I don't know what the function mean_value() does, but it's not getting anything different from one iteration to the next because the badly-named I is not changing inside the loop, so the very same I is going to the mean_value() function on each iteration. This does not make sense.
The other big, big problem is that your mean_value() function may well use the mean() function inside it. However, you overwrite the mean() function by assigning the result of mean_value() to it. Now perhaps the mean() function inside mean_value will still work, but maybe it won't. Either way, it's a horrible, bad, and unwise decision to name a variable/array after the name of a built-in function. Call it "theMeans" or something else, but certainly not "mean".
Also, flag is unnecessary. You can just simply use i instead of flag. But I'd actually use k instead of i or j, which are the imaginary variable.