We Keep Coding

Is it better to initialize a loop iterator once per scope?

I think the question should be self explanatory, and the language I'm thinking about right now is Java, but it probably applies across all languages.
That being said, basically what I'm talking about is whether this:
// Initialize first
int i = 0;
for (i = 0; i < x; i++) {
// do some stuff
}
for (i = 0; i < x; i++) {
// do some more stuff
}
for (i = 0; i < x; i++) {
// do other stuff
}
Is better than this:
// Initializing i in the for loop
for(int i = 0; i < x; i++) {
// do some stuff
}
for(int i = 0; i < x; i++) {
// do some more stuff
}
for(int i = 0; i < x; i++) {
// do other stuff
}
This is a performance question, and I'm talking about initializing once /per/ scope resolution.

I performed a performance test with x=10 to evaluate the performance difference between the in-loop declaration method and the out-of-loop declaration method.
Details: I ran the code 300x with in-loop first and then 300x with out-of-loop first. Each run, I recorded the total runtime in nanoseconds to execute each method 10,000 times. So, I recorded a total of 1200 observations (600 per method). To measure steady-state performance (vice startup performance), I removed the 20 observations from each data set that had the longest duration. (The mean runtime for the 20 startup observations was an order of magnitude larger than the mean runtime for all the other observations.)
Results: A single-factor ANOVA indicates that the in-loop declaration is faster than the out-of-loop declaration (p-value=8.12584E-07). The mean runtimes were 158635.4931 nanoseconds for in-loop and 166943.7397 nanoseconds for out-of-loop. From a practical standpoint, we're talking about a difference of ~0.01ms per 10,000 iterations.
Conclusion: Just use the in-loop declaration. #FallAndLearn also points out that the in-loop declaration is easier to maintain because the local variable i is declared with the smallest scope possible .

You first piece of code is better than second one because int is a value type and its value is stored in stack once you initialize it , later on you just assign that type a value again and again .
On the other hand (second piece of code )you are initializing i three times i.e. creating stack entries three times .
So the first piece of code is better than the second one , performance wise .

The scope of local variables should always be the smallest possible.
Hence if int i is not used outside the loop then second way is always better. More readable too.
Performance wise they are both same. From a maintenance perspective, second option is better.
Also, the answers to this question will depend on your requirement. If your code has other data dependent on i or have only three for loops statement.

Let's check out the disassembled code for the following snippet:
public class Test {
public static void main(String[] args) {
int i = 0;
for(i = 0; i < 3; i++){
//do some stuff
}
}
}
public class Test {
public Test();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iconst_0
3: istore_1
4: iload_1
5: iconst_3
6: if_icmpge 15
9: iinc 1, 1
12: goto 4
15: return
}
Now let's generate another one for the initialization of the control variable inside the loop:
public class Test {
public Test();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iload_1
3: iconst_3
4: if_icmpge 13
7: iinc 1, 1
10: goto 2
13: return
}
They're not the same, I'm not a byte-code expert but I can tell that the second one has less overhead. The first one pushes int constant twice (the two iconst_<i> instructions), and has two istore_<n> instructions, compared to one instruction in the second code.

Performance difference between using int a=a+1 and a++ in Java

Is there any performance difference between using int a=a+1 and a++ in Java?
If so which is better and why? Could you briefly explain me to understand this?

