How to calculate BMI using function?

Question

Calculate the body mass index. Body mass index compares your weight to your height, and is calculated by dividing your weight in kilograms by your height in meters squared. It gives you an idea of whether you're underweight, a healthy weight, overweight, or obese for your height.

Body Mass Index Categories:

1. Underweight = <18.5
2. Normal weight = 18.5–24.9
3. Overweight = 25–29.9
4. Obesity = BMI of 30 or greater

If you are underweight or overweight or obese, determine the ideal weight based on your height and age.

Estimate Ideal body weight in (kg) using Devine formula:

Males: IBW = 50 kg + 2.3 kg for each inch over 5 feet.

Females: IBW = 45.5 kg + 2.3 kg for each inch over 5 feet.

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

double calculateBMI(double weight, double height);

int main(void)
{
    printf("Calculate your BMI\n"); //Calculation of body mass index (BMI)

    double w, h, ret;
    double BMI = w / (h*h);
    ret = calculateBMI(BMI);

    printf("Enter your weight in kilograms:\n", w); //Input your weight in kilograms here
    scanf("%lf", &w);
    printf("Enter your height in metres:\n", h); //Input your height in metres here
    scanf("%lf", &h);
    printf("Your BMI is %lf\n", ret)

    printf("BMI categories:\n");
    if (BMI < 18.5)
    {
        printf("Your BMI is %lf and you are currently underweight.\n");
    }
    else if (BMI >= 18.5 && BMI <= 24.9)
    {
        printf("Your BMI is %lf and you are normal weight.\n");
    }
    else if (BMI >= 25 && BMI <= 29.9);
    {
        printf("Your BMI is %lf and you are currently overweight.\n");
    }
    else (BMI >= 30);
    {
        printf("Your BMI is %lf and you are obese.\n");
    }


    return 0;
}

double calculateBMI(double weight, double height)
{
    double result;
    result = weight / (height*height);

    return result;
}

Help I still don't know how to do the functions. Please help me.

Answer 1

You should use the parameters used in the method to calculate the BMI. Your variables BMI, height, and weight are not declared originally in the function. Instead you should declare BMI as double, and use height and weight as the function parameters.

Also, you need to return the value of BMI from the function. You are incorrectly returning the calculateBMI, which is not a valid identifier inside the function.

The working code would be :-

double calculateBMI(double weight, double height)
{
    double BMI = weight / (height*height);
    return BMI;
}

Also, you've not called the method calculateBMI() inside your main().

...
printf("Enter your weight in kilograms:\n"); //Input your weight in kilograms here
scanf("%lf", &weight);
printf("Enter your height in metres:\n"); //Input your height in metres here
scanf("%lf", &height);
// add below line in your code to call the function.
BMI = calculateBMI(height,weight);
printf("BMI categories:\n");
...

I'd also suggest you to read more about functions in C. You need some more basic knowledge of functions(practice hard too).

EDIT ---> Based on OP's comment, the final code would be :

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

double calculateBMI(double weight, double height);

int main(void)
{
printf("Calculate your BMI\n"); //Calculation of body mass index (BMI)

double w, h, BMI;
printf("Enter your weight in kilograms:\n", w); //Input your weight in kilograms here
scanf("%lf", &w);
printf("Enter your height in metres:\n", h); //Input your height in metres here
scanf("%lf", &h);
BMI = calculateBMI(w,h);
printf("Your BMI is %lf\n", BMI)

printf("BMI categories:\n");
if (BMI < 18.5)
{
    printf("Your BMI is %lf and you are currently underweight.\n");
}
else if (BMI >= 18.5 && BMI <= 24.9)
{
    printf("Your BMI is %lf and you are normal weight.\n");
}
else if (BMI >= 25 && BMI <= 29.9);
{
    printf("Your BMI is %lf and you are currently overweight.\n");
}
else (BMI >= 30);
{
    printf("Your BMI is %lf and you are obese.\n");
}


return 0;
}

double calculateBMI(double weight, double height)
{
    double result;
    result = weight / (height*height);
    return result;
}

Answer 2

Your function :

double calculateBMI(double w, double h)
{
    BMI = weight / (height*height);
    return calculateBMI;
}

Here you are calling the function and returning the function name "calculateBMI".

Return the proper value which you want.This is not a recursive function.