`truncate' undeclared (first use this function)

#include <iostream>
#include <math.h>

using namespace std;

#define CENTIMETERS_IN_INCH 2.54
#define POUNDS_IN_KILOGRAM 2.2

int main() {
    double feet, inches, centimeters, weight_in_kg, weight_in_lbs;
    
    // get height in feet and inches
    cout << "Enter height (feet): ";
    cin >> feet;
    cout << "Enter (inches): ";
    cin >> inches;
    
    // convert feet and inches into centimeters
    centimeters = ((12 * feet) + inches) * CENTIMETERS_IN_INCH;
    
    // round 2 decimal places and truncate
    centimeters = truncate(centimeters);
    
    printf("Someone that is %g' %g\" would be %g cm tall", feet, inches, centimeters);
    
    // weights for bmi of 18.5
    weight_in_kg = truncate(18.5 * centimeters);
    weight_in_lbs = round(weight_in_kg * POUNDS_IN_KILOGRAM);
    
    printf("18.5 BMI would correspond to about %g kg or %g lbs", weight_in_kg, weight_in_lbs);
    
    // weights for bmi of 25
    weight_in_kg = truncate(25 * centimeters);
    weight_in_lbs = round(weight_in_kg * POUNDS_IN_KILOGRAM);
    
    printf("25.0 BMI would correspond to about %g kg or %g lbs", weight_in_kg, weight_in_lbs);
    
    // pause output
    cin >> feet;
    
    return 0;
}

// round result
double round(double d) {
   return floor(d + 0.5);
}

// round and truncate to 1 decimal place
double truncate(double d) {
   return round(double * 10) / 10;
}