First of all, the Java Language Specification doesn't say anything about timing. But assuming we're using a typical compiler such as Suns javac we see that all of the above examples (a++, ++a, a += 1, a = a + 1) could either be compiled into something like:
iinc instruction, working on variables:
iload_<variable>
iinc <variable>, 1
istore_<variable>
iadd instuction, using the stack (here using variable 1 as a the storage):
iload_1
iconst_1
iadd
istore_1
It's up to the compiler to choose the best possible way to compile them. E.g. there is no difference between them. And it shouldn't be any difference between the statements - they all express the same thing - adding one to a number.
That beeing said, both the iinc and the iadd version can be compiled using the JIT to something fast and platform dependent, and in the end I would assume that a normal runtime compiles both versions into the same assembler code.
With my compiler, *jdk1.6.0_20* the "increment" methods even uses the same instruction.
public class Test {
public static void main(String[] args) {
int a = 0;
a = a + 1;
a += 1;
a++;
++a;
}
}
This is the disassembly:
Compiled from "Test.java"
public class Test extends java.lang.Object{
public Test();
Code:
0: aload_0
1: invokespecial #8; //Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iinc 1, 1 // a = a + 1;
5: iinc 1, 1 // a += 1;
8: iinc 1, 1 // a++;
11: iinc 1, 1 // ++a;
14: return
}

Looking at the generated bytecode:
public static void main(String[] args) {
int x = 1;
int y = 1;
int z = 1;
int a = 1;
int b = 1;
x = x + 1;
y++;
++z;
a += 1;
b += 2;
}
generates (use javap -c classname)
0: iconst_1
1: istore_1
2: iconst_1
3: istore_2
4: iconst_1
5: istore_3
6: iconst_1
7: istore 4
9: iconst_1
10: istore 5
12: iload_1
13: iconst_1
14: iadd
15: istore_1
16: iinc 2, 1
19: iinc 3, 1
22: iinc 4, 1
25: iinc 5, 2
28: return
So using (jdk1.6.0_18):
x = x + 1
creates
12: iload_1
13: iconst_1
14: iadd
15: istore_1
whereas
y++;
++z;
a += 1;
all result in
iinc
However, doing a rough performance test on my laptop resulted in next to no difference in the runtime between the two (sometimes ++x was quicker, sometimes x=x+1 was quicker), so I wouldn't worry about the performance implications.

No, there won't be any noticeable difference. Use what you find the most readable (which is a++, normally).
First rule of code optimization: don't.

The compiler should optimize and there should be no difference at all.
But keep in mind that prefix increment operator may be (it depends by the compiler) faster than the postfix equivalent (in C++ and C# also):
++a faster than a++ because the postfix operator must create a temporary variable.. think about their implementation:
prefix:
a = a + 1;
return a;
postfix:
int tmp = a;
a = a + 1;
return tmp;

a++ is much faster. It converts to INC command of assembler.But I think JVM will optimize a=a+1 so you don't need to care about that.

It's the same, and nowadays with compiler optimization should not be aware of that stuff, to increase your performance check other bigger issues like allocs :)

Numeric assignment should throw error

I was making some tests with numeric conversions and casts in Java and I found this strange behaviour (for me)
class Test {
public static void main(String[] args) {
//int y = 100000000000000; //does not compile
int x = 100000 * 1000000000; //compile (why?)
System.out.println(x); //x is 276447232
}
}
Basically x and y should be the same number: why does it compile?

Integer overflow goes undetected in Java, that's why the multiplication works fine. However, the literal you were stating was too large and thus is a compiler error.
I guess while the vast majority of Java compilers will precompute that value at compilation time that is not required by the JLS. Thus it cannot be an error, since then different compilers may compile a piece of code or yield errors – something that isn't so nice.
The Sun Java compiler actually performs the calculation as the disassembly shows:
Compiled from "x.java"
class x extends java.lang.Object{
x();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: ldc #2; //int 276447232
2: istore_1
3: getstatic #3; //Field java/lang/System.out:Ljava/io/PrintStream;
6: iload_1
7: invokevirtual #4; //Method java/io/PrintStream.println:(I)V
10: return
The important thing to note here is that for all intents and purposes the result must be the same as if it were computed during runtime. Therefore no compiler error can occur here.

Java deals with overflowing an integer not by an exception but by instead flipping over to negative values and continuing upward.
As an example, here's this code snippet:
int x = Integer.MAX_VALUE + 1;
System.out.println(x);
Java deals with it the best it can. That snippet outputs:
-2147483648
Which is the value of Integer.MIN_VALUE. You just have to be careful when you are dealing with really big numbers. That's why Java has a BigInteger class to deal with arbitrarily big numbers that go beyond the limit of integers.

