You can get the same output with for and while loops:
While:
$i = 0;
while ($i <= 10){
print $i."\n";
$i++;
};
For:
for ($i = 0; $i <= 10; $i++){
print $i."\n";
}
But which one is faster?
This question is related to
performance
for-loop
while-loop
They should be equal. The for loop you wrote is doing exactly the same thing that the while loop is doing: setting $i=0, printing $i, and incrementing $i at the end of the loop.
It shouldn't matter which is faster. If it does matter then benchmark it using your real code and see for yourself.
The answers to this other question might be useful as well: How to write more efficient code
Depends on the language and most likely its compiler, but they should be equivalent in most languages.
Some optimizing compilers will be able to do better loop unrolling with a for loop, but odds are that if you're doing something that can be unrolled, a compiler smart enough to unroll it is probably also smart enough to interpret the loop condition of your while loop as something it can unroll as well.
Isn't a For Loop technically a Do While?
E.g.
for (int i = 0; i < length; ++i)
{
//Code Here.
}
would be...
int i = 0;
do
{
//Code Here.
} while (++i < length);
I could be wrong though...
Also when it comes to for loops. If you plan to only retrieve data and never modify data you should use a foreach. If you require the actual indexes for some reason you'll need to increment so you should use the regular for loop.
for (Data d : data)
{
d.doSomething();
}
should be faster than...
for (int i = 0; i < data.length; ++i)
{
data[i].doSomething();
}
I used a for and while loop on a solid test machine (no non-standard 3rd party background processes running). I ran a for loop vs while loop as it relates to changing the style property of 10,000 <button> nodes.
The test is was run consecutively 10 times, with 1 run timed out for 1500 milliseconds before execution:
Here is the very simple javascript I made for this purpose
function runPerfTest() {
"use strict";
function perfTest(fn, ns) {
console.time(ns);
fn();
console.timeEnd(ns);
}
var target = document.getElementsByTagName('button');
function whileDisplayNone() {
var x = 0;
while (target.length > x) {
target[x].style.display = 'none';
x++;
}
}
function forLoopDisplayNone() {
for (var i = 0; i < target.length; i++) {
target[i].style.display = 'none';
}
}
function reset() {
for (var i = 0; i < target.length; i++) {
target[i].style.display = 'inline-block';
}
}
perfTest(function() {
whileDisplayNone();
}, 'whileDisplayNone');
reset();
perfTest(function() {
forLoopDisplayNone();
}, 'forLoopDisplayNone');
reset();
};
$(function(){
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
setTimeout(function(){
console.log('cool run');
runPerfTest();
}, 1500);
});
Here are the results I got
pen.js:8 whileDisplayNone: 36.987ms
pen.js:8 forLoopDisplayNone: 20.825ms
pen.js:8 whileDisplayNone: 19.072ms
pen.js:8 forLoopDisplayNone: 25.701ms
pen.js:8 whileDisplayNone: 21.534ms
pen.js:8 forLoopDisplayNone: 22.570ms
pen.js:8 whileDisplayNone: 16.339ms
pen.js:8 forLoopDisplayNone: 21.083ms
pen.js:8 whileDisplayNone: 16.971ms
pen.js:8 forLoopDisplayNone: 16.394ms
pen.js:8 whileDisplayNone: 15.734ms
pen.js:8 forLoopDisplayNone: 21.363ms
pen.js:8 whileDisplayNone: 18.682ms
pen.js:8 forLoopDisplayNone: 18.206ms
pen.js:8 whileDisplayNone: 19.371ms
pen.js:8 forLoopDisplayNone: 17.401ms
pen.js:8 whileDisplayNone: 26.123ms
pen.js:8 forLoopDisplayNone: 19.004ms
pen.js:61 cool run
pen.js:8 whileDisplayNone: 20.315ms
pen.js:8 forLoopDisplayNone: 17.462ms
Here is the demo link
Update
A separate test I have conducted is located below, which implements 2 differently written factorial algorithms, 1 using a for loop, the other using a while loop.
