help explain

[elegnat6]

i have a infill code,but the problem is that i don't understand why put ! in this code(!in.is_open()) i understand that
is_open to check whether the file has successfully been opened, if so prints out the content of the file, otherwise -like for example if the file to be opened doesn't exist- an error message is shown.

[Stewbond]

The ! is a logical "not".

You already know that:

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if ( in.is_open() ) { // do something }

if (the file "in" is open) { do something }

So the logical not, turns our statement into:

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if ( !in.is_open() ) { // do something }

if (the file "in" is not open ) { do something }