ``sum += (i + 2);``    ``sum += (i + 3);``    ``sum += (i + 4);``    ``sum += (i + 5);``    ``sum += (i + 6);``    ``sum += (i + 7);``  ``}`` ` `  ``// to mark the end of loop``  ``t = ``clock``() - t;`` ` `  ``cout << ``"Sum is: "` `<< sum << endl;``  ``cout << ``"Time taken by unrolling: "``       ``"(float)t / CLOCKS_PER_SEC << "` `seconds";``  ``return` `0;``}`

## Java

 `// Java program to compare ``// normal loops and loops ``// with unrolling technique`` ` `class` `GFG``{`` ` `    ``public` `static` `void` `main(String[] args)``    ``{``     ` `    ``// n is 8 lakhs``    ``int` `n = ``800000``;``     ` `    ``// t to note start time``    ``double` `t = (``double``)System.nanoTime();``     ` `    ``// to store the sum``    ``long` `sum = ``0``;``     ` `    ``// Normal loop``    ``for` `(``int` `i = ``1``; i <= n; i++)``        ``sum += i;``     ` `    ``// to mark end time``    ``t = (``double``)System.nanoTime() - t;``     ` `    ``System.out.println(``"sum is: "``+``        ``Double.toString(sum));``    ``System.out.println(``"time taken by normal loops:"` `+``        ``Double.toString(t / Math.pow(``10.0``, ``9.0``)));``     ` `    ``// to mark start time``    ``// of unrolling``    ``t = (``double``)System.nanoTime();``     ` `    ``// Unrolling technique ``    ``// (assuming that n is``    ``// a multiple of 8).``    ``sum = ``0``;``    ``for` `(``int` `i = ``1``; i <= n; i += ``8``) ``    ``{``        ``sum += i ;``        ``sum += (i + ``1``);``        ``sum += (i + ``2``);``        ``sum += (i + ``3``);``        ``sum += (i + ``4``);``        ``sum += (i + ``5``);``        ``sum += (i + ``6``);``        ``sum += (i + ``7``);``    ``}``     ` `    ``// to mark the end of loop``    ``t = (``double``)System.nanoTime() - t;``     ` `    ``System.out.println(``"sum is: "` `+``        ``Double.toString(sum));``    ``System.out.println(``"time taken by normal loops:"` `+ ``        ``Double.toString(t / Math.pow(``10.0``, ``9.0``)));``    ``}``}`` ` `// This code is contributed ``// by Harshit Saini`

## Python3

 `# Python program to compare ``# normal loops and loops ``# with unrolling technique``from` `timeit ``import` `default_timer as clock`` ` `if` `__name__ ``=``=` `"__main__"``:`` ` `    ``# n is 8 lakhs``    ``n ``=` `800000``;``     ` `    ``#t to note start time``    ``t ``=` `clock()``     ` `    ``# to store the sum``    ``sum` `=` `0``     ` `    ``# Normal loop``    ``for` `i ``in` `range``(``1``, n ``+` `1``):``        ``sum` `+``=` `i``         ` `    ``# to mark end time``    ``t ``=` `clock() ``-` `t``     ` `    ``print``(``"sum is: "` `+` `str``(``sum``))``    ``print``(``"time taken by normal "` `+` `                 ``"loops:"` `+` `str``(t))``     ` `    ``# to mark start ``    ``# time of unrolling``    ``t ``=` `clock()``     ` `    ``# Unrolling technique ``    ``# (assuming that n is ``    ``# a multiple of 8).``    ``sum` `=` `0``    ``for` `i ``in` `range``(``1``, n ``+` `1``, ``8``):``        ``sum` `+``=` `i``        ``sum` `+``=` `(i ``+` `1``)``        ``sum` `+``=` `(i ``+` `2``)``        ``sum` `+``=` `(i ``+` `3``)``        ``sum` `+``=` `(i ``+` `4``)``        ``sum` `+``=` `(i ``+` `5``)``        ``sum` `+``=` `(i ``+` `6``)``        ``sum` `+``=` `(i ``+` `7``)``     ` `    ``# to mark the ``    ``# end of loop``    ``t ``=` `clock() ``-` `t``     ` `    ``print``(``"Sum is: "` `+` `str``(``sum``))``    ``print``(``"Time taken by unrolling: "` `+` `                              ``str``(t))``                               ` `# This code is contributed``# by Harshit Saini`

## C#

 `// C# program to compare ``// normal loops and loops ``// with unrolling technique``using` `System;``using` `System.Diagnostics; ``     ` `class` `GFG``{``    ``private` `static` `double` `nanoTime() ``    ``{``        ``long` `nano = 10000L * Stopwatch.GetTimestamp();``        ``nano /= TimeSpan.TicksPerMillisecond;``        ``nano *= 100L;``        ``return` `nano;``    ``}``    ``public` `static` `void` `Main(String[] args)``    ``{``     ` `        ``// n is 8 lakhs``        ``int` `n = 800000;``         ` `        ``// t to note start time``        ``double` `t = nanoTime();``         ` `        ``// to store the sum``        ``long` `sum = 0;``         ` `        ``// Normal loop``        ``for` `(``int` `i = 1; i <= n; i++)``            ``sum += i;``         ` `        ``// to mark end time``        ``t = nanoTime() - t;``         ` `        ``Console.WriteLine(``"sum is: "``+ sum);``        ``Console.WriteLine(``"time taken by normal loops:"` `+``                              ``(t / Math.Pow(10.0, 9.0)));``         ` `        ``// to mark start time``        ``// of unrolling``        ``t = nanoTime();``         ` `        ``// Unrolling technique ``        ``// (assuming that n is``        ``// a multiple of 8).``        ``sum = 0;``        ``for` `(``int` `i = 1; i <= n; i += 8) ``        ``{``            ``sum += i ;``            ``sum += (i + 1);``            ``sum += (i + 2);``            ``sum += (i + 3);``            ``sum += (i + 4);``            ``sum += (i + 5);``            ``sum += (i + 6);``            ``sum += (i + 7);``        ``}``         ` `        ``// to mark the end of loop``        ``t = nanoTime() - t;``         ` `        ``Console.WriteLine(``"sum is: "` `+ (sum));``        ``Console.WriteLine(``"time taken by normal loops:"` `+ ``                              ``(t / Math.Pow(10.0, 9.0)));``    ``}``}`` ` `// This code is contributed by 29AjayKumar`

Output:
```Sum is: 320000400000
Time taken: 0.002645 seconds

Sum is: 320000400000
Time taken: 0.001799 seconds
```

Please refer loop unrolling for comparison of normal loops and loop unrolling.

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