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# Thread: 0 isn't 0 in C++ math

1. Junior Member
Join Date
Oct 2011
Posts
13

## 0 isn't 0 in C++ math

OK, i have a script, which calculate Y=1/(C-A). C is input, A is intetrval [Ap;Ag] with step Az;
And i have one problem, when c=2, A[-1.5;2.5] with step 0.1, when i want to say that 0 isn't possible in 1/0, the program colculate me: Y = -2.1e+06 . Programm is not English language, sorry. Problem in answer line 36.

Code:
```/*
*/
#include <iostream>
#include <cstdlib>
#include <iomanip>
using namespace std;
int main ()
{
float y, c, ap, az, ag;

cout << "Iveskite kintamojo C reiksme.\n";
cin >> c; //įvedame C reikšmę
do {
cin >> ap >> ag; //įvedame intervalo pradžią ir galą
} while (ap == ag); //išsireikalaujame, kad intervalo pradžia ir galas nesutaptų
do {
cout << "Iveskite intervalo kitimo zingsni, kuris nelygus 0.\n";
cin >> az; //įvedame intervalo kitimo žingsnį
} while (az == 0.0); //išsireikalaujame, kad intervalo kitimo ˛ingsnis būtų nelygus 0
if (ag > ap && az > 0.0) { //jei intervalas didėjantis IR žingsnis DIDESNIS už 0, skaičiuojame
cout << setfill('-') << setw(58) << "-" << endl;
cout << setfill(' ') << setw(8)<<"Sprendinio nr. " <<"|  "<<"Atsakymas\n";
cout << setfill('-') << setw(58) << "-" << setfill(' ') <<endl;
for (float z = 1.0, a = ap; a <= ag; a += az) {
if (a == c) { //jei vardiklis lygus 0
cout << setfill(' ') << setw(14) << z << ".  ";
cout << "Kai A = " << a <<", o " << c << " - " << a <<" = 0, apskaiciuoti\nnegalima, nes dalyba is 0 negalima.\n";
}
else {
y = 1.0 / (c - a); //skaičiuojame
cout << setfill(' ') << setw(14) << z << ".  " << "Y = "  << setprecision(3) << y << endl;
}
z++; //prisideame vienetuką prie sprendinio nr.
}

}
if (ag < ap && az < 0.0) { //jei intervalas mažėjantis IR žingsnis MAŽESNIS uz 0
cout << setfill('-') << setw(58) << "-" << endl;
cout << setfill(' ') << setw(8)<<"Sprendinio nr. " <<"|  "<<"Atsakymas\n";
cout << setfill('-') << setw(58) << "-" << setfill(' ') <<endl;
for (float z = 1.0, a = ap; a >= ag; a += az) {
if (a == c) { //jei vardiklis lygus 0
cout << setfill(' ') << setw(14) << z << ".  ";
cout << "Kai A = " << a <<", o " << c << " - " << a <<" = 0, apskaiciuoti\nnegalima, nes dalyba is 0 negalima.\n";
}
else {
y = 1.0 / (c - a); //skaičiuojame
cout << setfill(' ') << setw(14) << z << ".  " << "Y = "  << setprecision(3) << y << endl;
}
z++; //prisideame vienetuką prie sprendinio nr.
}

}
if (ag > ap && az < 0.0) { //jei intervalas didėjantis IR žingsnis MAŽESNIS uz 0
cout << setfill('-') << setw(58) << "-" << endl;
cout << "Apskaiciuoti negalime,\nnes intervalas didejantis,\no zingsnis mazejantis.\n";
}
if (ag < ap && az > 0.0) { //jei intervalas mažejantis IR žingsnis DIDESNIS už 0
cout << setfill('-') << setw(58) << "-" << endl;
cout << "Apskaiciuoti negalime,\nnes intervalas mazejantis,\no zingsnis didejantis.\n";
}
cout << setfill('-') << setw(58) << "-" << setfill(' ') <<endl;
system ("pause");
return 0;
}```
Code:
```Iveskite kintamojo C reiksme.
2
-1.5
2.5
Iveskite intervalo kitimo zingsni, kuris nelygus 0.
0.1
----------------------------------------------------------
Sprendinio nr. |  Atsakymas
----------------------------------------------------------
1.  Y = 0.286
2.  Y = 0.294
3.  Y = 0.303
4.  Y = 0.313
5.  Y = 0.323
6.  Y = 0.333
7.  Y = 0.345
8.  Y = 0.357
9.  Y = 0.37
10.  Y = 0.385
11.  Y = 0.4
12.  Y = 0.417
13.  Y = 0.435
14.  Y = 0.455
15.  Y = 0.476
16.  Y = 0.5
17.  Y = 0.526
18.  Y = 0.556
19.  Y = 0.588
20.  Y = 0.625
21.  Y = 0.667
22.  Y = 0.714
23.  Y = 0.769
24.  Y = 0.833
25.  Y = 0.909
26.  Y = 1
27.  Y = 1.11
28.  Y = 1.25
29.  Y = 1.43
30.  Y = 1.67
31.  Y = 2
32.  Y = 2.5
33.  Y = 3.33
34.  Y = 5
35.  Y = 10
36.  Y = -2.1e+06
37.  Y = -10
38.  Y = -5
39.  Y = -3.33
40.  Y = -2.5
41.  Y = -2
----------------------------------------------------------```
Last edited by justutiz; May 19th, 2013 at 01:05 PM.

2. ## Re: 0 isn't 0 in C++ math

Originally Posted by justutiz
OK, i have a script, which calculate ...
Scripts are not discussed in this Forum.
There is a Visual C++ Programming forum, and Visual C++ is code, not script.

