Difference between using a "-" and a "," in a sentence

I have noticed that some native english speakers use the "-" sign to explain some additional things in a sentence: eg

Sophie is going to the shop - which happened to be around the corner - to buy some groceries.

Before knowing this notation I would write the same sentence as follows:

Sophie is going to the shop, which happened to be around the corner, to buy some groceries.

To me the "-"-sign seems much clearer than a comma. Am I using the "-" correctly. If so what is the difference between a "," and a "-".

Solution 1:

Dashes, commas, and parenthesis are similar in that they are all used for parenthetical elements in a sentence, but they are are not identical.

The 'normal' is to use commas: the cat, which was dead, was in the middle of the road

Dashes are used when you want to 'shout' the parenthetical element, or it is surprising in some way: the cat - which was dead - was in the middle of the road

Dashes should also be used when the parenthetical element itself contains a comma: the cat - which was dead, and rather flat - was in the middle of the road

Parenthesis, which are slightly different in needing an opening and closing bracket, are used when you want to 'whisper' the parenthetical element: the cat (which was dead...) was in the middle of the road

There is no right or wrong as such; it is more of a style thing.

Solution 2:

I'll import what I posted on a now-closed thread:

Perhaps it would be better to address parentheses [as in OP's example] as parts of sentences before discussing the punctuation involved; it can be confusing that the word has such a dual role. Jose Carillo comments:

... information ... set off by the punctuation marks — whether by commas, dashes, or parentheses — is called a parenthetical [or simply parenthesis, EA], and its distinguishing characteristic is that the sentence remains grammatically and semantically correct even without it. A parenthetical is basically added information; however, it isn’t necessarily optional or semantically expendable. It may be needed to put the statement in a desired context, to establish the logic of the sentence, or to convey a particular tone or mood for the statement. In fact, the punctuation chosen for a parenthetical largely determines its optionality or importance to the statement.

Carillo goes on to discuss the choice between commas, dashes, or parentheses to set off parentheses (parentheticals). He may be over-analytical; it is true, however, that commas signal the least abrupt interruption to the matrix sentence and dashes the most abrupt.

Mark Nichol discusses different uses of parentheticals, though parentheses would not be preferred over commas say for all types in all situations.

Solution 3:

This piece of punctuation is called a dash. It is used to indicate a strong interruption in a sentence. In your example, a pair of commas would be the normal way to set off the non-defining relative clause. In general, you need a very good reason to use a pair of dashes.

From the advice of Larry Trask:

The dash has only one use: a pair of dashes separates a strong interruption from the rest of the sentence. (A strong interruption is one which violently disrupts the flow of the sentence.) Here are some examples:

• An honest politician — if such a creature exists — would never agree to such a plan.
• There was no other way — or was there?
• John, do you suppose you could — oh, never mind; I'll do it.

Solution 4:

Sometimes the em dash is indicated by two hyphens together, like this: --.

I think em dashes might be used more widely in America than in England. Here you can use a pair of them just as you have done, instead of parentheses or instead of commas around an appositive. The difference between em dashes and those marks often seems insignificant to me, though the Chicago Manual says the dash indicates a more abrupt change of thought.

You can also use a single em dash instead of a colon:

"We ate them all: bread, crackers, and chips."

"We ate them all--bread, crackers, and chips."

I'd say the choice in all cases is a matter of personal style, though dashes get confusing if you use more than two in a sentence.