Problem with taking inputs/calculating slope

This is a discussion on Problem with taking inputs/calculating slope within the C Programming forums, part of the General Programming Boards category; Hey all. Got a problem. I'm writing a program that takes the coordinates for 3 points and I'm supposed to ...

  1. #1
    Registered User toadkiwi's Avatar
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    Problem with taking inputs/calculating slope

    Hey all. Got a problem. I'm writing a program that takes the coordinates for 3 points and I'm supposed to identify whether or not the points in relation to eachother are collinear or not. For example, I have points

    (x1, y1) (x2, y2)(x3, y3)

    If the points were collinear, the slope between pt1 and pt2 would be the same as the slope between pt2 and pt3.

    My code so far gives me this error
    Code:
    collinear.c: In function `main':
    collinear.c:17: error: invalid lvalue in assignment
    And here's my code so far
    Code:
    #include <stdio.h>
    #include <math.h>
    
    main()
    {
    
    double x1;
    double y1;
    double x2;
    double y2;
    double x3;
    double y3;
    
    printf("Enter coordinates for 3 points--x1 y1 x2 y2 x3 y3\n");
            scanf("%lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3);
    
    if(((y2 - y1)/(x2 - x1)) = ((y3 - y2)/(x3 - x2)))
    {
            printf("The points entered are collinear\n");
    }
    
    else
    {
            printf("The points entered are not collinear\n");
    }
    
    }

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Comparison is done using ==, not =.

    Also, this will not work with vertical lines (if x1==x2 or x2==x3).

  3. #3
    Registered User toadkiwi's Avatar
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    Ahh, much thanks!

  4. #4
    CSharpener vart's Avatar
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    also note that main should be declared as
    int main(void)

    And your indentation style suffers...
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  5. #5
    Registered User toadkiwi's Avatar
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    My indentation?

    Any suggestions?

  6. #6
    Jack of many languages Dino's Avatar
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    Code:
    #include <stdio.h>
    #include <math.h>
    
    int main(void)
    {
    
    	double x1;
    	double y1;
    	double x2;
    	double y2;
    	double x3;
    	double y3;
    
    	printf("Enter coordinates for 3 points--x1 y1 x2 y2 x3 y3\n");
    	scanf("&#37;lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3);
    
    	if (((y2 - y1)/(x2 - x1) ) == ( (y3 - y2)/(x3 - x2)))
    	{
    		printf("The points entered are collinear\n");
    	}
    	else
    	{
    		printf("The points entered are not collinear\n");
    	}
    }
    Mac and Windows cross platform programmer. Ruby lover.

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    12/20: Mario F.:I never was, am not, and never will be, one to shut up in the face of something I think is fundamentally wrong.

    Amen brother!

  7. #7
    CSharpener vart's Avatar
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    just copied into VS and pressed Alt+F8
    Code:
    #include <stdio.h>
    #include <math.h>
    
    main()
    {
    	
    	double x1;
    	double y1;
    	double x2;
    	double y2;
    	double x3;
    	double y3;
    	
    	printf("Enter coordinates for 3 points--x1 y1 x2 y2 x3 y3\n");
    	scanf("&#37;lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3);
    	
    	if(((y2 - y1)/(x2 - x1)) = ((y3 - y2)/(x3 - x2)))
    	{
            printf("The points entered are collinear\n");
    	}
    	
    	else
    	{
            printf("The points entered are not collinear\n");
    	}
    	
    }
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  8. #8
    CSharpener vart's Avatar
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    Ups... you have mix of spaces and tabs
    Edited... was already beated... so i remove this intermediate state
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  9. #9
    Registered User toadkiwi's Avatar
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    Nice and organized. Thanks!

  10. #10
    Registered User
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    Code:
    {
        //code
    }
    not
    Code:
    {
    //code
    }
    edit: beat twice (maybe thrice)! ARG!!

  11. #11
    Registered User Bajanine's Avatar
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    You might want to check for division by 0!
    Favorite Quote:

    >For that reason someone invented C++.
    BLASPHEMY! Begone from my C board, you foul lover of objects, before the gods of C cast you into the void as punishment for your weakness! There is no penance for saying such things in my presence. You are henceforth excommunicated. Never return to this house, filthy heretic!



  12. #12
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by Bajanine View Post
    You might want to check for division by 0!
    Indeed, but don't write special case code for that. Instead make use of the following algebraic identity:
    Code:
    a / b == c / d   <==>   a * d == c * b
    This converts the divisions by zero into multiplications by zero, which as you know is not a problem.

    However in practical terms you still have another bug either way. The problem is to do with floating point inaccuracy and that fact that you can't compare values using ==.
    You should instead be usinfg fabs to test the values within an epsilon value of each other.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

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  13. #13
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by toadkiwi View Post
    My indentation?

    Any suggestions?
    http://cpwiki.sf.net/Indentation
    Maybe this can help?
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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