Originally Posted by justutiz
... which calculate Y=1/(C-A). C is input, A is intetrval [Ap;Ag] with step Az;
Did you, perhaps, mean "A is a value within intetrval [Ap;Ag]"?

Anyway, have a look at:
Floating point
What Every Computer Scientist Should Know About Floating-Point Arithmetic
and a lot of other articles/discussions here

3. Junior Member
Join Date
Oct 2011
Posts
13

## Re: 0 isn't 0 in C++ math

Originally Posted by VictorN
Scripts are not discussed in this Forum.
There is a Visual C++ Programming forum, and Visual C++ is code, not script.

Did you, perhaps, mean "A is a value within intetrval [Ap;Ag]"?

Anyway, have a look at:
Floating point
What Every Computer Scientist Should Know About Floating-Point Arithmetic
and a lot of other articles/discussions here
Im reading, and Im very new in C++, so I dont find how I can do the folowing that a == c, then a and c is floats.

4. ## Re: 0 isn't 0 in C++ math

Originally Posted by justutiz
... so I dont find how I can do the folowing that a == c, then a and c is floats.
In general you cannot just compare floats as "a == c". You have to implement some relative small (with respect to the "a" and "c") value ("delta") and compare like:
Code:
```if(abs(a - c) < delta)
{
...
}```

5. Junior Member
Join Date
Oct 2011
Posts
13

## Re: 0 isn't 0 in C++ math

but abs want me to use int, my a and c is floats

6. ## Re: 0 isn't 0 in C++ math

Well, I meant this abs, _abs64

However, you can use fabs instead.

7. ## Re: 0 isn't 0 in C++ math

when i want to say that 0 isn't possible in 1/0, the program colculate me: Y = -2.1e+06
Result of 1/0 is infinity, while programming languages operate with only finite figures. So you have the best result FPU was able to provide.

8. Junior Member
Join Date
Oct 2011
Posts
13

## Re: 0 isn't 0 in C++ math

Im still geting
Code:
```1      -1      0
1.11      -0.9      0
1.25      -0.8      0
1.43      -0.7      0
1.67      -0.6      0
2      -0.5      0
2.5      -0.4      0
3.33      -0.3      0
5      -0.2      0
10      -0.1      0
-1.34e+07      7.45e-08      0
-10      0.1      0
-5      0.2      0
-3.33      0.3      0
-2.5      0.4      0
-2      0.5      0
-1.67      0.6      0
-1.43      0.7      0
-1.25      0.8      0
-1.11      0.9      0```
trying everything, and still dont get how to fix it.

Code:
```#include <iostream>
#include <cstdlib>
#include <iomanip>
#include <math.h>

using namespace std;
int main ()
{
float y, c, ap, ag;
c = 0;
ap = -1;
ag = 1;
float az = 0.1;
if (ag > ap && az > 0.0) {
for (float a = ap; a <= ag; a += az) {
if ( fabsf(c - a) < 0.0 && fabsf(c - a) > 0.0 ) {
cout << "1/0 not posible";
}
else {
y = 1.0 / (c - a);
cout  << setprecision(4) << y <<"      " << a << "      " << c << endl;
}
}

}
else cout << "STOP";
system ("pause");
return 0;
}```

9. ## Re: 0 isn't 0 in C++ math

It's impossible to succeed with the condition fabsf(c - a) < 0.0. Victor meant something like fabsf(c - a) < 0.00001.

10. Junior Member
Join Date
Oct 2011
Posts
13

## Re: 0 isn't 0 in C++ math

Originally Posted by Igor Vartanov
Result of 1/0 is infinity, while programming languages operate with only finite figures. So you have the best result FPU was able to provide.
Yes, but why in my code -0.1 + 0.1 is not 0?

11. Junior Member
Join Date
Oct 2011
Posts
13

## Re: 0 isn't 0 in C++ math

Originally Posted by Igor Vartanov
It's impossible to succeed with the condition fabsf(c - a) < 0.0. Victor meant something like fabsf(c - a) < 0.00001.
Yes, it works, but maybe it is shortes way to compare c-a is 0?

12. ## Re: 0 isn't 0 in C++ math

Originally Posted by justutiz
Yes, but why in my code -0.1 + 0.1 is not 0?
I don't know what's wrong with your code, but mine works fine:
Code:
```#include <stdio.h>

int main()
{
float a = -0.1;
printf("%f", a +0.1);
return 0;
}```

D:\Temp\8>8.exe

-0.000000

13. Junior Member
Join Date
Oct 2011
Posts
13

## Re: 0 isn't 0 in C++ math

Originally Posted by Igor Vartanov
I don't know what's wrong with your code, but mine works fine:
Code:
```#include <stdio.h>

int main()
{
float a = -0.1;
printf("%f", a +0.1);
return 0;
}```
when add one time its true, but when you do it with cycle it looks something like this

a = -0.1 + 0.01000000001 and so on
and then you compare 0 with a it is looks something like this 0 != 0.0000000000001

14. ## Re: 0 isn't 0 in C++ math

Originally Posted by justutiz
Yes, it works, but maybe it is shortes way to compare c-a is 0?
Well, the big mistake you make is that you think math and programming are the same. Floats in programming have finite precision, so you must take certain precautions and tricks to succeed with your calculations. Like comparing with 0.00001, for example.

15. ## Re: 0 isn't 0 in C++ math

Originally Posted by justutiz
when add one time its true, but when you do it with cycle it looks something like this

a = -0.1 + 0.01000000001 and so on
and then you compare 0 with a it is looks something like this 0 != 0.0000000000001
Yep, see above: float calculations have finite precision. And the result might fluctuate a little.

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