I'm trying to determine the minimum range of floating-point types.
It's easy when using values from standard headers:
This code gives the following output:Code:#include <stdio.h>
#include <float.h>
main()
{
printf("Minimum range of float variable: %e\n", FLT_MIN);
printf("Minimum range of double variable: %e\n", DBL_MIN);
return 0;
}
Minimum range of float variable: 1.175494e-038
Minimum range of double variable: 2.225074e-308
How do I get to these same values using only direct computation?
What do you mean by "direct computation"? Why would you want to do that? I'm not sure it's possible, since you would have to basically start at zero and go up by FLT_MIN or DBL_MIN, but that involves using values from the standard headers.
Actually, you could try dividing by two repeatedly, but I don't know the floating point section of the C standard well enough to know if that will provide a well defined, portable solution.
I've at best a vague grasp of how floats are stored, but I don't see why it isn't possible -- it's going to be a consequence of the number of bits. Just you have to figure out what all dem bits mean ;)
Floating point - Wikipedia, the free encyclopedia
IEEE 754-2008 - Wikipedia, the free encyclopedia
Hi,Quote:
Originally Posted by anduril462
I'm trying to solve exercise 2-1 from "The C Programming Language", 2nd edition, which asks to:
"Write a program to determine the ranges of char, short, int, and long variables, both signed and unsigned,
by printing appropriate values from standard headers and by direct computation. Harder if you compute them:
determine the ranges of the various floating-point types."
I need to determine the minimum range of floating-point types using
basic mathematical operations like addition, subtraction, multiplication, division.
If this is any help, it appears that floats do "wrap around":Quote:
Originally Posted by ak22
Will give you the smallest positive value of a signed float. My idea is that the fact that it wraps around means that you could keep subtracting an exponentially (*10) increasing number from 0 until it becomes positive, then adding to that a number *5 until it is positive, then subtracting from that a number *2.5, etc, until you reach a point where only one iteration is required -- I'm not going to bother, and I can't say it is feasible or worthwhile.Code:#include <stdio.h>
int main(int argc, const char *argv[]) {
float n = 0.01, x;
while (n > 0.0f) {
x = n;
n *= 0.1f;
}
printf("%e\n", x);
return 0;
}
Which, BTW & honestly, there are more significant details to understand about floating point numbers than this. Eg, that they cannot exactly represent 0.1, because 0.1 is not an inverse power of 2. Try this:
The output is probably not what you would expect.Code:#include <stdio.h>
int main() {
float i;
for (i=0.0f; i<20; i+=0.1f) {
printf("%f\n",i);
}
return 0;
}
I just realized that the following two minimum values:
Minimum range of float variable: 1.175494e-038 and
Minimum range of double variable: 2.225074e-308
are actually 0. (I've put the %e specifier instead of %f).
So, I just had to find the maximum value for float and double, which was easy.
There's only one thing left that bothers me: I can't find the maximum value of a long double variable.
The following code:
prints the following:Code:#include <stdio.h>
#include <float.h>
main()
{
printf("Minimum range of long double variable: %f\n", LDBL_MIN);
printf("Maximum range of long double variable: %f\n", LDBL_MAX);
return 0;
}
Minimum range of long double variable: -0.000000
Maximum range of long double variable: -1.#QNAN0
Why does it print QNAN0 instead of max value?
Because printf by default uses 6 decimal places. Try this:
So...Code:printf("%f\n", 0.000000000001);
printf("%.20f\n", 0.000000000001);
No they aren't. %e is accurate, and printf is not a valid test (tho, if you try %.50f, you will see it). This is the real test:Quote:
I just realized that the following two minimum values:
Minimum range of float variable: 1.175494e-038 and
Minimum range of double variable: 2.225074e-308
are actually 0. (I've put the %e specifier instead of %f).
However, it is generally considered bad practice to use == with floating point numbers because of the abstraction involved in their representation (did you try the example at the end of post #6? That reveals a very important fact about floating point). Instead, in practice use a range with some granularity, eg:Code:if (x == 0.0f) ...
Code:// instead of == 1
if (x < 1.0001 && > 0.9999)
I use %Le as the format string for printf() with LDBL_MAX. %Lf works also, but the number of zeroes makes it quite a task to read!
This is a perfectly reasonable thing to do. In fact to test my own megafloat class that implemented 128-bit or greater floats, to confirm that the value returned as being its minimum is correct, I essentially compare it against the result of repeated multiplication by 0.5 until just before it becomes zero.
