Can a 'for' loop inside of a 'for' loop use the same counter variable name?

c loops for-loop nested
Uclydde picture Uclydde · Jul 27, 2018 · Viewed 13.1k times · Source

Can I use the same counter variable for a for loop inside of a for loop?

Or will the variables affect each other? Should the following code use a different variable for the second loop, such as j, or is i fine?

for(int i = 0; i < 10; i++)
{
  for(int i = 0; i < 10; i++)
  {
  }
}

Answer

Eric Postpischil picture Eric Postpischil · Jul 27, 2018

You may use the same name (identifier). It will be a different object. They will not affect each other. Inside the inner loop, there is no way to refer to the object used in the outer loop (unless you make special provisions for that, as by providing a pointer to it).

This is generally bad style, is prone to confusion, and should be avoided.

The objects are different only if the inner one is defined separately, as with the int i you have shown. If the same name is used without defining a new object, the loops will use the same object and will interfere with each other.

Related questions

For vs. while in C programming?

There are three loops in C: for, while, and do-while. What's the difference between them? For example, it seems nearly all while statements can be replaced by for statements, right? Then, what's the advantage using while?

Read More
How do I declare several variables in a for (;;) loop in C?

I thought one could declare several variables in a for loop: for (int i = 0, char* ptr = bam; i < 10; i++) { ... } But I just found out that this is not possible. GCC gives the following error: error: expected unqualified-id before 'char' …

Read More
Why is 'continue' statement ignoring the loop counter increment in 'while' loop, but not in 'for' loop?

Why does it tend to get into an infinite loop if I use continue in a while loop, but works fine in a for loop? The loop-counter increment i++ gets ignored in while loop if I use it after continue, …

Read More