1e-9 or -1e9, which one is correct?

Neither is more correct than the other. They just represent different values.

1e-9 is 0.000000001; the minus sign applies to the exponent.

-1e9 is -1000000000.0; the minus sign applies to the number itself.

The e (or E) means “times 10-to-the”, so 1e9 is “one times ten to the ninth power”, and 1e-9 means “one times ten to the negative ninth power”. In mathematical scientific notation, this is usually denoted by a superscript: 1 × 10-9 or -1 × 109. Programming languages adopted the e or E notation because it was easier to type and print than a superscript (and still is, for that matter). (I think this may have been introduced by Fortran in the 1950s, but I’m not sure of the exact history.)

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