write multiple files in C iterating over their names - c

Question

I am trying to write a bunch of files by iterating, doing some calculations and adding the index to the file's name, heres part of my code (I emphasized where the code stops compiling):
float AltAzCalc(int d, float t, float Lon, float RA, float Dec, float Lat){
FILE *in;
-----> char filename[30] = ("hiparcos_horizontal_%lf_%lf.csv",Lon,Lat);
in = fopen(filename, "w");
float PI = 3.14159265;// pi
float G = 6.5949997;
float Alt , Az;
float GST = G + 0.0657098244*d + 1.00273791*t;
if (GST > 24){
GST = GST - 24;
}
float LST = GST*360/24 + Lon;
Alt = (180/PI)*(asin(sin(PI*Dec/180)*sin(PI*Lat/180) + cos(PI*Dec/180)*cos(PI*Lat/180)*cos(PI*(LST-RA*360/24)/180)));
if(sin(PI*(LST-RA*360/24)/180) <= 0){
Az = (180/PI)*(acos((sin(PI*Dec/180)-(sin(PI*Alt/180)*sin(PI*Lat/180)))/(cos(PI*Alt/180)*cos(PI*Lat/180))));
}else{
Az = 360 - (180/PI)*(acos((sin(PI*Dec/180)-(sin(PI*Alt/180)*sin(PI*Lat/180)))/(cos(PI*Alt/180)*cos(PI*Lat/180))));
}
fprintf(in," %lf %lf \n",Alt,Az);
}
int main{
for(int i = -180 ; i < 181 ; i++){
for(int j = -180 ; j < 181 ; j++){
for(int k = 0; k < 119616 ; k++){
AltAzCalc(97,9.2,i,AscensionRecta.array[k],Declinacion.array[k],j);
}
}
}
}
I have used such syntaxes before only not adding any extra numbers that I want to change, which is a string literal and that's what its asking for, any idea how to fix this?

Answer 1

Two major things:
char filename[30] is probably too small in dimension to hold the name.
You need to use sprintf()/snprintf() to generate the filename.
Worthy to mention, %f is enough and recommended to print a float value.
Do something like (pseudo-code)
char filename[128] = {0}; //allocate enough memory
sprintf(filename, "hiparcos_horizontal_%f_%f.csv",Lon,Lat)); //Added missing semi-colon // use %f, that's enough and recommended, too.
Note: Always check the return value of fopen() for success before using the returned file pointer.

Answer 2

If you want a formatted string, use snprintf:
char filename[30];
snprintf(filename, sizeof filename, "hiparcos_horizontal_%f_%f.csv", Lon, Lat);
And %f instead of %lf to print a float.
Also note (as pointed out by #SouravGhosh) that [30] is too small to hild the entire filename.

Answer 3

What you have here it a comma-expression enclosed in parentheses. It contains three subexpressions—a string and two float variables—which are 'calculated' in odrer from left to right, then the result of the last subexpression becomes the result of the whole paren. Thus a result (value in this case) of a string and Lon variable remain unused, as compiler said. Then the result of Lat is inappropriate to initialize a char array.
The compiler has no idea you want a formatted string, you need to write it explicitly, as others said: first you need to prepare a buffer long enough for your file name:
char filename[66];
then fill it with data:
sprintf(filename, "hiparcos_horizontal_%lf_%lf.csv", Lon, Lat);
The formatting string itself contains 25 plain characters. Assuming both Lon and Lat will convert to no more than 20 characters each (but verify that with the sprintf description!) the total file name should not exceed 65 characters. Add one for the NUL terminator and you have the size of 66 in a declaration.
Oh, and don't forget to fclose a file once you finished writing to it!