MK27: It must be 0.5f though that you multiply it by because 0.5 can be represented exactly in floating point, but 0.1 cannot. Besides, multiplying by any value less than 0.5f e.g. 0.125f would potentially miss the smallest value.
MK27 is very much right though that the float min value is not equal to zero. Depending on your format string it may be displayed as merely rounded to zero.
Note that the same technique can be used to find the largest value. Simply multiply by 2.0f until the answer is infinity, which IIRC can be detected by (x-x) != 0
I think I figured out how to get the minimum values of floating-points.
The following code:
prints:Code:#include <stdio.h>
main()
{
float fl, last;
fl = 0.1;
while (fl > 0.0) {
last = fl;
fl = fl * 0.2;
}
printf("\nMinimum value of float variable is: %e\n", last);
double db, dblast;
db = 0.1;
while (db > 0.0) {
dblast = db;
db = db * 0.2;
}
printf("Minimum value of double variable is: %e\n", dblast);
return 0;
}
Minimum value of float variable is: 1.401298e-045
Minimum value of double variable is: 4.940656e-324
Are these values correct?
Values from float.h are different:
Minimum range of float variable: 1.175494e-038
Minimum range of double variable: 2.225074e-308
Okay two things:
One, you've completely ignored my post. To ensure that you get the correct answer, you need to use values that are exactly representable. So, with these changes it will correctly produce the minimum denormalised value.FTFYCode:#include <stdio.h>
int main(){
float fl, last;
fl = 1.f;
while (fl > 0.f) {
last = fl;
fl = fl * 0.5f;
}
printf("\nMinimum value of float variable is: %e\n", last);
double db, dblast;
db = 1.0;
while (db > 0.0) {
dblast = db;
db = db * 0.5;
}
printf("Minimum value of double variable is: %e\n", dblast);
return 0;
}
Secondly, I now recall that FLT_MIN and DBL_MIN give the minimum normalised value, rather than the minimum denormalised value.
One way to obtain the minimum normalised value is that you could start with a float that uses all bits of the significand as well as one which uses all but the last bit of the significand. This can be generated by adding successively smaller powers of two until the value does not change. The value before it did not change and the value before that are the ones with full significands and almost full significands.
Then repeatedly multiply both of these by 0.5 until they become equal.
Then multiple one of them by 2.f and you have the answer.
This process takes advantage of the fact that once the value becomes denomralised, the number of bits in the significand decreases, and in fact as I experienced, rounding also occurs.
Edit: It was slightly easier than I thought due to rounding. Corrected description above.
Thanks a lot, guys. I'm beginning to grasp how floating-point numbers work.
I will try to write a code for this. This looks a little harder.Quote:
One way to obtain the minimum normalised value is that you could start with a float that uses all bits of the significand as well as one which uses all but the last bit of the significand. This can be generated by adding successively smaller powers of two until the value does not change. The value before it did not change and the value before that are the ones with full significands and almost full significands.
Then repeatedly multiply both of these by 0.5 until they become equal.
Then multiple one of them by 2.f and you have the answer.
This process takes advantage of the fact that once the value becomes denomralised, the number of bits in the significand decreases, and in fact as I experienced, rounding also occurs.
I solved it!
this prints:Code:#include <stdio.h>
main()
{
float fl, last, last_b, power;
double db, lastdb, lastdb_b, power_b;
fl = 0;
power = 1;
while (fl != last) {
last_b = last;
last = fl;
fl = fl + (power / 2);
power = power / 2;
}
while (last != last_b) {
last = last * 0.5;
last_b = last_b * 0.5;
}
printf("\nMinimum value of denormalized float variable: %e", last_b);
db = 0;
power_b = 1;
while (db != lastdb) {
lastdb_b = lastdb;
lastdb = db;
db = db + (power_b / 2);
power_b = power_b / 2;
}
while (lastdb != lastdb_b) {
lastdb = lastdb * 0.5;
lastdb_b = lastdb_b * 0.5;
}
printf("\nMinimum value of denormalized double variable: %e\n", lastdb_b);
return 0;
}
Minimum value of denormalized float variable: 1.175494e-038
Minimum value of denormalized double variable: 2.225074e-308
(I didn't multiply with 2.f at the end, because somehow the compiler already produced the correct result)
Thanks to everyone who helped me.
This exercise was awesome!