write to variable filename

Hi

I want to be able to write to a file that the user has specified the name of. This is where i am

Code:

//the rest of the program

char *name[20], *filename;
        FILE *EulerData;

scanf("%s", name);

filename = ("%s.txt", name);

printf("%s", filename);//just to see if the program has added the .txt

for (i=0;i<100;i++)
{
EulerData = fopen(filename, "w");
        fprintf(EulerData, "%f %f %f\n", x[i], y[i], z[i]);
        fclose;
}

But it either crashes, or doesnt store.

Any ideas?

P.S if you are reading from a text file, how do you check if there is nothing in it or it doesnt exist?

cheers

You probably want to use sprintf() to produce a string from the input and the ".txt" extension (and most of the time, it's a good idea to check for an extension first, so that foo.txt doesn't become foo.txt.txt).

By the way, the fun of C and some of it's obscurer parts of it:

Code:

filename = ("%s.txt", name);

is the same as:

Code:

"%s.txt"; 
filename = name;

Of course having a text-string in the middle of the code produces absolutely nothing useful (although it is valid).

--
Mats

As an addition to matsp:
You probably want fclose(filehandle);. Not fclose;.

Also, you probably want to do the fopen and fclose outside the loop.

also your name and filename should be char arrays:

char name[20], filename[24];

instead of what you have

Note that file names could include spaces (in this case %s format is not suitable for entering them) and be a lot longer than 19 characters

Thanks guys for help, I have modified the function now to this:

Code:

int Euler(t, u, v, x0, y0, z0)

    float t, u, v, x0, y0, z0;
    {
        float x[100], y[100], z[100], dxdt_i, dydt_i, dzdt_i,dx, dy, dz, dxdt_new, dydt_new, dzdt_new, dt = 1;
        char name[20], filename[24];
        FILE *EulerData;
        int i, n;


        x[0] = x0;
        y[0] = y0;
        z[0] = z0;

        Sleep(1000);
        printf("\nPlease enter the name of the file to store the data in, (example: lorenz)\n");
        printf("If it doesnt exist, it will be created.\n\n");
        scanf("%s", name);


         sprintf(filename, "%s.txt",name);



        printf("\nStoring all Data in file '%s'\n", filename);

        Sleep(1000);
        printf("Beginning execution of improved Euler...\n\n");
        Sleep(1000);

        system("cls");
        for (i = 0; i<101; i++)
        {

        dxdt_i = t*(y[i] - x[i]);
        dydt_i = x[i]*(u - z[i]) - y[i];
        dzdt_i = (x[i]*y[i]) - (v*z[i]);

        dx = (t*(y[i] - x[i]))*dt;
        dy = (x[i]*(u - z[i]) - y[i])*dt;
        dz = ((x[i]*y[i]) - (v*z[i]))*dt;


        dxdt_new = t*(y[i] - (x[i] + dx));
        dydt_new = x[i]*(u - z[i]) - (y[i] + dy);
        dzdt_new = (x[i]*y[i]) - (v*(z[i] + dz));

        x[i+1] = x[i] +(dt/2)*(dxdt_i + dxdt_new);
        y[i+1] = y[i] +(dt/2)*(dydt_i + dydt_new);
        z[i+1] = z[i] +(dt/2)*(dzdt_i + dzdt_new);


        //printf("New x = %5.3f New y = %5.3f New z = %5.3f\n", x[i+1], y[i+1], z[i+1]);
        printf("%d\n", i);

    }
        EulerData = fopen(filename, "w");
        for (n = 0; n<101; n++)
        {
         printf("%d\n", n);
        fprintf(EulerData, "%f %f %f\n", x[n], y[n], z[n]);


        }
        fclose(filename);
    }

I added the lines
printf("%d\n", i);
and
printf("%d\n", n);

to see if it was actually running the for loops, which it is because i get the values printed to the console, but it crashes AFTER the n's have been printed. This tells me that the loop is running, but there is nothing in the textfile, which i suspect means it cant close the file, and thats why it crashes - why would that be?


EDIT - I had fclose(filename) not fclose(EulerData)

in reply to vart, should you never have a * before a character array, for instance in another function i have

char *option[100], *as[100], *bs[100], *cs[100], *Xos[100], *Yos[100], *Zos[100];//the s at the end of the name tell me it is stored as a string.

option is waiting for the user to type an option, "exit" etc, and the others are taking data from the user, should they not have the *?

Code:

int Euler(t, u, v, x0, y0, z0)

    float t, u, v, x0, y0, z0;

This style of C went out of fashion about 15 years ago.

If the array is intended to be a string then you should not have a * before it. If you want to store the address of a string in your array, then char * is indeed what you want. It is all about how you are using the array. Often, arrays do not have * in the type declaration, but there are many valid cases where you would do that.

--
Mats

OK so how is it done now, i read that method in the sams teach yourself c book.

Originally Posted by a.mlw.walker

OK so how is it done now, i read that method in the sams teach yourself c book.

Really?

Well, any new code in the last ten or so years uses:

Code:

int Euler(float t, float u, float v, float x0, float y0, float z0)
{
...
}

--
Mats

Oh OK, i thought you were going to tell me a completely new way of doing it, can you do

Code:

int Euler (float t, u, v, x0, y0, z0)
{
...
}

?

You need the type on each parameter, alas.

Also beware the dangers of scanf: SourceForge.net: Scanf woes - cpwiki
Or you can try my little experiment on some better C strings: New string functions

Be careful a.mlw.walker, if I am correct, I may be on the same course as you and doing the same program for Computational Techniques? Just be careful about putting your full code up here as people may be able to steal your code!

That said, I am at the same stage as you, I have one question... e are meant to run the program over the range 0<t<100, alas, the numbers soon become MASSIVE!

Are you sure there is not a typo is the assigment and instead we should run it up to 0<t<10?

I have done the calculations in excel and the numbers soon reach somewhere in the order of

+/- x*10^300 which is more or less as much as a double can take!

I'm stuck!

kryptica it sounds like we are on the same course. I was trying to take the code off but it wont let me. eaybaw