expected expression before 'long' - c

Question

I'm VERY new to programming, and I'm using a book to help me write some code that will solve ln(1+x) after the user inputs "x". My syntax is exactly the way the book has it in its example but I keep getting an error: expected expression before 'long' on line 28. Line 28 is the line that reads long double y = LOG(1+(x));.
#include <stdio.h>
#include <math.h>
#define LOG(X) _Generic((X),\
long double: log(1+(x))\
)
int main()
{
double x;
printf("Please enter a number from -1 to +1: ");
scanf("%f", &x);
long double y = LOG(1+(x));
printf("From C math library: ln(1+x) = %f\n", y);
}

Answer 1

Firstly, your generic macro is written for long double type. You supplied a double argument x. long double and double are two different types. You did not define a generic branch for double, which means that your macro will not compile for a double argument. Either change the type of x to long double or make your generic macro support double arguments.
Also, this is a very strange way to write a LOG macro. Why are you explicitly using 1 + (x) as log argument inside the macro? What if tomorrow you'll want to calculate LOG(2 + y)? Your macro will still insist on calculating log(1 + (x)) instead, which does not make any sense at all. A more sensible thing to do would be to just pass X to log
#define LOG(X) _Generic((X),\
long double: log(X)\
)
Secondly, in order to scanf a double value you need %lf format specifier. %f is for float. To printf a long double value you need %Lf format.

Answer 2

If you are new to programming, and trying to make an example work, I would not use macros at this stage but focus on the function and the data type. Start with a double which is the typical working type of the library interfaced by math.h. My example shows how to calculate the natural ln with the log() function.
As others have commented, scanf() needs a format specifier of %lf for a double, however printf() only needs the format specifier of %f for a double. Note that I have tested the return value from scanf() function as well as the number you input, to check that the input was well behaved.
Lastly, you invite numbers in the range -1 <= x <= 1 yet ln(0) is undefined, hence I have restricted the input range to exclude -1.
#include <stdio.h>
#include <math.h>
int main(void)
{
double x = 0, y;
printf("Please enter a number from -1 to +1: ");
if (1 != scanf("%lf", &x) || x <= -1 || x > 1) {
printf("I need -1 < number <= +1\n");
return 1;
}
y = log(x + 1);
printf("From C math library: ln(1+x) = %f\n", y);
return 0;
}
Here are some sample outputs:
Please enter a number from -1 to +1: -1
I need -1 < number <= +1
Please enter a number from -1 to +1: 0
From C math library: ln(1+x) = 0.000000
Please enter a number from -1 to +1: 1
From C math library: ln(1+x) = 0.693147