Java for loop vs. while loop. Performance difference?

Question

Assume i have the following code, there are three for loop to do something. Would it run fast if i change the most outer for loop to while loop? thanks~~

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Answer 1

It would only matter if you are using multi-thread or multiple processor programming. Then it would also depends on how you assign the loops to the various processors/threads.

Answer 2

While Loop and For loop are same

http://www.mkyong.com/java/while-loop-for-loop-and-iterator-performance-test-java/

Answer 3

Someone suggested to test while vs for loops, so I created some code to test whether while loops or for loops were faster; on average, over 100,000 tests, while loop was faster ~95% of the time. I may have coded it incorrectly, I'm quite new to coding, also considering if I only ran 10,000 loops they ended up being quite even in run duration.

edit I didn't shift all the array values when I went to test for more trials. Fixed it so that it's easier to change how many trials you run.

import java.util.Arrays;

class WhilevsForLoops {

 public static void main(String[] args) {

final int trials = 100; //change number of trials
final int trialsrun = trials - 1;

boolean[] fscount = new boolean[trials]; //faster / slower boolean
int p = 0; // while counter variable for for/while timers



while (p <= trialsrun) {
     long[] forloop = new long[trials];
     long[] whileloop = new long[trials];

     long systimeaverage; 
     long systimenow = System.nanoTime();
     long systimethen = System.nanoTime();

     System.out.println("For loop time array : ");
     for (int counter=0;counter <= trialsrun; counter++) {
         systimenow = System.nanoTime();
         System.out.print(" #" + counter + " @");
         systimethen = System.nanoTime();
         systimeaverage = (systimethen - systimenow);
         System.out.print( systimeaverage + "ns |");

         forloop[counter] = systimeaverage; 
     }

     int count = 0;
     System.out.println(" ");
     System.out.println("While loop time array: ");
     while (count <= trialsrun) {
         systimenow = System.nanoTime();
         System.out.print(" #" + count + " @");
         systimethen = System.nanoTime();
         systimeaverage = (systimethen - systimenow);
         System.out.print( systimeaverage + "ns |");

         whileloop[count] = systimeaverage;
         count++;
     }


     System.out.println("===============================================");
     int sum = 0;

     for (int i = 0; i <= trialsrun; i++) {
        sum += forloop[i];
     }

     System.out.println("for loop time average: " + (sum / trials) + "ns");

     int sum1 = 0;

     for (int i = 0; i <= trialsrun; i++) {
         sum1 += whileloop[i];
     }
     System.out.println("while loop time average: " + (sum1 / trials) + "ns");



     int longer = 0;
     int shorter = 0;
     int gap = 0;

     sum = sum / trials;
     sum1 = sum1 / trials; 

     if (sum1 > sum) {
        longer = sum1;
        shorter = sum;
     }
     else {
        longer = sum;
        shorter = sum1;
     }

     String longa;

     if (sum1 > sum) {
        longa = "~while loop~";
     }
     else {
         longa = "~for loop~";
     }

     gap = longer - shorter; 
     System.out.println("The " + longa + " is the slower loop by: " + gap + "ns");
     if (sum1 > sum) {
     fscount[p] = true; }
     else {
         fscount[p] = false;
     }
     p++;
}

    int forloopfc=0;
    int whileloopfc=0;

    System.out.println(Arrays.toString(fscount));

    for(int k=0; k <= trialsrun; k++) {
        if (fscount[k] == true) {
            forloopfc++; }
            else {
                whileloopfc++;}

    }

    System.out.println("--------------------------------------------------");

    System.out.println("The FOR loop was faster: " + forloopfc + " times.");
    System.out.println("The WHILE loop was faster: " + whileloopfc + " times.");
 }

}

Answer 4

Even if the hypothesis of the while loop being faster than the for loop were true (and it's not), the loops you'd had to change/optimize wouldn't be the outer ones but the inner ones, because those are executed more times.

Answer 5

you cant optimize it by changing it to while.

you can just increment speed very very very very little by changing the line

for (int k = 0; k < length - 1; k++) {

by

for (int k = 0; k < lengthMinusOne; k++) {

where lengthMinusOne is calculated before

this subtraction is just calculating almost (200x201/2) x (200-1) times and it is very little number for computer :)

Answer 6

The difference between for and while is semantic :

• In a while loop, you will loop as long as the condition is true, which can vary a lot, because you might, in your loop, modify variables using in evluating the while condition.
• Usually, in a for loop, you loop N time. This N can be variable, but doesn't move until the end of your N loop, as usually developpers doesn't modify variables evaluated in the loop condition.

It is a way to help other to understand your code. You are not obliged not to modify for loop variables, but it is a common (and good) practice.