I can't understand: what's the difference between complicated and complex?
They seem to be used interchangeably. Are they actually different at all?
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Complex is used to refer to the level of components in a system. If a problem is complex, it means that it has many components. Complexity does not evoke difficulty.
On the other hand, complicated refers to a high level of difficulty. If a problem is complicated, there might be or might not be many parts but it will certainly take a lot of hard work to solve.
I am forced by my nature to add a more pedantic answer to the list. The content of this answer does not reflect my personal opinion, nor does it reflect common usage, but it does explain why the two words are not precisely interchangeable in all circumstances.
Complexity is intrinsic. Something is complex if it involves a lot of [metaphorical] moving parts even when considered as a Platonic ideal.
Complication is extrinsic. Something is complicated by external influences, or because of external influences.
Pedantically, something can be complex without being complicated, or complex because it is complicated. (Things are rarely complicated without also being complex.) In realms where precision is important, there is often a distinct division between the terms. In medicine, for instance, a broken bone may be described as a complex fracture because the fracture is complicated by breaking the skin, inviting the risk of infection.
Here endeth the pedantry.
In common use, complex is more usually used in a technical sense. Complicated is more likely to appear in everyday language among the general population since most complexity in everyday life is complicated in some way. Complicated may not always be precisely correct, but most people don't spend their lives with a dictionary in one hand and a thesaurus in the other looking for le seul mot juste -- they already have a word that means what they want to say.
In the Zen of Python, we have an aphorism
Simple is better than complex. Complex is better than complicated.
It's a handy guideline when you are designing something.
One subtle distinction between the two is that where the complexity was in some sense introduced, it is probably more common to use "complicated" than "complex." For example,
The instructions were way too complicated.
It is of course true that the instructions were also complex, but the silent implication in using "complicated," the past participle of "to complicate," is that somebody caused the complexity, as opposed to it being inherent in the subject. As a result, "complicated" sometimes has a negative connotation, effectively meaning "unnecessarily complex."
Quantum mechanics is an inherently complex subject, but the textbook was an even tougher slog because of the author's complicated explanations.
However, as others have said, there is a large overlap in the use of the two words.
Also, the idiomatic retort "It's complicated" is a sort of defense or apology for not being able to give a simple, and often expected, answer:
"What were you doing with Teresa? Don't you love your wife?"
"Yes, but ... it's complicated."
You can use them interchangeably.
Complicated: consisting of a lot of different parts or details and therefore difficult to understand. Examples:
1- The rules of the game seemed very complicated.
2- I didn’t realize programming the VCR would be so complicated.
3- The brain is like a very powerful, very complicated computer.
4- a complicated issue
Complex: a complex process, relationship etc is difficult to understand because it has a lot of parts that are all connected in different ways. Examples:
1- The chemical processes involved are extremely complex.
2- the complex relationship between government and the media
These words have a really large overlap, but they each have some distinct uses.
Complicated can refer to things that are difficult to explain because they are unclear rather than because they have a lot of parts. For example, Facebook's "It's complicated" relationship status could refer to a relationship that is one-to-one, but not well-defined.
Complex also has some technical meanings, as in "Complexity theory". This is related to the idea that the behavior of a system cannot be fully understood even if the parts are understood. This meaning is much more specific than what is conveyed by "complicated".
As reported by the NOAD, the meaning of those words are:
1. consisting of many interconnecting parts or elements; intricate: a complicated stereo system.
• involving many different and confusing aspects: a long and complicated saga.
2. Medicine involving complications: complicated appendicitis.
1. consisting of many different and connected parts: a complex network of water channels.
• not easy to analyze or understand; complicated or intricate: a complex personality, the situation is more complex than it appears.
2. Mathematics denoting or involving numbers or quantities containing both a real and an imaginary part.
3. Chemistry denoting an ion or molecule in which one or more groups are linked to a metal atom by coordinate bonds.
The meaning of complex partially overlaps the meaning of complicated; the difference is that complex is also used as noun, and in specific contexts with a specific meaning (see Chemistry, and Mathematics).
Complex is generally used in technical situations, where a problem has a lot of components and aspects.
Complicated is more used in social situations.
Of course, you could say that "I am in a relationship and it is complex" and "This project's architecture is complicated". It would not be wrong, but it sounds a bit off to the ear.
The usage of these terms is different, that's all.
There can be no better explanation than in this TED talk by Eric Berlow (only three minutes, so go watch it!).
To quote the salient bit:
A well-crafted baguette fresh out of the oven is complex. But a curry/onion/green olive/poppy/cheese bread is complicated.
(That said, I actually disagree with Eric Berlow on the topic of network complexity. I’m a bioinformatician, we deal with complex interaction networks (mainly of the cell), and let me tell you: are they ever complicated. It’s safe to say that we (the scientists) understand almost nothing about these networks and reducing or interpreting these complex networks like Eric Berlow does in his talk is simply not possible. I suspect that this talk is more show than anything else.)
In the context of their use as an adjective in practice, the truth is that there is no clear distinction between the two terms.
There are a few contexts in which one is more idiomatic than the other. For example, while I don’t use Facebook anymore, I recall a dropdown menu for relationship status including an option for “it’s complicated.” At least in American English, “it’s complex” would be somewhat unidiomatic, although people would get your meaning, and hardly anyone would even pause, let alone correct you.
Some answers here indicate that one of the terms refers to the abundance of constituent parts, whereas the other refers to a level of difficulty inherent in understanding the subject. However, if you look at a dictionary definition, you will often find both definitions listed, and/or one word defined using the other. Here’s a screenshot of the Google search result for “complex definition” which demonstrates both, which is attributed to Oxford Languages:
Furthermore, any system with a sufficient number of constituent parts will also be inherently difficult to understand, so in a sense the two definitions are practically synonymous.
I contend that there is no correct answer to this question which provides clear separation between the two terms.
Unless you encounter both terms in a single technical document, and that document provides a glossary which clarifies the intended definitions, you would do well to treat them as synonyms.