which of these for loop styles is more efficient ( was "Plz Help...." )

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February 1st, 2006, 03:33 AM

handshakeit

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which of these for loop styles is more efficient ( was "Plz Help...." )

Hi there,

Tell me which is more efficient of these two
FOR LOOPS

for(int i=0;i<10;i++);
or
for(int i=0;i<10;++i);

pls:-Justify your Answers.

Thanx
Devs

February 1st, 2006, 04:41 AM

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Hello and welcome to Devshed.

Please use an informative thread title in future. For example, in this case, something like "which of these for loops is more efficient" would be a good choice.

Second, what language are you looking at? I'm guessing this is highly language dependant.

Third, premature optimisation is the root of all evil. Something like this is going to provide extremely minor benefits, if any, in most cases.

Fourth - have you tried benchmarking it yourself?

--Simon

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February 1st, 2006, 08:08 AM

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There should be no difference in a compiled language.

There should be very little (IF ANY) difference in an interpreted language and it would be impossible to tell which would be more efficient. Let the compiler optimise for you, you have to read the code easily, not the computer.

As said above, benchmark it yourself.

Comments on this post

Yawmark agrees!

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February 1st, 2006, 10:14 AM

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The ++i will be more efficient, but I won't specify the reason why for the next few days, because this question smells an awful lot like a cut-and-paste homework assignment. I'll also say that when using integer variables, it may not matter for good optimizing compilers because they'll probably optimize it the correct way anyway.

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February 7th, 2006, 03:27 PM

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Now that a week has passed since the last post, I'll answer this question now. The answer should be fairly obvious if you work with operator overloading. See the notes to overload ++ and -- in this tutorial

Note that for a post-increment operator, it returns a value instead of a reference

Quote:

As to why the function returns Complex instead of Complex &, this is fairly easy to explain: The post-increment operator is supposed to return the current value of the object and then increment it. Hence, we store the current value in a temporary variable, increment the members and then return the temporary variable (which holds the previous value of the object).

What this means is that, with ++i, it needs to
(1) find the object (i) in memory,
(2) increment it and
(3) return a reference to that object.

In case of i++, it needs to
(1) find the object,
(2) copy it to a temporary location,
(3) increment the original object and
(4) return the value from the temporary location.

The extra operation is in copying to a temporary location. This doesn't take much effort for an integer, but takes more for a larger class object.

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February 13th, 2006, 07:40 AM

Yawmark

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Note that in Java (1.5), there is no difference in the bytecode:

Java Code:

Original - Java Code

public void pre() {
    for(int i = 0; i < 10; ++i);
}
 
// public void pre();
//   Code:
//    0:    iconst_0
//    1:    istore_1
//    2:    iload_1
//    3:    bipush  10
//    5:    if_icmpge   14
//    8:    iinc    1, 1
//    11:   goto   2
//    14:   return
    
public void post() {
    for(int i = 0; i < 10; i++);
}
 
// public void post();
//   Code:
//    0:    iconst_0
//    1:    istore_1
//    2:    iload_1
//    3:    bipush  10
//    5:    if_icmpge   14
//    8:    iinc    1, 1
//    11:   goto   2
//    14:   return

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February 14th, 2006, 05:11 PM

LyonHaert

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Same in C#. When the value of the incrementer (-or?) is not used (like in for loops, and other times when the incrementation is the whole statement), then the IL is the same whether you use post- or pre-incrementation.

Code:

// C#
int i = 0;
i++; // or ++i, doesn't matter

// MSIL
// (Comments are explanatory, not actually part of MSIL)
.locals init ([0] int32 i)
IL_0000:  ldc.i4.0  // load value 0
IL_0001:  stloc.0  // store in location 0 (i)
IL_0002:  ldloc.0  // load location 0 (i)
IL_0003:  ldc.i4.1  // load value 1
IL_0004:  add
IL_0005:  stloc.0  // store in location 0

However, when you use the value of i, then things are only slightly different between post- and pre-incrementation.

Code:

givens

// C#
int a;
int i = 2;

// MSIL
.locals init ([0] int32 a, [1] int32 i)
IL_0000:  ldc.i4.2
IL_0001:  stloc.1

Code:

post-incrementation

// C#
a = i++; // a == 2, i == 3

// MSIL
IL_0002:  ldloc.1
IL_0003:  dup
IL_0004:  ldc.i4.1
IL_0005:  add
IL_0006:  stloc.1
IL_0007:  stloc.0

Code:

pre-incrementation

// C#
a = ++i; // i == 3, a == 3

// MSIL
IL_0002:  ldloc.1
IL_0003:  ldc.i4.1
IL_0004:  add
IL_0005:  dup
IL_0006:  stloc.1
IL_0007:  stloc.0

No much difference in the IL, just whether dup is used before or after the add.

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