Here is the code:
function runPerfTest() {
"use strict";
function perfTest(fn, ns) {
console.time(ns);
fn();
console.timeEnd(ns);
}
function whileFactorial(num) {
if (num < 0) {
return -1;
}
else if (num === 0) {
return 1;
}
var factl = num;
while (num-- > 2) {
factl *= num;
}
return factl;
}
function forFactorial(num) {
var factl = 1;
for (var cur = 1; cur <= num; cur++) {
factl *= cur;
}
return factl;
}
perfTest(function(){
console.log('Result (100000):'+forFactorial(80));
}, 'forFactorial100');
perfTest(function(){
console.log('Result (100000):'+whileFactorial(80));
}, 'whileFactorial100');
};
(function(){
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
runPerfTest();
console.log('cold run @1500ms timeout:');
setTimeout(runPerfTest, 1500);
})();
And the results for the factorial benchmark:
pen.js:41 Result (100000):7.15694570462638e+118
pen.js:8 whileFactorial100: 0.280ms
pen.js:38 Result (100000):7.156945704626378e+118
pen.js:8 forFactorial100: 0.241ms
pen.js:41 Result (100000):7.15694570462638e+118
pen.js:8 whileFactorial100: 0.254ms
pen.js:38 Result (100000):7.156945704626378e+118
pen.js:8 forFactorial100: 0.254ms
pen.js:41 Result (100000):7.15694570462638e+118
pen.js:8 whileFactorial100: 0.285ms
pen.js:38 Result (100000):7.156945704626378e+118
pen.js:8 forFactorial100: 0.294ms
pen.js:41 Result (100000):7.15694570462638e+118
pen.js:8 whileFactorial100: 0.181ms
pen.js:38 Result (100000):7.156945704626378e+118
pen.js:8 forFactorial100: 0.172ms
pen.js:41 Result (100000):7.15694570462638e+118
pen.js:8 whileFactorial100: 0.195ms
pen.js:38 Result (100000):7.156945704626378e+118
pen.js:8 forFactorial100: 0.279ms
pen.js:41 Result (100000):7.15694570462638e+118
pen.js:8 whileFactorial100: 0.185ms
pen.js:55 cold run @1500ms timeout:
pen.js:38 Result (100000):7.156945704626378e+118
pen.js:8 forFactorial100: 0.404ms
pen.js:41 Result (100000):7.15694570462638e+118
pen.js:8 whileFactorial100: 0.314ms
Conclusion: No matter the sample size or specific task type tested, there is no clear winner in terms of performance between a while and for loop. Testing done on a MacAir with OS X Mavericks on Chrome evergreen.
I was wondering the same thing so i googled and ended up here. I did a small test in python (extremely simple) just to see and this is what I got:
For:
def for_func(n = 0):
for n in range(500):
n = n + 1
python -m timeit "import for_func; for_func.for_func()" > for_func.txt
10000 loops, best of 3: 40.5 usec per loop
While:
def while_func(n = 0):
while n < 500:
n = n + 1
python -m timeit "import while_func; while_func.while_func()" > while_func.txt
10000 loops, best of 3: 45 usec per loop
Set the loop iterations to 10,000.
Find the time in milliseconds>Run Loop>find time in milliseconds and subtract the first timer.
Do it for both codes, what ever one has the lowest milliseconds it runs faster. You might want to run the test multiple times and average them out to reduce the likelihood of background processes influencing the test.
You are likely to get really similar times on both of them, but I am interested to see if one is always just slightly faster.
If that were a C program, I would say neither. The compiler will output exactly the same code. Since it's not, I say measure it. Really though, it's not about which loop construct is faster, since that's a miniscule amount of time savings. It's about which loop construct is easier to maintain. In the case you showed, a for loop is more appropriate because it's what other programmers (including future you, hopefully) will expect to see there.
As for infinite loops for(;;) loop is better than while(1) since while evaluates every time the condition but again it depends on the compiler.
That will depend on the language implementation of said loop, compiler and what not.