Which of the following Java coding fragments is better?

This isn't meant to be subjective, I am looking for reasons based on resource utilisation, compiler performance, GC performance etc. rather than elegance. Oh, and the position of brackets doesn't count, so no stylistic comments please.
Take the following loop;
Integer total = new Integer(0);
Integer i;
for (String str : string_list)
{
i = Integer.parse(str);
total += i;
}
versus...
Integer total = 0;
for (String str : string_list)
{
Integer i = Integer.parse(str);
total += i;
}
In the first one i is function scoped whereas in the second it is scoped in the loop. I have always thought (believed) that the first one would be more efficient because it just references an existing variable already allocated on the stack, whereas the second one would be pushing and popping i each iteration of the loop.
There are quite a lot of other cases where I tend to scope variables more broadly than perhaps necessary so I thought I would ask here to clear up a gap in my knowledge. Also notice that assignment of the variable on initialisation either involving the new operator or not. Do any of these sorts of semi-stylistic semi-optimisations make any difference at all?

The second one is what I would prefer. There is no functional difference other than the scoping.
Setting the same variable in each iteration makes no difference because Integer is an immutable class. Now, if you were modifying an object instead of creating a new one each time, then there would be a difference.
And as a side note, in this code you should be using int and Integer.parseInt() rather than Integer and Integer.parse(). You're introducing quite a bit of unnecessary boxing and unboxing.
Edit: It's been a while since I mucked around in bytecode, so I thought I'd get my hands dirty again.
Here's the test class I compiled:
class ScopeTest {
public void outside(String[] args) {
Integer total = 0;
Integer i;
for (String str : args)
{
i = Integer.valueOf(str);
total += i;
}
}
public void inside(String[] args) {
Integer total = 0;
for (String str : args)
{
Integer i = Integer.valueOf(str);
total += i;
}
}
}
Bytecode output (retrieved with javap -c ScopeTest after compiling):
Compiled from "ScopeTest.java"
class ScopeTest extends java.lang.Object{
ScopeTest();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public void outside(java.lang.String[]);
Code:
0: iconst_0
1: invokestatic #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
4: astore_2
5: aload_1
6: astore 4
8: aload 4
10: arraylength
11: istore 5
13: iconst_0
14: istore 6
16: iload 6
18: iload 5
20: if_icmpge 55
23: aload 4
25: iload 6
27: aaload
28: astore 7
30: aload 7
32: invokestatic #3; //Method java/lang/Integer.valueOf:(Ljava/lang/String;)Ljava/lang/Integer;
35: astore_3
36: aload_2
37: invokevirtual #4; //Method java/lang/Integer.intValue:()I
40: aload_3
41: invokevirtual #4; //Method java/lang/Integer.intValue:()I
44: iadd
45: invokestatic #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
48: astore_2
49: iinc 6, 1
52: goto 16
55: return
public void inside(java.lang.String[]);
Code:
0: iconst_0
1: invokestatic #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
4: astore_2
5: aload_1
6: astore_3
7: aload_3
8: arraylength
9: istore 4
11: iconst_0
12: istore 5
14: iload 5
16: iload 4
18: if_icmpge 54
21: aload_3
22: iload 5
24: aaload
25: astore 6
27: aload 6
29: invokestatic #3; //Method java/lang/Integer.valueOf:(Ljava/lang/String;)Ljava/lang/Integer;
32: astore 7
34: aload_2
35: invokevirtual #4; //Method java/lang/Integer.intValue:()I
38: aload 7
40: invokevirtual #4; //Method java/lang/Integer.intValue:()I
43: iadd
44: invokestatic #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
47: astore_2
48: iinc 5, 1
51: goto 14
54: return
}
Contrary to my expectations, there was one difference between the two: in outside(), the variable i still took up a register even though it was omitted from the actual code (note that all the iload and istore instructions point one register higher).
The JIT compiler should make short work of this difference, but still you can see that limiting scope is a good practice.
(And with regards to my earlier side note, you can see that to add two Integer objects, Java must unbox both with intValue, add them, and then create a new Integer with valueOf. Don't do this unless absolutely necessary, because it's senseless and slower.)

