strtof() function misplacing decimal place

Question

I have a string "1613894376.500012077" and I want to use strtof in order to convert to floating point 1613894376.500012077. The problem is when I use strtof I get the following result with the decimal misplaced 1.61389e+09. Please help me determine how to use strof properly.

The conversion was successful. It's a matter of how you're displaying it. — dbush, Mar 19 '21 at 22:52
Floats only have approximately 6 or 7 decimal digits of accuracy, so 1.61389 x 10^9 seems about right. — Mike Vine, Mar 19 '21 at 22:52

chux - Reinstate Monica · Answer 1 · 2021-03-20T15:08:48.910

4

A typical float is 32-bit and can only represent exactly about 2³² different values. "1613894376.500012077" is not one of those.

"1.61389e+09" is the same value as "1613890000.0" and represents a close value that float can represent.

The 2 closest floats are:

1613894272.0 
1613894400.0 // slightly closer to 1613894376.500012077

Print with more precision to see more digits.

edited Mar 20 '21 at 15:08

answered Mar 19 '21 at 22:55

chux - Reinstate Monica

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Thank you, I switched to `strtod()` and in order to properly print with more precision used `std::fixed` and `std::setprecision(9)`. – GigaWatts Mar 21 '21 at 01:45

score 1 · Answer 2 · answered Mar 19 '21 at 23:01

The decimal point is not misplaced. The notation “1.61389e+09” means 1.61389•10⁹, which is 1,613,890,000., which has the decimal point in the correct place.

The actual result of strtof in your computer is probably 1,613,894,400. This is the closest value to 1613894376.500012077 that the IEEE-754 binary32 (“single”) format can represent, and that is the format commonly used for float. When you print it with %g, the default is to use just six significant digits. To see it with more precision, print it with %.999g.

score 0 · Answer 3 · answered Mar 21 '21 at 16:09

The number 1613894376.500012077 is equivalent (the same number up to the precision of the machine as 1.61389e+09.) The e+09 suffix means that the decimal point is located nine decimal digits right the place it has been placed (or that the number is multiplied by 10 to the ninth power). This is a common notation in computer science called scientific notation.

strtof() function misplacing decimal place

3 Answers3