Most compiler will compile to the exact same executable code for example in CIL (.NET) they definitely do.
Source: vcsjones @ http://forums.asp.net/t/1041090.aspx
Either way, the body of the loop is where the processing time will be spent not the way you iterate.
As others have said, any compiler worth its salt will generate practically identical code. Any difference in performance is negligible - you are micro-optimizing.
The real question is, what is more readable? And that's the for loop (at least IMHO).
I find the fastest loop is a reverse while loop, e.g:
var i = myArray.length;
while(i--){
// Do something
}
In C#, the For loop is slightly faster.
For loop average about 2.95 to 3.02 ms.
The While loop averaged about 3.05 to 3.37 ms.
Quick little console app to prove:
class Program
{
static void Main(string[] args)
{
int max = 1000000000;
Stopwatch stopWatch = new Stopwatch();
if (args.Length == 1 && args[0].ToString() == "While")
{
Console.WriteLine("While Loop: ");
stopWatch.Start();
WhileLoop(max);
stopWatch.Stop();
DisplayElapsedTime(stopWatch.Elapsed);
}
else
{
Console.WriteLine("For Loop: ");
stopWatch.Start();
ForLoop(max);
stopWatch.Stop();
DisplayElapsedTime(stopWatch.Elapsed);
}
}
private static void WhileLoop(int max)
{
int i = 0;
while (i <= max)
{
//Console.WriteLine(i);
i++;
};
}
private static void ForLoop(int max)
{
for (int i = 0; i <= max; i++)
{
//Console.WriteLine(i);
}
}
private static void DisplayElapsedTime(TimeSpan ts)
{
// Format and display the TimeSpan value.
string elapsedTime = String.Format("{0:00}:{1:00}:{2:00}.{3:00}",
ts.Hours, ts.Minutes, ts.Seconds,
ts.Milliseconds / 10);
Console.WriteLine(elapsedTime, "RunTime");
}
}
I also tried to benchmark the different kinds of loop in C#. I used the same code as Shane, but I also tried with a do-while and found it to be the fastest. This is the code:
using System;
using System.Diagnostics;
public class Program
{
public static void Main()
{
int max = 9999999;
Stopwatch stopWatch = new Stopwatch();
Console.WriteLine("Do While Loop: ");
stopWatch.Start();
DoWhileLoop(max);
stopWatch.Stop();
DisplayElapsedTime(stopWatch.Elapsed);
Console.WriteLine("");
Console.WriteLine("");
Console.WriteLine("While Loop: ");
stopWatch.Start();
WhileLoop(max);
stopWatch.Stop();
DisplayElapsedTime(stopWatch.Elapsed);
Console.WriteLine("");
Console.WriteLine("");
Console.WriteLine("For Loop: ");
stopWatch.Start();
ForLoop(max);
stopWatch.Stop();
DisplayElapsedTime(stopWatch.Elapsed);
}
private static void DoWhileLoop(int max)
{
int i = 0;
do
{
//Performe Some Operation. By removing Speed increases
var j = 10 + 10;
j += 25;
i++;
} while (i <= max);
}
private static void WhileLoop(int max)
{
int i = 0;
while (i <= max)
{
//Performe Some Operation. By removing Speed increases
var j = 10 + 10;
j += 25;
i++;
};
}
private static void ForLoop(int max)
{
for (int i = 0; i <= max; i++)
{
//Performe Some Operation. By removing Speed increases
var j = 10 + 10;
j += 25;
}
}
private static void DisplayElapsedTime(TimeSpan ts)
{
string elapsedTime = String.Format("{0:00}:{1:00}:{2:00}.{3:00}", ts.Hours, ts.Minutes, ts.Seconds, ts.Milliseconds / 10);
Console.WriteLine(elapsedTime, "RunTime");
}
}
and these are the results of a live demo on DotNetFiddle:
Do While Loop:
00:00:00.06While Loop:
00:00:00.13For Loop:
00:00:00.27
Source: Stackoverflow.com