The second one is far better because the first style is should only be used in C code as its mandatory. Java allows for inline declarations to minimize the scope of variables and you should take advantage of that. But you code can be further improved:
int total = 0;
for (String str: stringList) {
try {
total += Integer.valueOf(str);
} catch(NumberFormationException nfe) {
// more code to deal with the error
}
}
That follows the Java code style convention. Read the full guide here:
http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html

It makes no significant difference apart from on the last iteration, when the reference is cleared quicker in the second example (and that would be my preference - not so much for that reason, but clarity.)
Keep the scope to the minimum possible. The hotspot VM does escape analysis to determine when references are no longer accessible, and on the basis of this allocates some objects on the stack rather than on the heap. Keeping scope as small as possible aids this process.
I would ask why you're using Integer instead of a simple int...or perhaps it's just by way of example?

The second is far better. Scoping variables as narrowly as possible makes the code far easier to read and maintain, which are much more important overall than the performance differences between these examples, which are trivial and easily optimized away.

Neither. Integer.valueOf(0); will use a reference to a cached 0. :)

Well, in this case, you're instantiating an Integer primitive every single time you say i = Integer.parseInt(str) (where i is an Integer), so (unless Java knows how to optimize it), both cases are almost equally inefficient. Consider using int instead:
int total = 0;
for (String str : string_list)
{
int i = Integer.parseInt(str);
total += i;
}
Now we're back to the question of whether to put the int declaration on the inside or outside. Assuming the Java compiler has a lick of decent optimization, I'd say it doesn't matter. Efficiency aside, it is considered good practice to declare variables as close as possible to their use.

The second one is the preferable of the two for readability, maintainability, and efficiency.
All three of these goals are achieved because you are succinctly explaining what you are doing and how your variables are being used. You are explaining this clearly to both developers and the compiler. When the variable i is defined in the for block everyone knows that it is safe to ignore it outside of the block and that the value is only valid for this iteration of the block. This will lead the to the Garbage Collector being able to be able to easily mark this memory to be freed.
I would suggest not using Integer for intermediate values. Accumulate the total as an int and after the loop create the Object or depend on auto-boxing.

Assuming you have positive numbers in your list and you're serious with
I am looking for reasons based on
resource utilisation, compiler
performance, GC performance etc.
rather than elegance.
You should implement it by yourself like:
import java.util.ArrayList;
import java.util.List;
public class Int {
public static void main(String[] args) {
List<String> list = new ArrayList<String>();
list.add("10");
list.add("20");
int total = 0;
for (String str : list) {
int val = 0;
for (char c : str.toCharArray()) {
val = val * 10 + (int) c - 48;
}
total += val;
}
System.out.print(total);
}
}
The only GC relevant thing would be toCharArray() which could be replaced by another loop using charAt()

The question of what variable scope to use is a readability issue more than anything else. The code is better understood when every variable is restricted to the scope where it is actually used.
Now, if we inspect the technical consequences of using wide/narrow scopes, I believe that there IS a performance/footpring advantage with narrow scopes. Consider the following method, where we have 3 local variables, belonging to one global scope:
private static Random rnd = new Random();
public static void test() {
int x = rnd.nextInt();
System.out.println(x);
int y = rnd.nextInt();
System.out.println(y);
int z = rnd.nextInt();
System.out.println(z);
}
If you diassemble this code (using javap -c -verbose {class name} for example), you will see that the compiler reserves 3 slots for local variables in the stack frame structure of the test() method.
Now, suppose that we add some artificial scopes:
public static void test() {
{
int x = rnd.nextInt();
System.out.println(x);
}
{
int y = rnd.nextInt();
System.out.println(y);
}
{
int z = rnd.nextInt();
System.out.println(z);
}
}
If you diassemble the code now, you will notice that the compiler reserves only 1 slot for local variables. Since the scopes are completely independent, each time x,y or z are used, the same slot #0 is used.
What does it mean?
1) Narrow scopes save stack space
2) If we are dealing with object variables, it means that the objects may become unreachable faster, therefore are eligible for GC sooner than otherwise.
Again,note that these 2 "advantages" are really minor, and the readability concern should be by far the most important concern.

