无法通过函数指针进入函数。
我正在编写一个程序，该程序基于用于美国和欧盟标准的输入（身体质量指数计算器）。 我的观点是使用一个函数“ calcMethod”来计算BMIndex，但是尝试将其他函数的指针分配给该函数，则会导致错误“被称为对象不是函数或函数指针”。任何帮助表示赞赏。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>
float calcEU(float inputMass, float inputHeight)
{
float BMIndexF;
BMIndexF = inputMass / (inputHeight * inputHeight);
return BMIndexF;
}
float calcUS(float inputMass, float inputHeight)
{
float BMIndexF;
BMIndexF = 703 * inputMass / (inputHeight * inputHeight);
return BMIndexF;
}
int main()
{
float BMIndex , inputMass , inputHeight;
float heightColumn, massRow;
float *calcMethod ;
int Mod = 0;
int countRow , countColumn;
char unitStandard[2] , metricUnitH[2] , metricUnitM[2];
printf("Your measure units? EU (kg, m) or US (lb, in) \n");
gets(unitStandard);
if(strcmp(unitStandard, "EU") == 0)
{
Mod = 1;
strcpy(metricUnitH, "me");
strcpy(metricUnitM, "kg");
float (*calcMethod)(float , float) = &calcEU;
}
else if (strcmp(unitStandard, "US") == 0)
{
Mod = -1;
strcpy(metricUnitH, "in");
strcpy(metricUnitM, "lb");
float (*calcMethod)(float , float) = &calcUS;
}
else
{
printf("Wrong Input");
exit(-1);
}
printf("Introduce your body mass:\n");
scanf("%f", &inputMass);
printf("Introduce your height:\n");
scanf("%f", &inputHeight);
printf("\n");
for(countRow = 0; countRow <= 5; countRow++)
{
for(countColumn = 0; countColumn <= 5; countColumn++)
{
heightColumn = inputHeight - 0.1 * (3 - countRow);
massRow = inputMass - 1 * (3 - countColumn);
if(countRow == 0 && countColumn == 0) printf("H / M|");
if(countRow == 0 && countColumn != 0) printf("%.0f%s |", massRow , metricUnitM);
if(countColumn == 0 && countRow != 0) printf("%.1f%s |", heightColumn , metricUnitH);
if(countRow != 0 && countColumn !=0)
{
//this line causes error
BMIndex = (*calcMethod)(massRow , heightColumn);
printf("%.2f |", BMIndex);
}
}
printf("\n");
}
return 0;
}
注释行导致错误：被调用对象不是函数或函数指针
期望它不会引发错误并按预期工作。
答案 0 :(得分：2)
问题是您声明了float *calcMethod;-指向浮点数的指针，而不是指向函数的指针。然后，您可以在内部块中将其重新声明为函数指针，但这仅是在这些块中-您尝试在其中调用它是在尝试调用浮点指针。
解决方法是首先将其声明为函数指针：
float (*calcMethod)(float, float);
然后，当您决定使用哪个时，不要重新声明它，只需分配它即可：
calcMethod = calcUS;
或
calcMethod = calcEU;
您也不需要*来通过指针进行调用-您只需使用
BMIndex = calcMethod(massRow , heightColumn);
答案 1 :(得分：1)
您在calcMethod函数中有三个main()变量。第一个是一个指向float变量的指针（显然不是函数指针）。另外两个是函数指针，但是它们仅存在于其代码块之内。
如果仅将calcMethod定义为一个函数指针，则无论代码中是否存在其他错误，都可以使用。
这是三个变化：
int main()
{
float BMIndex , inputMass , inputHeight;
float heightColumn, massRow;
float (*calcMethod)(float , float); // ****** CHANGE #1 HERE
int Mod = 0;
int countRow , countColumn;
char unitStandard[2] , metricUnitH[2] , metricUnitM[2];
printf("Your measure units? EU (kg, m) or US (lb, in) \n");
gets(unitStandard);
if(strcmp(unitStandard, "EU") == 0)
{
Mod = 1;
strcpy(metricUnitH, "me");
strcpy(metricUnitM, "kg");
calcMethod = &calcEU; // ****** CHANGE #2 HERE
}
else if (strcmp(unitStandard, "US") == 0)
{
Mod = -1;
strcpy(metricUnitH, "in");
strcpy(metricUnitM, "lb");
calcMethod = calcUS; // ****** CHANGE #3 HERE
}
else
{
printf("Wrong Input");
exit(-1);
}
printf("Introduce your body mass:\n");
scanf("%f", &inputMass);
printf("Introduce your height:\n");
scanf("%f", &inputHeight);
printf("\n");
for(countRow = 0; countRow <= 5; countRow++)
{
for(countColumn = 0; countColumn <= 5; countColumn++)
{
heightColumn = inputHeight - 0.1 * (3 - countRow);
massRow = inputMass - 1 * (3 - countColumn);
if(countRow == 0 && countColumn == 0) printf("H / M|");
if(countRow == 0 && countColumn != 0) printf("%.0f%s |", massRow , metricUnitM);
if(countColumn == 0 && countRow != 0) printf("%.1f%s |", heightColumn , metricUnitH);
if(countRow != 0 && countColumn !=0)
{
BMIndex = (*calcMethod)(massRow , heightColumn);
printf("%.2f |", BMIndex);
}
}
printf("\n");
}
return 0;
}
答案 2 :(得分：0)
主要问题是您在嵌套块中声明了不同的calcMethod变量，并更改了错误的变量。另外，您尝试调用的外部calcMethod变量甚至都不是函数指针：
float *calcMethod ;
//assigning the pointer to function
//this subprogram is nested in main
if(Mod == 1) //Mod is just a variable
{
strcpy(metricUnitH, "me");
strcpy(metricUnitM, "kg");
// N.B. This is a different calcMethod variable!
float (*calcMethod)(float , float) = &calcEU;
} // N.B. calcMethod variable in previous block no longer exists!
if(Mod == -1)
{
strcpy(metricUnitH, "in");
strcpy(metricUnitM, "lb");
// N.B. This is a different calcMethod variable!
float (*calcMethod)(float , float) = &calcUS;
} // N.B. calcMethod variable in previous block no longer exists!
//the calcMethod is called
//this is nested in 2 for(s) and an if
// N.B. the calcMethod variable in this block is uninitialized, and
// it is not even a function pointer (it is a pointer to float) so
// this will not even compile....
BMIndex = (*calcMethod)(massRow , heightColumn);
解决方案是将外部calcMethod变量声明为函数指针，并更改内部块以将其分配给外部块的calcMethod变量，而不是声明一个新变量：
float (*calcMethod)(float, float) = calcUS; // default to "US"
//assigning the pointer to function
//this subprogram is nested in main
if(Mod == 1) //Mod is just a variable
{
strcpy(metricUnitH, "me");
strcpy(metricUnitM, "kg");
calcMethod = calcEU;
}
if(Mod == -1)
{
strcpy(metricUnitH, "in");
strcpy(metricUnitM, "lb");
calcMethod = calcUS;
}
//the calcMethod is called
//this is nested in 2 for(s) and an if
BMIndex = (*calcMethod)(massRow , heightColumn);
在calcMethod语句之一未设置的情况下，我将calcUS初始化为if。或者，您可以将其初始化为NULL并在调用calcMethod之前检查是否为错误情况：
float (*calcMethod)(float, float) = NULL; // default to not set
// ...
if (calcMethod == NULL)
{
/* ERROR: calcMethod hasn't been set. */
/* DO SOMETHING! */
}
else
{
BMIndex = (*calcMethod)(massRow , heightColumn);
}