Opening a user-defined file

Hi. This is probably a really simple question but I can't find how to do it in any of my notes!


I am writing a program in C and part of it needs to create a file, the name of which is inputted by the user. I have a char string called filename, which the user chooses using scanf. I then need to create the file with "filename".xls and add data to it. My file pointer is called 'file'. I'm guessing I need to use a line something like:

file=fopen("%s.xls",filename "w");

or:

file=fopen("(filename).xls","w");

But neither of these work!

Can anyone tell me what to do?

Cheers.

Chris

Code:

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char *fullfilename = malloc(strlen(filename) + 5);
if ( fullfilename == NULL )
  /* Do error handling here but make sure you don't continue until fullfilename is not null */

strcpy (fullfilename, filename);
strcat (fullfilename,".xls");
file = fopen(fullfilename, "w");

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Thanks for that! It's working now :D

OK now this one really is a stupid question:

How do I use pi in calculations?

I've included math.h, and i've tried M_PI but it doesn't work


Thanks again!


Chris

Quote:

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but it doesn't work

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Post your code

impinduc=2*3.142*workingfreq*indctnce;


I just want to use the exact value of pi instead of 3.142!

Quote:

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Originally Posted by mr_diss

I've included math.h, and i've tried M_PI but it doesn't work

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M_PI is nonstandard.

Quote:

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Originally Posted by mr_diss

I just want to use the exact value of pi instead of 3.142!

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Try this.

Code:

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impinduc = 2 * (4.0 * atan(1.0)) * workingfreq * indctnce;

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Thanks that works a bit better, is there no constant for just pi?

I presume that's using the inverse tan of 1 or something is it?

I would define it myself so:

Code:

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#define PI 3.1415926536    /* as many digits as I can remember */

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or, following Dave's suggestion (I quite like that)

Code:

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#define PI (4.0 * atan(1.0))

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