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Over the years I've been running into many examples (esp. in software source code comments) where people use the word "similar" when, from the context, it seems to me that they really mean "identical".

So my question is: Is it possible to use "similar" interchangeably with "identical"?

And if not do you maybe have any insights why this appears to be an at least not uncommon error?

Added OP's comment: "The most recent example I came across was: "Warn about similar names between variables, events and functions". Yet the code in question was checking for equality not similarity."

closed as off-topic by Edwin Ashworth, David, Davo, NVZ, RaceYouAnytime Jul 28 '17 at 14:58

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    Can you share some specific examples where you saw this usage? Are you sure it's not meant to be ironic or humorous? – The Photon Jul 27 '17 at 18:12
  • They are similar and not identical. However, let's consider that many technical people have English as a second or third language. Also, the words get a similar point across, but similar is safer. No one will say "You're completely wrong, they are not identical!" – Yosef Baskin Jul 27 '17 at 18:14
  • @ThePhoton The most recent example I came across was: "Warn about similar names between variables, events and functions". Yet the code in question was checking for equality not similarity. – UloPe Jul 27 '17 at 18:25
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    I think this is a case of nobody spends any more time than absolutely necessary to write comments in code. If I wrote "similar" and a second later I realize I should have said "equal", only 10% chance I go back and change it. Even less if the code is already committed to version control. – The Photon Jul 27 '17 at 18:37
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    Please include the research you’ve done. Dictionary definitions go a long way towards answering this. – Edwin Ashworth Jul 27 '17 at 19:01
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Similar does not mean identical! And technical people should know this, from geometry. For example, all equilateral triangles are similar; all have all three angles equal to sixty degrees (and all three sides of equal length), but their sizes can be different. Two equilateral triangles are identical only if they are of the same size.

Similarity (geometry):

Two geometrical objects are called similar if they both have the same shape, or one has the same shape as the mirror image of the other. More precisely, one can be obtained from the other by uniformly scaling (enlarging or reducing), possibly with additional translation, rotation and reflection. This means that either object can be rescaled, repositioned, and reflected, so as to coincide precisely. with the other object.

If we allow similar to mean identical in a non-technical context, we are opening a door to chaos.

  • Examining the maths sense undermines your argument, as identical (congruent) shapes are indeed similar. Just as all squares are rectangles. There is an overlap. – Edwin Ashworth Jul 27 '17 at 21:54
  • The example given is exactly why I would expect this usage, "Warn about similar names between variables, events and functions." Just as an initial impression, I would expect this to check for names that are almost identical - maybe they have different capital letters. Which is likely to cause problems in many programming contexts and should be checked for. – Tyrannosaur Jul 27 '17 at 22:25
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Similar and identical have different meaning to be precise. But if you have noticed people using it, that could be because many times people start using the first word that comes to their mind and they don't evaluate that word to be precisely what they are trying to say.

Alternatively - it is also true that you can use similar for identical anyway because anythings which are identical are definitely similar and the person using this word is not sure if those things are really identical (how do you know just by looking at what is visible, two identical tennis balls might be different as in one of them bounces lower than the other). So it is safer to use "similar" than "identical" in those situations.

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    Your comment made me think about this in terms of set theory. One could say that "identical items" are a strict subset of "similar items". – UloPe Jul 27 '17 at 18:45
  • Correct, I agree with your statement :) – V_Singh Jul 28 '17 at 22:50
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I looked up similar and identical in the Oxford dictionary, and found the following definitions:

similar (adj) Having a resemblance in appearance, character, or quantity, without being identical.

identical (adj): Similar in every detail; exactly alike.

Each of the definitions use the other word as part of the definition, and this shows that the two words are definitely very related. If you look at the parts in italics, though, it becomes apparent that these words have a big difference. If you notice, the definition of similar specifically states, "without being identical", so that makes it pretty obvious that the two words are not the same.

So what is the difference?

Similar refers to when things have some things in common, yet differ in some way. Even if they have almost every detail matching, some tiny detail will not be the same.

thing 1 and 2

Identical means that the things are exactly the same, with absolutely no deviations.

thing 1 and 1

Are these words interchangeable?

No. They have two different meanings. If the items are different in any way, they can only be similar. If they match in every single aspect, they are identical.

Why do people mistakenly confuse the two words?

Well, why do people make any mistake? Many times it is just because they are uniformed and don't know better. Other times is because they don't think too much before choosing words to use. The words are after all very...um similar, so if a person is not too familiar with this vocabulary, or simply does not put much though into their actions, they can make an honest mistake.

  • Hello, Tot. This should have been presented by OP (which might well have rendered the question irrelevant). – Edwin Ashworth Jul 27 '17 at 21:49
  • @EdwinAshworth I agree that had the OP looked up the definitions, it would have been clear that the two words have a difference. Just from the definitions, however, it may not be clear what exactly the difference is. The rest of my answer is an attempt to make it clearer what the difference between the two words is. – Tot Zam Jul 27 '17 at 21:54
  • But ELU is aimed at proficient Anglophones, who should be able to distinguish the two definitions you give with ease. The mathematical sense of 'similar' is applicable to identical (congruent) shapes, so there is some overlap. The only reason I didn't close-vote the question on general reference grounds is that there may be some other area of overlap. Perhaps OED lists the words as synonyms at some level. (But OP should have shown some research.) – Edwin Ashworth Jul 27 '17 at 22:53

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