Second one since you want to keep the scope of your variables as "inner" as possible. The advantage of smaller scope is less chance for collision. In your example, there's only a few lines so the advantage might not be so obvious. But if it's larger, having the smaller-scope variables definitely is more beneficial. If someone else later has to look at the code, they would have to scan all the way back to right outside the method definition to know what i is. The argument is not much different than that of why we want to avoid global variable.

Will reassigning a variable in every iteration of a loop affect performance?

Consider the following two ways of writing a loop in Java to see if a list contains a given value:
Style 1
boolean found = false;
for(int i = 0; i < list.length && !found; i++)
{
if(list[i] == testVal)
found = true;
}
Style 2
boolean found = false;
for(int i = 0; i < list.length && !found; i++)
{
found = (list[i] == testVal);
}
The two are equivalent, but I always use style 1 because 1) I find it more readable, and 2) I am assuming that reassigning found to false hundreds of times feels like it would take more time. I am wondering: is this second assumption true?
Nitpicker's corner
I am well aware that this is a case of premature optimization. That doesn't mean that it isn't something that is useful to know.
I don't care which style you think is more readable. I am only interested in whether one has a performance penalty compared to the other.
I know that style 1 has the advantage of allowing you to also put a break; statement in the if block, but I don't care. Again, this question is about performance, not style.

Well, just write a micro benchmark:
import java.util.*;
public class Test {
private static int[] list = new int[] {1, 2, 3, 4, 5, 6, 7, 8, 9} ;
private static int testVal = 6;
public static boolean version1() {
boolean found = false;
for(int i = 0; i < list.length && !found; i++)
{
if(list[i] == testVal)
found = true;
}
return found;
}
public static boolean version2() {
boolean found = false;
for(int i = 0; i < list.length && !found; i++)
{
found = (list[i] == testVal);
}
return found;
}
public static void main(String[] args) {
// warm up
for (int i=0; i<100000000; i++) {
version1();
version2();
}
long time = System.currentTimeMillis();
for (int i=0; i<100000000; i++) {
version1();
}
System.out.println("Version1:" + (System.currentTimeMillis() - time));
time = System.currentTimeMillis();
for (int i=0; i#lt;100000000; i++) {
version2();
}
System.out.println("Version2:" + (System.currentTimeMillis() - time));
}
}
On my machine version1 seems to be a little bit faster:
Version1:5236
Version2:5477
(But that's 0.2 seconds on a 100 million iterations. I wouldn't care about this.)
If you look at the generated bytecode there are two more instructions in version2 which probably cause the longer execution time:
public static boolean version1();
Code:
0: iconst_0
1: istore_0
2: iconst_0
3: istore_1
4: iload_1
5: getstatic #2; //Field list:[I
8: arraylength
9: if_icmpge 35
12: iload_0
13: ifne 35
16: getstatic #2; //Field list:[I
19: iload_1
20: iaload
21: getstatic #3; //Field testVal:I
24: if_icmpne 29
27: iconst_1
28: istore_0
29: iinc 1, 1
32: goto 4
35: iload_0
36: ireturn
public static boolean version2();
Code:
0: iconst_0
1: istore_0
2: iconst_0
3: istore_1
4: iload_1
5: getstatic #2; //Field list:[I
8: arraylength
9: if_icmpge 39
12: iload_0
13: ifne 39
16: getstatic #2; //Field list:[I
19: iload_1
20: iaload
21: getstatic #3; //Field testVal:I
24: if_icmpne 31
27: iconst_1
28: goto 32
31: iconst_0
32: istore_0
33: iinc 1, 1
36: goto 4
39: iload_0
40: ireturn

