How to "zoom in" on the one number before the decimal?

The "floor" of x gives you the integer part obtained by rounding down, so e.g. floor(12345.67) = 12345. (I'm purposely sweeping the issue of recurring 9s in the decimal under the rug here.)

Assuming N is a nonnegative integer, the remainder of N when divided by 10 (often written N % 10) is the last digit, so e.g. 12345 % 10 = 5.

So the last digit before the decimal point of a nonnegative real number x is floor(x) % 10.

If x is negative then you can just flip the sign by taking the absolute value |x| and then apply the above, so the formula that works for all real numbers x is floor(|x|) % 10.
Modulo 10 should do it.
Is this a programming problem? In that case the modulo operation will probably help you though to the best of my knowledge most programming languages implement modulo in integer form i.e. you will lose everything after the decimal. If you absolutely need everything after the decimal and the width after the decimal point is fixed then you form a dummy variable by multiplying your original variable with 10\^(precision) and then doing a modulo like: (dummy variable) % 10\^ (precision + 1)

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