c - 三个函数中的指针

Question

我是 C 的新手，我正在尝试创建一个程序来计算银行账户的复利。

我正在使用三个带指针的函数，一个用于获取数据，一个用于计算，一个用于将数据打印到表格中。我假设我对指针的混淆导致计算值在表中打印不正确。

就目前而言，我没有收到任何错误，但收到了一些关于我的指针“默认为 int”的警告。

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

//Function declarations
void GETDATA(double* StartAmnt, float* IntrRate, int* NumYears, int* StartYear);

void Mathemagic (double StartAmnt, float IntrRate, int NumYears, int StartYear,
                 float* IntrEarned, float* PercentGained,long double* FutureValue,
                 int* FutureYear);

void PRINTTABLE (double StartAmnt, float IntrRate, int NumYears, int StartYear,
                 float IntrEarned, float PercentGained,long double FutureValue,
                 int FutureYear);

int main(void)
{

    //Local Declarations
    double StartAmnt;
    float IntrRate;
    float IntrEarned;
    float PercentGained;
    long double FutureValue;

    int NumYears;
    int StartYear;
    int FutureYear;

    //Statements
    GETDATA (&StartAmnt, &IntrRate, &StartYear, &NumYears);

    Mathemagic (StartAmnt, IntrRate, StartYear, NumYears, &IntrEarned,
                &PercentGained, &FutureValue, &FutureYear);

    PRINTTABLE (StartAmnt, IntrRate, NumYears, StartYear, IntrEarned,
                PercentGained, FutureValue, FutureYear);


    return 0;
}//main

void GETDATA(double* StartAmnt, float* IntrRate, int* NumYears, int* StartYear)
{
    //Statements
    printf("COP 2220-51014 Project 2: Michael Walt\n\n");
    printf("Enter a Starting amount (dollars and cents): ");
    scanf("%lf", StartAmnt);
    printf("Enter an Interest rate (ex. 2.5 for 2.5%):   ");
    scanf("%f", IntrRate);
    printf("Enter the Number of years (integer number):  ");
    scanf("%d", NumYears);
    printf("Enter the Starting year (four digits):       ");
    scanf("%d", StartYear);
    return;
}//GETDATA


void Mathemagic (double StartAmnt, float IntrRate, int NumYears, int StartYear,
                 float* IntrEarned, float* PercentGained,long double* FutureValue,
                 int* FutureYear)
{
    //Statements

    *FutureValue = StartAmnt*pow((1+(IntrRate/100)),NumYears);

    *PercentGained =((*FutureValue - StartAmnt)/StartAmnt)*100;

    *IntrEarned = (*FutureValue-StartAmnt);

    *FutureYear = (StartYear+NumYears);

    return;
}//Mathemagic

PRINTTABLE(StartAmnt, IntrRate, NumYears, StartYear, IntrEarned,
           PercentGained, FutureValue, FutureYear)
{
    printf("\n+-----------------------------+--------------+\n");
    printf("| Description                 |  Input Data  |\n");
    printf("|-----------------------------+--------------|\n");
    printf("| Starting amount             | $  %.2f  |\n", StartAmnt);
    printf("| Interest rate               |        %f%% |\n", IntrRate);
    printf("| Number of Years             |        %d     |\n", NumYears);
    printf("| Starting year               |     %d     |\n", StartYear);
    printf("+-----------------------------+--------------+\n");
    printf("| Future value                |   Results    |\n");
    printf("|-----------------------------+--------------|\n");
    printf("| In %d the balance will be | $  %f  |\n", FutureYear, FutureValue);
    printf("| Interest earned             | $   %f  |\n", IntrEarned);
    printf("| Total percent gained        |       %f%% |\n", PercentGained);
    printf("+-----------------------------+--------------+\n");
    return;
}

Answer 1

主要问题是您的PRINTTABLE 函数有错误的定义:

PRINTTABLE(StartAmnt, IntrRate, NumYears, StartYear, IntrEarned,
       PercentGained, FutureValue, FutureYear)

虽然它之前被声明为

void PRINTTABLE (double StartAmnt, float IntrRate, int NumYears, int StartYear,
                 float IntrEarned, float PercentGained,long double FutureValue,
                 int FutureYear);

如果您启用所有警告(几乎所有编译都必须启用)，您会看到许多默认为“int” 的警告:

*.c:76:1: warning: return type defaults to 'int' [-Wreturn-type]
 PRINTTABLE(StartAmnt, IntrRate, NumYears, StartYear, IntrEarned,
 ^
*.c:76:1: warning: conflicting types for 'PRINTTABLE' [enabled by default]
*.c:12:6: note: previous declaration of 'PRINTTABLE' was here
 void PRINTTABLE (double StartAmnt, float IntrRate, int NumYears, int StartYear,
      ^

因为您对 PRINTTABLE 有不同的声明和定义，所以编译器将使用您代码中出现的最后一个。在 C99 之前，如果声明中没有类型，所有参数都默认为 int，并且返回值也隐含为 int，造成类型冲突上面的警告。

这也是为什么在错误的转换和格式说明符(调用 undefined behavior)中有很多其他警告的原因

*.c:82:5: warning: format '%f' expects argument of type 'double', but argument 2 has type 'int' [-Wformat=]
     printf("| Starting amount             | $  %.2f  |\n", StartAmnt);

StartAmnt 是 int 由于隐含的类型规则，您将其打印为 double

有关规则的更多信息，请阅读

• C function calls: Understanding the "implicit int" rule
• implicit int in c language
• implicit declaration warning: What are the built-in functions?

另一个问题是你在这一行错过了一个百分号

printf("Enter an Interest rate (ex. 2.5 for 2.5%):   ");
                                               ↑

您还在另一个地方使用了错误的格式说明符。 FutureValue 的类型为 long double 但您要按 %f 打印它。它应该是 %Lf。

printf("| In %d the balance will be | $  %f  |\n", FutureYear, FutureValue);

实际上，在大多数应用程序中，long double(如果可用)的更高精度是不必要的。 long double 也可能占用更多空间，并且在许多情况下速度要慢得多，因为精度和使用旧的 x87 FPU 或软件浮点库。我也不知道你为什么要这样混合使用 float、double 和 long double，这只会浪费你在类型之间转换的时间并在其他情况下降低精度。一般来说，double 是推荐的类型，即使像 1.23 这样没有后缀的浮点字面量也是 double。 float 最常用于存储，因为精度非常有限。所有非 double 字面值都必须使用后缀

并且不要像这样对函数或变量使用全部大写