Comment about nitpicks corner:
If you're really concerned with absolute performance, putting a break in and removing the "&& !found" will give you theoretically better performance on #1. Two less binary ops to worry about every iteration.
If you wanted to get really anal about optimization without using breaks then
boolean notFound = true;
for(int i = 0; notFound && i < list.length; i++)
{
if(list[i] == testVal)
notFound = false;
}
will run faster in the average case than the existing option #1.
And of course it's personal preference, but I prefer to never put any extra evaluations inside the head of a for loop. I find it can cause confusion while reading code, because it's easy to miss. If I can't get the desired behavior using break/continues I will use a while or do/while loop instead.

Actually, the "if" will slow your program down more than assignment due to the pipeline.

It depends on what compiler you use since different compilers might do different optimizations.

I believe style 2 is ever-so-slightly faster - say 1 clock cycle or so.
I'd rewrite it into the following, though, if I were tackling it:
for(i=0; i<list.length && list[i]!=testval; i++);
boolean found = (i!=list.length);

It seems to me that if you expect your value to be found before the end of the list, you'd be better off with #2 - as it short circuits the check with !found in the loop conditional. Assuming you put a break in, the 1st option (the only sensible thing, IMO), then pseudo assembly would look something like:
Option 1:
start:
CMP i, list.length
JE end
CMP list[i], testval
JE equal
JMP start
equal:
MOV true, found
end:
Option 2:
start:
CMP i, list.length
JE end
CMP true, found
JE end
CMP list[i], testval
JE equal
JNE notequal
equal:
MOV true, found
JMP start
notequal:
MOV false, found
JMP start
end:
I'd say Option 1 is superior here, as it's about 1/3rd less instructions. Of course, this is without optimizations - but that'd be compiler and situation specific (what is found doing after this? can we just optimize it away all together?).

Here is another style
for(int i = 0; i < list.length; i++)
{
if(list[i] == testVal)
return true;
}
return false;

I think both alternatives leave something to be desired, from a performance point of view.
Think about how many tests (which are almost always jumps) you do per iteration, and try to minimize the amount.
The solution by Matt, to return out when the answer is found, reduces the number of tests from three (loop iterator, found-test in loop, actual comparison) to two. Doing the "found"-test essentially twice is wasteful.
I'm not sure if the classic, but somewhat obfuscating, trick of looping backwards is a win in Java, and not hot enough at reading JVM code to figure it out right now, either.

I would say that in 98% of systems, it does not matter. The difference, if there is any, is hardly noticeable unless that loop is the main portion of code and is running a mindnumbing number of times.
Edit: That is ofcourse assuming that it is not being already optimized by the compiler.

Any decent compiler would keep found in a register for a duration of the loop and so the cost is absolutely negligible.
If the second style is done without a branch then it would be preferable, since the CPU's pipeline will not get disrupted as much ... but that depends on how the compiler uses the instruction set.

This will only be measurable in code which is extremely performance-sensitive (simulators, emulators, video encoding software, etc.) in which case you probably want to manually inspect the generated code anyway to make sure that the compiler actually generates sensible code.

To be sure, you should compile both versions (say with latest compiler from Sun) and examine the generated bytecode with an appropriate tool... That's the only reliable way to know for sure, everything else is wild guess.

boolean found = false;
for(int i = 0; i < list.length && !found; i++)
{
if(list[i] == testVal)
found = true;
}
I don't see a break statement in the block.
Other than that, I prefer this style. It improves readability and thereby the chance that a maintainer mis-reading and mis-fixing it.