What does the constant E do in the c language
The documentation for floating-point constants is here.
The form 1e2 or 1E2 means 1 times ten to the power of 2, and you’re perfectly correct that 2E-1 means 2 times ten to the power of -1, or 0.2.
It’s based on the scientific E notation linked by Eugene.
When I run the code it prints a weird number
That’s just because you used the wrong format specifier. These are documented here, and you can see you should use one of eEfFgG for doubles.
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