在C中的fprintf上以不同的名称保存多个文件

Question

I want to save multiple .dat files but apears the warning: "format '%f' expects argument of type 'double', but argument 3 has type 'double *' [-Wformat=]" and the files come out empty.

FILE *f1;
double hist[N];
double delta_t = 0.25;
int n_periodos = 0;
char name[100]; 
sprintf(name,"testeT%f.dat",n_periodos*delta_t);
f1 = fopen (name,"w");
fprintf(f1,"%lf",hist); //The problem is here

Answer 1

Your problem in the last line is you are passing hist (which is an array of double ) to fprintf where you have used the %lf conversion specifier which expects a double as its argument (not an array of double )

When you declare an array in C, on access, the array is converted to a pointer to the first element in the array C11 Standard - 6.3.2.1 Lvalues, arrays, and function designators(p3) . (the exceptions are stated there -- when use with sizeof , _Alignof , or with the unary & operator, or when initialized with a string literal ) None of those apply here.

So with your declaration of:

double hist[N];

hist is an array of double. When you use hist in:

fprintf (f1, "%lf", hist);

hist is converted to a pointer to the first element in the array (eg the address of the first element) which has a type 'double*' . To correct the problem, you need to dereference the pointer (generally done with an array by using [element] following the varaible, eg

fprintf (f1, "%lf", hist[0]);  /* or element 1, 2, 3, .... */

This will make your types consistent.

You could rewrite your code (thought it is still unclear what N is), to eliminate the issue and correct some other shortcomings (identified in the comments below)

#include <stdio.h>

#define MAXC  100   /* if you need a constant, #define on (or more) */
#define NHIST  32   /* it is unclear where N came from in your code */

int main (void) {

    FILE *f1 = NULL;            /* initialize all variables and help */
    double hist[NHIST] = {0.0}; /* avoid Undefined Behavior :)       */
    double delta_t = 0.25;
    int n_periodos = 0;
    char name[MAXC] = ""; 

    /* use snprintf to protect the bounds of 'name' */
    snprintf (name, MAXC - 1, "testeT%f.dat", n_periodos*delta_t);

    f1 = fopen (name, "w");     /* call fopen */
    if (f1 == NULL) {           /* validate file is open for writing */
        perror ("fopen-name");
        return 1;
    }

    fprintf (f1, "%lf", hist[0]);
}

Look things over and let me know if you have further questions.

Answer 2

When you call runtime functions it is important to check the return values to see if they were successful or not.

Another thing to keep in mind is that float and doubles are not exact values so using them as part of a file name is not good.

So check the return value

f1 = fopen(name, "w");
if (f1 != NULL)
{
  ...
  fclose(f1);
}
else
{
  perror(name); // or write out the error
}

Note also that variables declared are not necessary 0 if you declare them in a function, they can have arbitrary values so you need to initialize them

double hist[N] = {0};

When you write the hist[] to a file you cannot use fprintf like that, you should loop through the values writing them one value at a time, fprintf can't handle writing an array like you wrote.

for (int i = 0; i < N; ++i)
{
  fprintf(f1, "%lf\n", hist[i]); // added \n as delimiter
}

Answer 3

hist is an array of doubles (or technically a pointer, double* , as its passed into fprintf), but you are attempting to write a single double value to file, not an array. You probably want something like this to write the entire array:

for (int i = 0; i < N; i++)
{
    fprintf(f1, "%f", hist[i]);
}

Or just a single value:

fprintf(f1, "%f", hist[0]);

Also, in your sample code, hist is an uninitialized array. What gets written to file will likely not be what you expect.

Answer 4

regarding:

fprintf(f1,"%lf",hist);

in C, a reference to the name of an array 'degrades' to the address of the first byte of the array.

Much better to use:

fprintf(f1,"%lf",hist[0]);

as that will output the contents of the first entry in the array.