For example:
for (i = 0; i < 100; i++)
{
string myvar = "";
// Some logic
}
Do it make performace or memory leak?
Why i do this, because i don't want "myvar" accessible outside the for..loop.
It is any performance monitor, i can compare the execute time between two snippet or whole program ?
thanks you.
No, variables are purely for the programmer's convenience. It doesn't matter where you declare them. See my answer to this duplicate question for more details.
Perhaps you could check out an old test that I once did regarding another conversation. Variable declaration. Optimized way
The results turned out that it was faster to redefine but not as easy on memory.
My simple test. I initialised an object 100,000,000 times and it is was apparently faster to create a new one instead of re-using an old one :O
string output = "";
{
DateTime startTime1 = DateTime.Now;
myclass cls = new myclass();
for (int i = 0; i < 100000000; i++)
{
cls = new myclass();
cls.var1 = 1;
}
TimeSpan span1 = DateTime.Now - startTime1;
output += span1.ToString();
}
{
DateTime startTime2 = DateTime.Now;
for (int i = 0; i < 100000000; i++)
{
myclass cls = new myclass();
cls.var1 = 1;
}
TimeSpan span2 = DateTime.Now - startTime2;
output += Environment.NewLine + span2.ToString() ;
}
//Span1 took 00:00:02.0391166
//Span2 took 00:00:01.9331106
public class myclass
{
public int var1 = 0;
public myclass()
{
}
}
To provide a real life example:
I just finished writing a .obj model loader which, of course, contains some nested loops. I declared all my variabeles above my loops but then I started wondering the same thing as the OP and found this thread. So I tried and moved all variable declarations to the first point in my loop where I use them and actually saw a small performance gain. A model that previously took 380 ms on average to load (370-400 ms actually) now consistently loads about 15-20 ms faster. This is just about 5% but nontheless an improvement.
My loop construct consists of just a foreach-loop and a nested for-loop but also contains a lot of if-statements. I moved 5 variable declarations, most of which are arrays of strings and ints.
UPDATE:
if your are using the same type of variable as the loop you could do what I originally sugested:
for (int i = 0, myvar = 0; i < 100; i++) {
//some logic
}
otherwise don't worry about it as others already suggested
@phoog, thx for checking the answer
I believe this would be a performance problem. Since the VM would need to allocate memory to store a reference to the String every loop. Even though the reference may be to the same String instance, allocating memory every time it goes around the loop would not be preferable.
Yes, that would create a memory usage problem. I don't believe it would result in a memory leak as the Garbage Collector would eventually collect the unused objects, but that will have a negative impact on the performance of the application.
I would recommend you use a string builder declared outside the scope of the for loop.
It won't exactly decrease performance or cause a memory leak, but it is still something to be cautious of.
Strings are immutable, which means that once created, they can't be changed. By creating a new string inside the loop, you are creating at least n string variables. If you're trying to do string manipulation inside a loop, you should consider using a StringBuilder instead.
Unless the compiler somehow optimizes your code, declaring a variable inside a for loop will require the allocation of new variables and the collection of old ones. That being said, the compiler does a really good job at optimizing your code.
If you want a quick way to test your two scenarios, use the StopWatch class to measure how much time it takes to execute each case. My guess is that this difference will be nonexistent to negligible.
