skrtbhtngr already answered the stated question, but I'd like to address the underlying problem.
Apply the Unix philosophy. Use one tool to generate the data, and another to consume it -- here, to compute the integral using the trapezoid rule.
The easiest format you can use is the same supported by Gnuplot:
- Empty lines are ignored
- Lines beginning with
# are ignored, and can be used for comments
- Each line defines one sample
Essentially, you could describe a sine curve, very roughly, using
#x sin(x)
0.000 0.000000000
0.100 0.099833417
0.200 0.198669331
0.300 0.295520207
0.400 0.389418342
0.500 0.479425539
0.600 0.564642473
0.700 0.644217687
0.800 0.717356091
0.900 0.783326910
1.000 0.841470985
1.100 0.891207360
1.200 0.932039086
1.300 0.963558185
1.400 0.985449730
1.500 0.997494987
1.600 0.999573603
1.700 0.991664810
1.800 0.973847631
1.900 0.946300088
2.000 0.909297427
2.100 0.863209367
2.200 0.808496404
2.300 0.745705212
2.400 0.675463181
2.500 0.598472144
2.600 0.515501372
2.700 0.427379880
2.800 0.334988150
2.900 0.239249329
3.000 0.141120008
3.100 0.041580662
3.200 -0.058374143
3.300 -0.157745694
3.400 -0.255541102
3.500 -0.350783228
3.600 -0.442520443
3.700 -0.529836141
3.800 -0.611857891
3.900 -0.687766159
4.000 -0.756802495
4.100 -0.818277111
4.200 -0.871575772
4.300 -0.916165937
4.400 -0.951602074
4.500 -0.977530118
4.600 -0.993691004
4.700 -0.999923258
4.800 -0.996164609
4.900 -0.982452613
5.000 -0.958924275
5.100 -0.925814682
5.200 -0.883454656
5.300 -0.832267442
5.400 -0.772764488
5.500 -0.705540326
5.600 -0.631266638
5.700 -0.550685543
5.800 -0.464602179
5.900 -0.373876665
6.000 -0.279415498
6.100 -0.182162504
6.200 -0.083089403
You could use eg. awk to generate that, like I did:
awk 'BEGIN { printf "#x sin(x)\n" ; for (x=0.0; x<6.3; x+=0.1) printf "%.3f %11.9f\n", x, sin(x) }'
If you save that to a file (appending > data.txt to the above command), you can plot it in Gnuplot using
plot "data.txt" using 1:2 notitle with lines
Such data is easy to read in a C program. Because I only use POSIX.1 systems (Linux, BSDs, macOS), and POSIX.1 provides the very useful getline() function -- it lets you read in lines of any length, dynamically allocating a large enough buffer --, this particular implementation also requires POSIX.1 support. In other words, it works basically everywhere except in Windows.
#define _POSIX_C_SOURCE 200809L
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
/* Read x y(x) values, one pair per line, from a stream.
The arrays are dynamically allocated, and pointers stored
to *xptr and *yptr. The size of the arrays is stored at *nptr.
They are initially cleared to NULL/zero.
The function returns 0 if success, an errno error code otherwise:
EINVAL: Invalid function parameters
EIO: Read error
ENOMEM: Out of memory
EBADMSG: Malformed line
*/
int read_xy(double **xptr, double **yptr, size_t *nptr, FILE *in)
{
/* Line input buffer variables. */
char *line = NULL;
size_t size = 0;
ssize_t len;
/* Data array variables. */
double *x = NULL;
double *y = NULL;
size_t n = 0; /* Entries in x[] and y[] */
size_t nmax = 0; /* Entries allocated */
/* Temporary variables. */
double xval, yval, *newx, *newy;
/* We clear the output parameters to NULL or zero,
in case the caller is careless and does not check
the return value. Clearing them ensures they do
not contain garbage in such a case. */
if (xptr)
*xptr = NULL;
if (yptr)
*yptr = NULL;
if (nptr)
*nptr = 0;
/* We need in and nptr, and at least one of xptr and yptr. */
if (!in || !nptr || (!xptr && !yptr))
return errno = EINVAL;
/* If an error has already occurred in 'in',
we do not even try to read from it. */
if (ferror(in))
return EIO;
while (1) {
/* Read next input line. */
len = getline(&line, &size, in);
/* End of input or error? */
if (len < 0)
break;
/* Skip empty and comment lines. */
if (len == 0 ||
line[0] == '\n' || (line[0] == '\r' && line[1] == '\n') ||
line[0] == '#')
continue;
/* Parse the line. */
if (sscanf(line, " %lf %lf", &xval, &yval) != 2)
break;
/* Need to grow the dynamically allocated arrays? */
if (n >= nmax) {
/* Allocation policy.
