Question

I need to count digits from a float number and keep the number.
I can use scanf() with %f or %c but not %s, and I can use getchar().

I can use getchar but I will loose the number.

Answer 1

Why will you lose the number with getchar?

1. Read characters with getchar until you hit whitespace/enter/end of input
2. Collect them all into a single string
3. Use strtod to make sure it’s a valid floating point value
4. Count digits in the string – either before, or after the point, whatever you need.

If you’re limited to only getchar:

1. Read chars one by one with getchar
2. Keep a state of where you are in the number: before decimal point, at decimal point, or after
3. Keep counting the digits as long as it’s a valid floating point number (i.e. 1 or more digits, then optionally a decimal point with 1 or more digits after it)
4. Collect the digits into a floating point number by shifting powers of 10 (i.e. before decimal point multiply by 10.0 and add new number, after decimal point divide by a growing power of 10 and add).