Double float, what does it actually mean?

Question

So it says that you need to use "double" before float for long expressions (8 bytes). And then it says 4.21 can be a double float but i thought 8 bytes/double float would mean something like a float with many digits like 0.002348.

Answer 1

Not necessarily.
https://www.doc.ic.ac.uk/~eedwards/compsys/float/
Floating point numbers are represented in a similar way to numbers in scientific notation i.e. 5 × 10², only in binary instead. It depends on how many bits are needed the represent the number so the decimal representation might not be indicative of how much information you need to store. (in any case, usually the full amount of bytes is used once you declare the data type)