We allocate room for at least 16 doubles,
then double the size up to 1048576 (=2^20),
then adjust to the next full multiple of 1048576.
This is not 'the best', but it is robust,
and not too wasteful.
*/
if (n < 16)
nmax = 16;
else
if (n < 1048576)
nmax = n * 2;
else
nmax = (n | 1048575) + 1048576;
/* Note: realloc(NULL, size) is equivalent to malloc(size).
If the realloc() call fails, it returns NULL,
but the original array is still valid.
Also note that free(NULL) is safe, and does nothing.
*/
newx = realloc(x, nmax * sizeof x[0]);
newy = realloc(y, nmax * sizeof y[0]);
if (newx)
x = newx;
if (newy)
y = newy;
if (!newx || !newy) {
/* One or both of the allocations failed. */
free(line);
free(x);
free(y);
return ENOMEM;
}
}
/* Save the parsed values to the arrays. */
x[n] = xval;
y[n] = yval;
n++;
}
/* We no longer need the line buffer. */
free(line);
/* Did a read error occur? */
if (ferror(in)) {
free(x);
free(y);
return EIO;
}
/* Was there no data to read? */
if (n < 1) {
free(x);
free(y);
return 0;
}
/* Reallocate the arrays to their exact sizes
(actually, allow for one extra double at the end,
because it is often useful to copy the initial
ones there if the data is considered cyclic).
*/
nmax = n + 1; /* One extra just because it is so often useful. */
newx = realloc(x, nmax * sizeof x[0]);
newy = realloc(y, nmax * sizeof y[0]);
if (newx)
x = newx;
if (newy)
y = newy;
if (!newx || !newy) {
free(x);
free(y);
return ENOMEM;
}
/* Save the array pointers. */
if (xptr)
*xptr = x;
else
free(x);
if (yptr)
*yptr = y;
else
free(y);
/* Save the number of samples read. */
*nptr = n;
/* If feof(in) is true, then we read everything
up to end of input. Otherwise, we stopped at
a line we could not parse.
*/
if (!feof(in))
return EBADMSG;
return 0;
}
That function, or something like it, should be in the course materials for every numerical computation course. They are just so darn useful. This particular one has no inherent limits to the size of data it can read, other than possible memory allocation limits set by the system administrator for each process. I know for a fact that it happily reads billions of lines of data, successfully, if you just have enough RAM.
Using the function is very simple. Here is an example main() that just reads such data from standard input -- remember that you can make it read from a file by appending < file to the command when you run it --, and prints the data out.
int main(void)
{
double *x, *y;
size_t i, n;
int result;
result = read_xy(&x, &y, &n, stdin);
switch (result) {
case 0: /* No errors */
break;
case EBADMSG:
if (n > 1)
fprintf(stderr, "Invalid line after %zu data samples.\n", n);
else
fprintf(stderr, "Cannot parse first input line.\n");
return EXIT_FAILURE;
case ENOMEM:
fprintf(stderr, "Out of memory.\n");
return EXIT_FAILURE;
case EIO:
fprintf(stderr, "Read error.\n");
return EXIT_FAILURE;
case EINVAL:
fprintf(stderr, "Invalid parameters to the read_xy() function!\n");
return EXIT_FAILURE;
default:
fprintf(stderr, "%s.\n", strerror(result));
return EXIT_FAILURE;
}
printf("Read %zu samples:\n", n);
for (i = 0; i < n; i++)
printf("%.9f %.9f\n", x[i], y[i]);
return EXIT_SUCCESS;
}
Note that most of it is the switch (result) { .. } error-reporting code. This example is very careful to tell you, if an error occurs; the reason is, you, as the user, need to know when the program knows they might spew garbage, and will in real life prefer for the programs to abort than spew that garbage silently -- perhaps making you believe it is valid.
While it is possible to modify the above code to work even on Windows (you replace the getline() with fgets(), for example, and hope that the buffer size you use for it will suffice; and also may have to change some of the errno error codes). However, there is a reason there are no Windows machines in the Top 500 supercomputer list: POSIXy systems (Unix and Linux) are just better suited for scientific computing. So, if you do intend to work on some scientific computing, you might as well just set up a Linux or BSD virtual machine, and do your development there.
