The function log(x) in C is always giving the same value

Question

Basically, I am trying to find the logarithm of a number using the log(x) function in the math.h library of C. But, for any and every value that I enter for the variable x, I end up with the same answer: -722.259365. Where is the problem with my code below?

I am kind of new to C, so please don't mind if this is too silly a mistake.

#include<stdlib.h>
#include<math.h>
void PrintLogarithm(double x){
    if(x<=0){
        printf("Positive numbers only please.\n");
        return;
    }
    double Result = log(x);
    printf("%f\n", Result);
}
int main(){
    double x;
    scanf("%f", &x);
    PrintLogarithm(x);
    return 0;
}

I am kind of new to C, so please don't mind if this is too silly a mistake.

Answer 1

First of all, you are missing an include

#include <stdio.h>

Secondly, you are not using the correct format specifier. You are using doubles, so you should use %lf. If you compile with the -Wformat flag, your compiler will warn you about this by telling you something like this:

/cplayground/code.cpp:16:17: warning: format specifies type 'float *' but the argument has type 'double *' [-Wformat]
    scanf("%f", &x);
           ~~   ^~
           %lf

If you fix these 2 problems, your program should finally work as expected.

#include <stdlib.h>
#include <math.h>
#include <stdio.h>

void PrintLogarithm(double x) {
    if(x<=0) {
        printf("Positive numbers only please.\n");
        return;
    }

    double Result = log(x);
    printf("%lf\n", Result);
}

int main() {
    double x;
    scanf("%lf", &x);

    PrintLogarithm(x);

    return 0;
}

Edit : As commenters pointed out, printf() works fine with either %lf or %f as the format specifier.