如何修复此Python BMI计算器？

Question

This is a BMI calculator that I wrote in python

print('BMI calculator V1')

name = str(input("What's your name?"))
weight = int(input("Your weight in Kilograms"))
height = float(input("Your height in Metres"))

def function(w, h):    #function here is the bmi calculator
     bmi = w / h ** 2
     return("Your BMI is " + str(bmi))

bmi_user = function(weight, height)  
print(bmi_user)  

if bmi_user < 18:
     print(name + "," + "you are underweight")
elif bmi_user > 25:
     print(name + "," + "you are overweight")
else:
     print(name + "," + "you are normal")

It shows the following error when I run the code

line 15, in if float(bmi_user) < 18:
ValueError: could not convert string to float:

Answer 1

The error message is clear: You can't do comparisons between a string and a double.

Look at what your function returns: a string.

def function(w, h):    #function here is the bmi calculator
     bmi = w / h ** 2
     return("Your BMI is " + str(bmi))

bmi_user = function(weight, height) 

You'll do better with this:

def bmi_calculator(w, h):    
     return w / h ** 2 

Answer 2

Fix it by not returning a string from your calculation. You should give this How to debug small programs (#1) a read and follow it to debug your code.

print('BMI calculator V1')

name = str(input("What's your name?"))
weight = int(input("Your weight in Kilograms"))
height = float(input("Your height in Metres"))

def calcBmi(w, h):    # function here is the bmi calculator
     bmi = w / h ** 2
     return bmi        # return a float, not a string

bmi_user = calcBmi(weight, height)  # now a float
print(f'Your BMI is: {bmi_user:.2f}')   # your output message

if bmi_user < 18:
     print(name + "," + "you are underweight")
elif bmi_user > 25:
     print(name + "," + "you are overweight")
else:
     print(name + "," + "you are normal")

function is not a really good name, I changed it to calcBmi .

Answer 3

Your function def function(w, h): returns a string as below.

return("Your BMI is " + str(bmi))

This cannot be compared with an integer as specified in your statements like below.

if bmi_user < 18:

The below line also will be an error

elif bmi_user > 25:

Change your function as below, it will work

def function(w, h):    #function here is the bmi calculator
    bmi = w / h ** 2
    return bmi