#include <math.h>
#include <stdio.h>
#include <stdlib.h>
 
double round_to_n_digits_1(double x, int n)
{ 
    double scale = pow(10.0, ceil(log10(fabs(x))) + n);
 
    return round(x * scale) / scale;
}
 
double round_to_n_digits_2(double x, int n)
{ 
    char buff[32];
 
    sprintf(buff, "%.*g", n, x);
 
    return atof(buff);
}
 
int main(void)
{
    double x = 0.0501;
 
    printf("%g\n", round_to_n_digits_1(x, 1));
    printf("%g\n", round_to_n_digits_2(x, 1));
 
    return 0;
}

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