Trying to create an exp(x) program

This is a discussion on Trying to create an exp(x) program within the C Programming forums, part of the General Programming Boards category; Hey all. I'm experimenting with a program that is in relation to an assignment I have. All I'm trying to ...

  1. #1
    Registered User toadkiwi's Avatar
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    Trying to create an exp(x) program

    Hey all. I'm experimenting with a program that is in relation to an assignment I have. All I'm trying to do is have the user input a number, and then have the program raise "e" (as in 2.71828...) to that number. I've got something, but it's not working like I need it to. I know my program doesnt include it yet, but I want to have a printf statement saying something like "exp(x) is <answer>". Can anyone see what I'm doing wrong?

    Code:
    #include <stdio.h>
    #include <math.h>
    
    int main()
    {
            double x;
            double b;
    
            printf("Enter an exponent for base e\n");
            scanf("&#37;lf", &x);
    
            b = double exp(x);
            return(b);
    }
    Last edited by toadkiwi; 02-28-2008 at 06:37 PM.

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Once you take out the erroneous double in "b = double exp(x);" everything looks like it should work.

  3. #3
    Registered User toadkiwi's Avatar
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    I had tried that and I get a crazy error:

    Code:
    Undefined                       first referenced
     symbol                             in file
    exp                                 /var/tmp//ccsUdNwM.o
    ld: fatal: Symbol referencing errors. No output written to a.out
    collect2: ld returned 1 exit status

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    And of course, somewhere along the way you should remember that to include the math library using gcc you need to add "-lm" to the end of your compilation line.

  5. #5
    Registered User toadkiwi's Avatar
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    Forgot about that. But now there's a new problem. The program just returns exactly what I input. I.e. after it asks for a number, I input 6 and it returns the number 6.

  6. #6
    Registered User toadkiwi's Avatar
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    Ok I figured it out:

    Code:
    #include <stdio.h>
    #include <math.h>
    
    main()
    {
            double x;
            double b;
    
            printf("Enter an exponent for base e\n");
            scanf("%lf", &x);
    
            b = exp(x);
            printf("exp(%f) is equivalent to %lf\n", x, b);
    }
    But one last thing. The final printf (example) reads something like
    Code:
    exp(2.000000) is equivalent to 7.389056
    Is there a way to make it so that the program doesn't spit out so many decimal places? I.e. can it just say 2.00 instead of 2.0000000...?

  7. #7
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    printf uses &#37;f for doubles.

    you can use the field width thing:

    %1.2f

  8. #8
    Registered User toadkiwi's Avatar
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    Thanks!

  9. #9
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    what do you mean by add -lm to to the end of the compilation line

  10. #10
    Woof, woof! zacs7's Avatar
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    -l = GCC link 'flag', m = math library. So you're linking against the math library, ie support for math.h functions.

  11. #11
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    so would it be like

    gcc-lm <filename>

  12. #12
    C++ Witch laserlight's Avatar
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    More like:
    gcc -lm <filename>

    Without the space you are invoking the (presumably non-existent) gcc-lm instead of gcc.
    C + C++ Compiler: MinGW port of GCC
    Version Control System: Bazaar

    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  13. #13
    uint64_t...think positive xuftugulus's Avatar
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    Whenever on a Unix compliant system and need to know how to do something with a program, the first thing to do is 'man xxx' where xxx is the program you want to use. It might be a little difficult at first as man pages are mostly technical, but the first lines in the man page contain the various syntax rules to invoke xxx.
    Code:
    ...
        goto johny_walker_red_label;
    johny_walker_blue_label: exit(-149$);
    johny_walker_red_label : exit( -22$);
    A typical example of ...cheap programming practices.

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