How do I solve C wrong math result ?

Question

1.00/5 (1 vote)

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I will freely admit not  being computing math guru, but this one got me stomped. 
I just cannot see what I am  doing wrong. 

Code 
float hypotenuse;
				hypotenuse = 1.0 - .1;			// OK
				printf("\nhypotenuse of  circle %i %f", (int) R + 1,
						hypotenuse);

				hypotenuse = 1.0 - (float) radius; // wrong result 
				printf("\nhypotenuse of  circle %i %f", (int) R + 1,
						hypotenuse);

Results 
radius  difference between  base and circle 0 0.100000
hypotenuse of  circle 1 0.900000            OK 
hypotenuse of  circle 1 1.090909            why not 0.9 ??

Text copied from LibreOffice.

What I have tried:

No, I did not ask Mrs Google and RTFM already.

Posted 17-Apr-20 7:35am

Vaclav_

Updated 18-Apr-20 6:52am

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Comments

Patrice T 17-Apr-20 13:45pm

Post a piece of code that we can run.
What is the value of radius ?

phil.o 17-Apr-20 17:07pm

A circle doesn't have a hypotenuse; a right triangle does.

Richard MacCutchan 18-Apr-20 4:12am

What are the values of R and radius? And what on earth is the hypotenuse of a circle?

KarstenK 18-Apr-20 4:12am

never compute in printf statement. Use a variable, there are enough of it :-O

3 solutions

Solution 1

If I dummy in the variables you don't show defintions or values for:

C++

#include <stdio.h>

int main()
    {
    float hypotenuse;
    float radius =  0.1;
    int R = 666;
    hypotenuse = 1.0 - .1;			// OK
    printf("\nhypotenuse of  circle %i %f", (int) R + 1,
    		hypotenuse);
    
    hypotenuse = 1.0 - (float) radius; // wrong result 
    printf("\nhypotenuse of  circle %i %f", (int) R + 1,
    		hypotenuse);
    
    return 0;
    }

I get what I expected:

hypotenuse of  circle 667 0.900000                                                                 
hypotenuse of  circle 667 0.900000                                                                 
                                                                                                   
...Program finished with exit code 0

So i think you need to look at the content of your radius using the debugger for your system. I suspect it's not what you think it is...

Posted 17-Apr-20 8:15am

OriginalGriff

Solution 3

Would it be appropriate to ask to remove / delete this thread?
It now longer matters HERE.
Moved to real discussion forum.

Posted 18-Apr-20 6:53am

Vaclav_

Comments

CHill60 20-Jul-20 12:38pm

You can remove your own question which will delete the entire thread. Hover over your post and click the icon that looks like a dustbin

Vaclav_ 20-Jul-20 22:11pm

Sorry, no dustbin in original post, just in "Solutions". Do not want to do something upsetting to the forum by deleting them.

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Vaclav_ · Accepted Answer · 2020-04-18T04:11:00

Before some police gets uptight , I suggest to move this to real discussion forum.

I am sorry, due to some technical difficulties with local internet provider part of the post got missed.
I generally check inputs, to prevent GIGO.

So to get back to solution and skip all the superfluous fluff - if I add

radius = .1;

as indicated - all is well.

????

This is for those who wanted to see the code.

C++

	// radius difference
				float radius_new  = (float) IMG_circle[1].radius - (float) IMG_circle[0].radius;
				printf("\nradius  differece between  base and cicrcle %i %f", R,
						radius_new);
				float hypotenuse;
				float hypotenuse_new;
				//radius = .1;
				hypotenuse = 1.0 - .1;			//- (double) radius;
				//printf("\nhypotenuse of  cicrcle %i %f", (int) R,
				//		hypotenuse);
// BUG
				float base = 1.0;
				hypotenuse_new = base - (float) radius_new; //  (float) radius;
				hypotenuse_new = base -( (float) IMG_circle[1].radius - (float) IMG_circle[0].radius);
				printf("\nBase circle radius     %f ",(float) IMG_circle[0].radius);
				printf("\nCurrent circle radius  %f ",(float) IMG_circle[1].radius);
				printf("\ndiff (hypotenuse_new)  %f ",(float) IMG_circle[1].radius - (float) IMG_circle[0].radius);
				printf("\ndiff (hypotenuse_new)  %f ",(float) IMG_circle[0].radius - (float) IMG_circle[1].radius);
				printf("\nnew hypotenuse of  circle of radius %f %i %f", radius_new, (int) R,
						(float) hypotenuse_new);

And here is current output

radius  differece between  base and cicrcle 0 0.100000
Base circle radius     1.000000 
Current circle radius  0.909091 
diff (hypotenuse_new)  -0.090909 
diff (hypotenuse_new)  0.090909 
new hypotenuse of  circle of radius -0.090909 0 1.090909
hypotenuse of  cicrcle 1 0.810000

Plase note that the first printf prints POSITIVE radius - which I assumed was correct.
Using SAME values the "current circle radius" CHANGED.

I am really not sure where is my error and how to solve it.
And that is just beginning of the calculations...

C++

// radius difference
            float radius = IMG_circle[1].radius - IMG_circle[0].radius;
            printf("\nradius  differece between  base and cicrcle %i %f", R,
                    radius);
            float hypotenuse;
            hypotenuse = 1.0 - .1;          //- (double) radius;
            printf("\nhypotenuse of  cicrcle %i %f", (int) R + 1,
                    hypotenuse);

                           radius = .1;

            hypotenuse = 1.0 - (float) radius;
            printf("\nhypotenuse of  cicrcle %i %f", (int) R + 1,
                    hypotenuse);

radius  differece between  base and cicrcle 0 0.100000
hypotenuse of  cicrcle 1 0.900000
hypotenuse of  cicrcle 1 1.090909