how to read a digit of a floating point number?????

This is a discussion on how to read a digit of a floating point number????? within the C Programming forums, part of the General Programming Boards category; how do we read a particular digit/digits of a floating point number??? ive jus started out with C....hardly know anything ...

  1. #1
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    Unhappy how to read a digit of a floating point number?????

    how do we read a particular digit/digits of a floating point number???

    ive jus started out with C....hardly know anything and i was presented with this problem....

    im supposed to write a program which accepts a floating point number from the user....and displays the rightmost digit of the integral part....(the digit at ones place)..

    next..it asked me to modify the program such that two od the rightmost digits of the integral part of the number were displayed...

    ill be very thankful for any help...

  2. #2
    Dr Dipshi++ mike_g's Avatar
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    I'd convert it to a string, read it until reaching a dot or null char then get the character before it.

  3. #3
    Deathray Engineer MacGyver's Avatar
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    Few ways of doing it:

      1. I think you can call floor() on the float, and then cast it to an int. This should take something like 34.54 and convert it to 34.
      2. Get the remainder of the int divided by 10 (Hint: % operator) 34 / 10 = 3, R = 4
      3. Print.

      1. Read in float as a string with fgets().
      2. Find the '.' char.
      3. If it exists, print the char before it.
      4. If it doesn't exist, print the last char of the string.

  4. #4
    Dr Dipshi++ mike_g's Avatar
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    1. I think you can call floor() on the float, and then cast it to an int. This should take something like 34.54 and convert it to 34.
    Would copying the float to an int not automatically floor it? Thats what I have been doing (i think).

  5. #5
    Deathray Engineer MacGyver's Avatar
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    I think you're right, but for whatever reason I was under the impression the floor() was needed. I don't remember what prompted me to think that.

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    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by mike_g View Post
    Would copying the float to an int not automatically floor it? Thats what I have been doing (i think).
    Consider negative numbers.

    EDIT: Also, modf() is what you want.

  7. #7
    Malum in se abachler's Avatar
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    The string method using sprintf() is probably the best. just read until you see the period or reach teh end of the string

    Code:
     
    void func(double input){
         char temp[256];
         int index;
     
         sprintf((char*)temp , "%f" , input);
         index =0;
         while((temp[index] != 0) && (index<256) && (temp[index] != '.')) index++;
         index--;
     
         // index now points at the ones place
     
     
         return;
         }
    Until you can build a working general purpose reprogrammable computer out of basic components from radio shack, you are not fit to call yourself a programmer in my presence. This is cwhizard, signing off.

  8. #8
    Frequently Quite Prolix dwks's Avatar
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    You could make your job easier with &#37;.0f:
    Code:
    double get_first_digit(double number) {
        char buffer[BUFSIZ];
    
        sprintf(buffer, "%.0f", number);
    
        if(*buffer) {
            return buffer[strlen(buffer) - 1] - '0';
        }
    
        return 0;
    }
    But anyway, using a string is a bad idea in my opinion. You use lots of extra memory, and sprintf()'s probably pretty slow too. The mathematical way isn't much harder, if any.

    I think you're right, but for whatever reason I was under the impression the floor() was needed. I don't remember what prompted me to think that.
    Casting a floating point number to an int would indeed have the same effect as flooring it. But using floor() and modf() instead of a cast to an integral type is a much better idea, because a floating point number can easily store values that cannot be represented in any integral type. If you casted one of those values to (int), you'd get an integer overflow.

    So floor() exists so that you can round a number without risk of overflow.

    Anyway, you could also use something like this:
    Code:
    double get_first_digit(double number) {
        return floor(floor(number/10)*10 - number);
    }
    That's probably a bad way to do it . . . perhaps something like this, then.
    Code:
    double get_first_digit(double number) {
        return floor(fmod(number, 10));
    }
    To get the first decimal digit, of course.
    dwk

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    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by dwks View Post
    Casting a floating point number to an int would indeed have the same effect as flooring it.
    Again though, not always. Consider a negative number. On my system, (int)(-3.5) == -3, not -4. A floor() operation would have taken that value to -4.0.

  10. #10
    Deathray Engineer MacGyver's Avatar
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    Quote Originally Posted by brewbuck View Post
    Again though, not always. Consider a negative number. On my system, (int)(-3.5) == -3, not -4. A floor() operation would have taken that value to -4.0.
    Floor()ing the float under that condition is not what the OP wants I take it. Straight casting might be better if he chooses to do conversions to an int.

    The string method still sounds good. lol...

  11. #11
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by MacGyver View Post
    Floor()ing the float under that condition is not what the OP wants I take it. Straight casting might be better if he chooses to do conversions to an int.
    How about:

    Code:
    int last_digit_of_integral_part(float v)
    {
        if(v < 0.0) v = -v;
        return (int)((v * 0.1 - (int)(v * 0.1)) * 10.0);
    }

  12. #12
    Frequently Quite Prolix dwks's Avatar
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    I like
    Code:
    double get_first_digit(double number) {
        if(number < 0) {
            return ceil(fmod(number, 10));
        }
        else {
            return floor(fmod(number, 10));
        }
    }
    Or if the digit alone was required, without a sign:
    Code:
    floor(fmod(fabs(number), 10));
    [edit] The C99 round() would work too, replacing floor() and ceil().
    Code:
    round(fmod(number, 10));
    [/edit]
    Last edited by dwks; 07-13-2007 at 04:39 PM.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
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  13. #13
    Algorithm Dissector iMalc's Avatar
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    Using floor explicitly also ensures correct operation if the /QIfist compiler option is turned on. (Which I currently do for my software 3D engine)
    My homepage
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  14. #14
    Registered User linuxdude's Avatar
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    what about
    Code:
    printf("%1.0f\n",num);
    this has a width of 1 and since no decimals all the width will be in the ones place right?
    similarly
    Code:
    printf("%2.0f\n",num);

  15. #15
    Deathray Engineer MacGyver's Avatar
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    Code:
    #include <stdio.h>
    
    int main(void)
    {
    	float fNum = 123.456;
    	
    	printf("fNum = &#37;1.0f\n",fNum);
    	
    	return 0;
    }
    Output:

    Code:
    fNum = 123
    In other words, no. That's wrong.

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