What is a “side effect?”

Question

I haven’t clearly understood the concept of side effect.

• What is side effect in programming?
• Is it programming language dependent?
• Is there such a thing as external and internal side effects?

Please give some example of causes that create side effects.

Answer 1

A side effect refers simply to the modification of some kind of state - for instance:

• Changing the value of a variable;
• Writing some data to disk;
• Enabling or disabling a button in the User Interface.

Contrary to what some people seem to be saying:

• A side effect does not have to be hidden or unexpected (it can be, but that has nothing to do with the definition as it applies to computer science);

• A side effect has nothing to do with idempotency. An idempotent function can have side effects, and a non-idempotent function may have no side effects (such as getting the current system date and time).

It's really very simple. Side effect = changing something somewhere.

P.S. As commenter benjol points out, several people may be conflating the definition of a side effect with the definition of a pure function, which is a function that is (a) idempotent and (b) has no side-effects. One does not imply the other in general computer science, but functional programming languages will typically tend to enforce both constraints.

Answer 2

Any operation which modifies the state of the computer or which interacts with the outside world is said to have a side effect. See http://en.wikipedia.org/wiki/Side_effect_(computer_science).

For example, this function has no side effects. You can call it as often as you like, and you will always get the same results for the same arguments:

int square(int x) { return x * x; }

In contrast, calling these functions will give you different results depending upon the order in which you call them, because they change something about the state of the computer:

int n = 0;
int next_n() { return n++; }
int set_n(int newN) { return n = newN; }      

This function has the side effect of writing data to output. You don't call the function because you want its return value; you call it because you want the side effect:

int Write(const char* s) { return printf("Output: %s\n", s); }

Answer 3

A side-effect is when an operation has an effect on a variable/object that is outside the intended usage.

It can happen when you make a call to a complex function that has a side-effect of altering some global variable, even though that was not the reason you called it (maybe you called it to extract something from a database).

I admit I'm having trouble coming up with a simple example that doesn't look totally contrived, and examples from stuff I've worked on are waaaay too long to post here (and since it's work related, I probably shouldn't anyway).

One example I've seen (a while ago) was a function that opened a database connection if the connection was in a closed state. The problem was that it was supposed to close the connection at the end of the function, but the developer forgot to add that code. So here, there was an unintended side effect: calling a procedure was supposed to only do a query and the side effect was that the connection remained open and if the function was called twice in a row, an error would be raised saying the connection was already open.

---

Ok, so since everyone's giving examples now, I think I will too ;)

/*code is PL/SQL-styled pseudo-code because that's what's on my mind right now*/

g_some_global int := 0; --define a globally accessible variable somewhere.

function do_task_x(in_a in number) is
begin
    b := calculate_magic(in_a);
    if b mod 2 == 0 then
        g_some_global := g_some_global + b;
    end if;
    return (b * 2.3);
end;

The function do_task_x has a primary effect of returning the result of some calculations, and a side effect of possibly modifying a global variable.

Of course, which is the primary and which is the side effect could be open to interpretation and might depend on actual usage. If I call this function for the purpose of modifying the global and I discard the returned value than I'd say that modifying the global is the primary effect.

Answer 4

Here is a simple example:

int _totalWrites;
void Write(string message)
{
    // Invoking this function has the side effect of 
    // incrementing the value of _totalWrites.
    _totalWrites++;
    Debug.Write(message);
}

The definition of side effect is not specific to programming so simply imagine the side effects of your meds or eating too much food.

Answer 5

In programming a side effect is when a procedure changes a variable from outside its scope. Side effects are not language dependent. There are some classes of languages which aim to eliminate side effects (pure functional languages), but I'm not sure if there are any which require side effects, but I could be wrong.

As far as I know, there are no internal and external side effects.

Answer 6

A side effect is things that happen in code that aren't obviously apparent.

For example lets say you have this class

public class ContrivedRandomGenerator {
   public int Seed { get; set; }

   public int GetRandomValue()
   {
      Random(Seed);
      Seed++;
   }
}

When you initially create the class you give it a seed.

var randomGenerator = new ContrivedRandomGenerator();
randomGenerator.Seed = 15;
randomGenerator.GetRandomValue();

You don't know the internals, you just expect to get a random value, and you would expect the randomGenerator.Seed to still be 15... but it's not.

The function call had the side effect of changing the Seed value.