Simple loop, which one I would get more performance and which one is recommended? defining a variable inside a loop or outside of it?

Question

Variable outside of the loop

int number = 0;
for(int i = 0; i < 10000; i++){
     number = 3 * i;
     printf("%d",number);
}

or Variable inside of the loop

for(int i = 0; i < 10000; i++){
     int number = 3 * i;
     printf("%d",number);
}

Which one is recommended and which one is better in performance?

Edit:

This is just an example to exhibit what I mean, All I wanna know is if defining a variable inside a loop and outside a loop means the same thing , or there's a difference.

Answer 1

Time to learn something early: any optimization you could make on something like this will be irrelevant in the face of printf.

Printf will be very, very slow. You could quintuple the math and get no measurable speed decrease. It's just the nature of printing to the terminal.

As for your edited question, there is no difference defining it in the loop or out. Imagine that

for (i = 0; i < 500; i++) {
  int a = i * 3;
}

is just the same as

int forloop::a; // This doesn't work, the idea is to show it just changes the scope
for (i = 0; i < 500; i++) {
  a = i * 3;
}

They will produce identical code, unless you start needing to use that variable outside of the loop it is defined in, because it is defined in the local scope of the loop. So...more like this:

int forloop::a;    // Still not valid code, just trying to show an explanation
namespace forloop {
for (i = 0; i < 500; i++) {
  a = i * 3;
}
} // namespace forloop

If this is unclear please let me know I'll go into more detail or explain differently.

Answer 2

Do not bother you with performance at first: make it safe before everything.

I would just quote Scott Meyers (Effective C++) for your concern: "Postpone declarations as far as you can". Thus, the second pattern is safer.

Example:

int j = 0;
for(int i = 0; i < 10000; i++){
    j = 3 * i;
    printf("%d",j);
}
...
// Use of j out of control!!!
int k = j * 5;

Now with the second pattern:

for(int i = 0; i < 10000; i++){
    int j = 3 * i;
    printf("%d",j);
}
...
// j not declared at this point.
// You get informed of the mistake at compile time, which is far much better.
int k = j * 5; 

Answer 3

You do have a C++ tag, and you mention "declaring a string" in the question. Therefore there might be a performance difference (and yes, the printf could swamp it). Declaring a non-simple variable means calling a constructor, which might mean a non-trivial amount of work. In that case, declaring it inside of the loop could be hiding significant work in what appears to be an innocent declaration.

In general, the answer is that if you really care about performance - and treating the sample code as only an example of the difference between two places to declare a variable - then for non-simple variables, it is better to declare it outside the loop, unless the semantics require a fresh version of a temporary at each iteration.

There are likely many other places first to look at if performance is an issue, but one consideration is always moving loop invariants out of loops, especially if it is much easier for you to tell that it is invariant than for the compiler. And what looks like a declaration, can, in C++, fall into that category.

If, for (silly) example, you have

int k = 43;
for ( int i = 0; i < N; ++i )
    {
    int j = 17 + k; // k was previously defined outside the loop, but doesn't change in it
    l = j * j; // l was also declared outside the loop
    }

any good optimizing compiler can recognize that k is constant, and that j is always assigned 60, and l is assigned 3600 N times, and the loop can simply be removed and replaced with a single assignment to l. Here k and j are both loop invariants.

But a not-quite-so-good compiler might miss even one link in that chain, and wind up creating the loop.

It gets harder for the compiler to figure things out when you have

 Foo k( 43 );  // a class that takes an int argument to its constructor
 for( int i = 0; i < N; ++i )
    {
    Bar j( k ); // a Bar takes an int argument, adds 17 and stores it.
    l = j.squared();
    }

Same invariants. Not as easy to detect without looking inside the workings of bar; and if the constructor and squared method aren't inline, we've just made it slower.

Answer 4

In this case, printf("%d", i * 3) would be better than defining the variable at all.

Answer 5

To answer your question and not nitpicking:

The difference between the 2 variants is, that you are declaring your variable number in different "variable environments" - by which I mean that the scope changes. A variable environment is given by your curly braces { ... }. Everytime you open a new curly brace like this { ... { ... } ... }, you declare a new variable environment inside the old one, which means, that if you declare numbers like so:

{ ... { int numbers; ... } ... }

this variable will only be visible or existent in the innermost environment. So

{ ... { int numbers; ... } ... do_something(numbers); ... }

will give a compiler error.

And to your concerns about performance: Neither variant is better performing. Most, if not all compilers will give the same assembly.