Thread: mysterious numbers in a float

  1. #1
    Registered User
    Join Date
    Jun 2007

    mysterious numbers in a float

    Hi everyone,
    I just began to learn c, and here's the first problem I encountered,
    the text is about the 3 data types (int, float, char). Here particularly, it talks about precision in floats. Just to test it, I set up a simple code, Here:

    adrian@Adriansys:~/ccode$ cat floating.c
    #include <stdio.h>
      float num1 = 91.273;
      printf("\n%.1f\n", num1);
      printf("%.2f\n", num1);
      printf("%.3f\n", num1);
      printf("%.6f\n", num1);
      printf("%f\n", num1);
      printf("%.14f\n", num1);
    adrian@Adriansys:~/ccode$ ./floating

    And the question is, if num1 was made to be equal to 91.273, where are these numbers after .273 coming from? (The 0's are obvious, but the 26245........)

    Thanks for all the help,

  2. #2
    Deathray Engineer MacGyver's Avatar
    Join Date
    Mar 2007
    Floating point numbers are not exact when dealing with binary just like the decimal system cannot handle all possible decimal numbers in that representations (such as 10/3 ).

    Depending upon the system used, floating point numbers may be represented in many different fashions. One such fashion is the IEEE representation of binary numbers.

  3. #3
    Registered User
    Join Date
    May 2007
    This is a classic issue - every programmer encounters it.

    It's been covered so many times, you're likely to get links to some reference on the material everywhere you ask.

    They key summary is that floats and doubles are approximations. Occasionally they return an exact value (like 3.0), and on other occasions they seem to refuse to accept an exact value (like 2.99). While there are means of dealing with the issue, there is no means of "correcting" a double or float (in a way that's consistent - it can be maddening).

    Two caveats in dealing with this fact. First, if you require precision, you'll need to use other means (large integers, such as storing values in pennies instead of dollars with decimals), or you'll have to accept the minor error. This means, two, that when comparing values, you often must consider a small range instead of a simple equality.

  4. #4
    Registered User
    Join Date
    Jun 2007
    Thanks for the help guys,
    Now everything seems to be clear.

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