sscanf problem - very simple im sure!!

[scott452]

Hi, here's my code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(int argc, char* argv[])

{
	double a,valid_input;
	
	valid_input=sscanf(argv[1], "%lg", &a);
	printf("a= %.14lg\n", a);
	return(0);
}

When I put: assgn1A 1.69, I expect the printf to give me a=1.69000000000000 but it gives me 1.689999991233153 or something similar!

What am I doing wrong?

Thanks, Scott

[brewbuck]

You're doing nothing wrong except assuming that floating point values can precisely represent any arbitrary number.

[hk_mp5kpdw]

Floating point variables are incapable of representing some values precisely.

[scott452]

Thanks for replies guys

So how can I fix this so that if i put in a number that is say up to 5 decimal places, it will print that number exactly?

Thanks, Scott

[slingerland3g]

Just curious, what compiler are you using?

[scott452]

gcc?

[dwks]

printf() handles both floats and doubles with %g (unlike *scanf(), which requires you to tell it whether you're passing a float or a double). So you can just use a format specifier like

%.5g

in your printf() call.

[scott452]

hmm okay. I think i've oversimplified my problem

Here's what I'm trying to do:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(int argc, char* argv[])
{
	double a, b, c, disc;
	double valid_input_1, valid_input_2, valid_input_3;
	
	valid_input_1=sscanf(argv[1], "%lg", &a);
	valid_input_2=sscanf(argv[2], "%lg", &b);
	valid_input_3=sscanf(argv[3], "%lg", &c);
	
	disc=(b*b-4*a*c);
	
	printf("a equals %.5lg\n",a);
	printf("b equals %.5lg\n",b);
	printf("c equals %.5lg\n",c);
	printf("Disciminant equals %.5lg\n",disc);
	printf("b*b=%.5lg\n",(b*b));
	printf("-4ac=%.5lg\n", (-4*a*c));
	
}

if i put a=0.25, b=1.69, and c=2.8561 (1.69 squared) - i get this out:

a equals 0.25
b equals 1.69
c equals 2.8561
Disciminant equals -4.4409e-16
b*b=2.8561
-4ac=-2.8561

Now disc should equal 0 but it doesn't!
Is it still not possible??

Thanks, Scott

[tabstop]

0.25 can be exactly represented. 1.69 and 2.8561 can not, although you can get awfully close. Doing math with almost the right numbers gives almost the right answers.

[scott452]

ah okay, so it's not doable basically. Thanks for reply!

Scott