c++cfloating-pointprintf

What does floating point error -1.#J mean?


Recently, sometimes (rarely) when we export data from our application, the export log contains float values that look like "-1.#J". I haven't been able to reproduce it so I don't know what the float looks like in binary, or how Visual Studio displays it.

I tried looking at the source code for printf, but didn't find anything (not 100% sure I looked at the right version though...).

I've tried googling but google throws away any #, it seems. And I can't find any lists of float errors.


Solution

  • It can be either negative infinity or NaN (not a number). Due to the formatting on the field printf does not differentiate between them.

    I tried the following code in Visual Studio 2008:

    double a = 0.0;
    printf("%.3g\n", 1.0 / a);  // +inf
    printf("%.3g\n", -1.0 / a); // -inf
    printf("%.3g\n", a / a);    //  NaN
    

    which results in the following output:

    1.#J
    -1.#J
    -1.#J
    

    removing the .3 formatting specifier gives:

    1.#INF
    -1.#INF
    -1.#IND
    

    so it's clear 0/0 gives NaN and -1/0 gives negative infinity (NaN, -inf and +inf are the only "erroneous" floating point numbers, if I recall correctly)