How do I write only the decimal places in C language?

Question

If I have 2.55, how do I write only .55 and skip 2 in programming language?

@WeatherVane shouldn't that be `trunc(number)` in case `number` is negative? — Dmitri, Sep 23 '15 at 16:43
@Dmitri `trunc` is non-standard. But I take the point, if negative `number - ceil(number)` — Weather Vane, Sep 23 '15 at 16:44
OP why accept the the answer with output of `"0.55"` when `".55"` was the required output in the post? — chux - Reinstate Monica, Sep 23 '15 at 17:30
possible duplicate of [How to separate float into an integer and a fractional part?](http://stackoverflow.com/questions/23993898/how-to-separate-float-into-an-integer-and-a-fractional-part) — Mark Dickinson, Sep 23 '15 at 18:39

ameyCU · Answer 1 · 2015-09-23T17:22:54.777

2

Well you can do this to store it in another variable -

 double a=2.55,b;
 b =a-(long)a;                   // subtracting  decimal part from a
 printf("%.2f\n",b);

As pointed out by Mark Dickinson Sir in comment that this is not safe . So you can make use of function modf from <math.h>-

For example -

double a=-2.55,b,i;
b =modf(a,&i);                   // i will give integer part and b will give fraction part
printf("%.2f\n",b);

edited Sep 23 '15 at 17:22

answered Sep 23 '15 at 16:37

ameyCU

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As with @Mureinik's answer, this gives undefined behaviour for large `double`s. – Mark Dickinson Sep 23 '15 at 17:17
@MarkDickinson Sir ,i have updated my anser .Thanks for response . – ameyCU Sep 23 '15 at 17:23

score 2 · Answer 2 · answered Sep 23 '15 at 17:54

Use double modf(double value, double *iptr) to get the factional part. Use round() to get the best value near the requested precision.

double GetDecimalPlaces(double x, unsigned places) {
  double ipart;
  double fraction = modf(x, &ipart);
  return fraction;
  // or 
  double scale = pow(10.0, places);
  return round(fraction * scale)/scale;
}

void GetDecimalPlaces_Test(double x, unsigned places) {
  printf("x:%e places:%u -->", x, places);
  printf("%#.*f\n", places, GetDecimalPlaces(x, places));
  // Additional work needed if leading '0' is not desired.
}

int main(void) {
  GetDecimalPlaces_Test(2.55, 2);
  GetDecimalPlaces_Test(-2.55, 2);
  GetDecimalPlaces_Test(2.05, 2);
  GetDecimalPlaces_Test(0.0, 2);
  GetDecimalPlaces_Test(0.0005, 2);
}

Output

x:2.550000e+00 places:2 -->0.55
x:-2.550000e+00 places:2 -->-0.55
x:2.050000e+00 places:2 -->0.05
x:0.000000e+00 places:2 -->0.00
x:5.000000e-04 places:2 -->0.00

score 1 · Accepted Answer · answered Sep 23 '15 at 16:38

1

One dirty trick is to cast your double to an int to get only the whole number. You can then subtract the two to get only the decimal part:

double d = 2.55;
double remainder = d - (int)d;
printf ("%.2f\n", remainder);

answered Sep 23 '15 at 16:38

Mureinik

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This gives undefined behavour for large `d`, as a result of the cast to `int` overflowing. (C99 6.3.1.4p1: "If the value of the integral part cannot be represented by the integer type, the behavior is undefined.") Why not use the `modf` function from the math library? – Mark Dickinson Sep 23 '15 at 17:09

Andreas DM · Answer 4 · 2015-09-23T17:43:56.133

1

double values are not perfectly precise, so small rounding errors can get introduced. You can store the total number in an Integer. You can for example divide by 100 to get the value before the . and use % modulus to get the decimal values.

Example:

int main()
{   
    int num = 255;

    printf("%d.%d\n", num / 100, num % 100); // prints 2.55
    printf(".%d", num % 100);                // prints .55

    return 0;
}

This fails with negative numbers, but you can easily add cases to handle that.

edited Sep 23 '15 at 17:43

answered Sep 23 '15 at 17:22

Andreas DM

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OP did not mention "money" (although he did not expand on any particular use as well). Nevertheless, your sugessted code fails for negative numbers and `num = 101` (although both can be trivially fixed). – Jongware Sep 23 '15 at 17:28
@Jongware `num = 101` will display `.1` as expected, or did you mean that it doesn't display `.01` ? – Andreas DM Sep 23 '15 at 17:45
Detail: `double` _values_ are precise. It is `double` math that introduces the rounding errors. – chux - Reinstate Monica Sep 23 '15 at 17:59

How do I write only the decimal places in C language?

4 Answers4