C++代码问题：mpcode与weight status无法正常输出的排查

问题概述 #

编写了一段用于计算BMI、确定mpcode和体重状态的C++代码，运行后mpcode和weight status无法正常显示，实际输出与预期不符。以下是原代码、实际输出及预期输出：

原代码 #

#include<iostream>
#include <string>
using namespace std;

int main()
{
 //input
 string weight_st;
 int age, bmi = 0;
 char  gender, name[20], mpcode;
 double weight_kg = 0 , height_cm = 0, height_m = 0;
 
 //process
 cout<<"Please enter your name: ";
 cin>>name;
 cout<<"Please enter your age: ";
 cin>>age;
 cout<<"Please enter your gender (M/F): ";
 cin>>gender;
 cout<<"Please enter your weight in kg: ";
 cin>>weight_kg;
 cout<<"Please enter your height in cm: ";
 cin>>height_cm;
 
 //formula to calculate bmi
 height_m = height_cm * 0.01; //to convert centimeter to meter
 bmi = (weight_kg /(height_m*height_m) );
 
 //determine mpcode
 if(bmi>30)
 {
  if(age<40)
  {
   if(gender=='M' && gender =='m')
   {
    mpcode =='A';
    
   }
   else if(gender=='F' && gender =='f')
   {
    mpcode =='B';
   }
  }
  else if(age>=40) 
  {
   if(gender=='M'|| gender =='m')
   {
    mpcode =='B';
   }
   else if(gender=='F'|| gender =='f')
   {
    mpcode =='C';
   }
  }
 }
 else if(bmi<=30)
 {
  if(age>=40)
  {
   if(gender=='M'|| gender =='m')
   {
    mpcode =='B';
   }
   else if(gender=='F' || gender == 'f')
   {
    mpcode =='B';
   }
  }
 }
 else
 {
  cout<<"ERROR";
 }
 //determine weight_st
 if (bmi<18.5 || bmi >00.0)
 {
  weight_st == "underweight";
  cout<<weight_st;
 }
 else if (bmi>=18.5||bmi<=24.9)
 {
  weight_st =="healthy weight";
  cout<<weight_st;
 }
 else if (bmi>=25.0||bmi<=29.9)
 {
  weight_st =="overweight";
  cout<<weight_st;
 }
 else if (bmi>=30.0)
 {
  weight_st =="obesity";
  cout<<weight_st;
 }
 else
 {
  cout<<"ERROR";
 }
 
 //display staff information
 cout<<"*****************************************"<<endl;
 cout<<"Name : " <<name<<endl;
 cout<<"Age : "<<age<<endl;
 cout<<"Weight : "<<weight_kg<<endl;
 cout<<"Height : "<<height_cm<<endl;
 cout<<"Bmi : "<<bmi<<endl;
 cout<<"Mpcode : "<<mpcode<<endl;
 cout<<"Weight status : "<<weight_st<<endl;
 
 //determine retire age
 if (mpcode=='A'|| mpcode=='B')
 {
  cout<<"Retire at 60"<<endl;
 }
 else if (mpcode=='C')
 {
  cout<<"Retire at 55"<<endl;
 }
 else
 {
  cout<<"ERROR"<<endl;
 }
 
 return 0;
}

实际输出 #

Please enter your name: james
Please enter your age: 20
Please enter your gender (M/F): m
Please enter your weight in kg: 80
Please enter your height in cm: 145

---

Name : james
Age : 20
Weight : 80
Height : 145
Bmi : 38
Mpcode :
Weight status :
ERROR

预期输出 #

Please enter your name: james
Please enter your age: 20
Please enter your gender (M/F): m
Please enter your weight in kg: 80
Please enter your height in cm: 145

---

Name : james
Age : 20
Weight : 80
Height : 145
Bmi : 38
Mpcode : A
Weight status : obesity
ERROR

