Thread: problems with printf

  1. #1
    Registered User
    Join Date
    Mar 2012

    problems with printf

    i have the following code:

    #include <stdio.h>
    float z(float x,float y)
    	double g;
    	g = -x*x + -y*y + 2;
    	printf("%f", &g);
    	return g;
    int main()
    	float x;
    	float y;
    	double s;
    	x = 0;
    	y = 0;
    	s = z(x,y);
    	return 0;
    When I try compiling with gcc on a linux mint distribution, I get the following warning:
    Lab5Exercise1.c: In function ‘z’:
    Lab5Exercise1.c:11: warning: format ‘%f’ expects type ‘double’, but argument 2 has type ‘double *’

    Since it is just a warning, an output file was created which returns 0.000000 when it should be 2 if my math is correct.

    Before this, i had a loop to test several coordinates pair into the function, but the returned values where really off from where they should be, so i made this basic test code.

    Thx in advance, Donaldgx

  2. #2
    C++ Witch laserlight's Avatar
    Join Date
    Oct 2003
    You want to print a double, so print it, not a pointer to a double:
    printf("%f", g);
    You probably confused this with scanf, where you want to read a double, so you need to pass a pointer to a double so that your double variable is modified.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
    Programming Wraith GReaper's Avatar
    Join Date
    Apr 2009
    "%f" in printf expect the value of a double, not the pointer of a double. That's what your compiler is saying.
    Devoted my life to programming...

  4. #4
    Registered User
    Join Date
    Mar 2012
    Thanks for the help, it works now.

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