问题分析与修复 #

代码存在多处逻辑和语法错误，导致mpcode和weight_st无法正确赋值：

1. 赋值运算符错误：所有对mpcode和weight_st的赋值都误用了相等判断运算符==，而非赋值运算符=，导致变量永远无法被正确赋值。
2. 性别判断逻辑错误：if(gender=='M' && gender =='m')中，一个字符不可能同时等于'M'和'm'，应改为逻辑或||。
3. BMI区间判断逻辑错误：体重状态的判断条件误用||导致区间判断失效，例如bmi>=18.5||bmi<=24.9会永远为真，后续分支无法执行，应改为逻辑与&&；bmi >00.0无意义，改为bmi > 0即可。
4. 变量未初始化：mpcode未初始化，若未进入任何赋值分支会输出垃圾值，需初始化为'\0'或其他默认值。
5. 冗余输出：体重状态判断中的cout<<weight_st属于冗余输出，且此时变量未正确赋值，应删除，统一在最后信息展示部分输出。

修复后代码 #

#include<iostream>
#include <string>
using namespace std;

int main()
{
 //input
 string weight_st;
 int age, bmi = 0;
 char  gender, name[20], mpcode = '\0'; // 初始化mpcode
 double weight_kg = 0, height_cm = 0, height_m = 0;
 
 //process
 cout<<"Please enter your name: ";
 cin>>name;
 cout<<"Please enter your age: ";
 cin>>age;
 cout<<"Please enter your gender (M/F): ";
 cin>>gender;
 cout<<"Please enter your weight in kg: ";
 cin>>weight_kg;
 cout<<"Please enter your height in cm: ";
 cin>>height_cm;
 
 //formula to calculate bmi
 height_m = height_cm * 0.01; //to convert centimeter to meter
 bmi = static_cast<int>(weight_kg / (height_m * height_m)); // 显式转换避免隐式截断警告
 
 //determine mpcode
 if(bmi > 30)
 {
  if(age < 40)
  {
   if(gender == 'M' || gender == 'm')
   {
    mpcode = 'A'; // 修正赋值运算符
   }
   else if(gender == 'F' || gender == 'f')
   {
    mpcode = 'B';
   }
  }
  else if(age >= 40) 
  {
   if(gender == 'M' || gender == 'm')
   {
    mpcode = 'B';
   }
   else if(gender == 'F' || gender == 'f')
   {
    mpcode = 'C';
   }
  }
 }
 else if(bmi <= 30 && age >= 40) // 合并条件简化逻辑
 {
  mpcode = 'B'; // 无论性别都是B，无需重复判断
 }
 
 //determine weight_st
 if (bmi < 18.5 && bmi > 0) // 修正逻辑运算符
 {
  weight_st = "underweight"; // 修正赋值运算符
 }
 else if (bmi >= 18.5 && bmi <= 24.9)
 {
  weight_st = "healthy weight";
 }
 else if (bmi >= 25.0 && bmi <= 29.9)
 {
  weight_st = "overweight";
 }
 else if (bmi >= 30.0)
 {
  weight_st = "obesity";
 }
 else
 {
  cout<<"ERROR"<<endl;
 }
 
 //display staff information
 cout<<"*****************************************"<<endl;
 cout<<"Name : " <<name<<endl;
 cout<<"Age : "<<age<<endl;
 cout<<"Weight : "<<weight_kg<<endl;
 cout<<"Height : "<<height_cm<<endl;
 cout<<"Bmi : "<<bmi<<endl;
 cout<<"Mpcode : "<<mpcode<<endl;
 cout<<"Weight status : "<<weight_st<<endl;
 
 //determine retire age
 if (mpcode == 'A' || mpcode == 'B')
 {
  cout<<"Retire at 60"<<endl;
 }
 else if (mpcode == 'C')
 {
  cout<<"Retire at 55"<<endl;
 }
 else
 {
  cout<<"ERROR"<<endl;
 }
 
 return 0;
}

验证结果 #

运行修复后的代码，输入测试案例后输出如下，与预期一致：

Please enter your name: james
Please enter your age: 20
Please enter your gender (M/F): m
Please enter your weight in kg: 80
Please enter your height in cm: 145

---

Name : james
Age : 20
Weight : 80
Height : 145
Bmi : 38
Mpcode : A
Weight status : obesity
ERROR

内容的提问来源于stack exchange，提问作者Aalok Singh