在C中动态分配内存时出错

Question

I am trying to calculate the determinant of an NxN matrix. This piece of code gives an error in lines where I try to allocate the memory dynamically.

error: a value of type "int" cannot be assigned to an entity of type "float *"

error: a value of type "int" cannot be assigned to an entity of type "float **"

double CalcDeterminant( float **mat, int order)
{
    float **minor;
    unsigned short i;
    float det = 0;
    // order must be >= 0
    // stop the recursion when matrix is a single element
    if( order == 1 )
        return mat[0][0];

    // the determinant value
    // allocate the cofactor matrix

    **minor = malloc((order-1) * sizeof(float *));
    for(i=0;i<order-1;i++)
        minor[i] = malloc((order-1) * sizeof(float));**

    //float *mat2d = malloc( rows * cols * sizeof( float ));
    for(i = 0; i < order; i++ )
    {
        // get minor of element (0,i)
        GetMinor( mat, minor, 0, i , order);
        // the recusion is here!

        det += (i%2==1?-1.0:1.0) * mat[0][i] * CalcDeterminant(minor,order-1);
        //det += pow( -1.0, i ) * mat[0][i] * CalcDeterminant( minor,order-1 );
    }

    // release memory
    for(i=0;i<order-1;i++)
        free(minor[i]);
    free(minor);
    return det;
}

Answer 1

You need to add the line #include <stdlib.h> so that malloc() is properly declared.

As it stands, the compiler is being very lax (C89 mode) and allowing implicit function declarations, so when the compiler comes across malloc() , it assumes that it is a function that returns an int , instead of the correct void * .

You need to change your compilation options until the compiler complains more loudly. For example, if you use GCC, you should consider:

gcc -std=c99 -Wall -Wextra -Wstrict-prototypes -Wmissing-prototypes ...

You may prefer, or even need, to use -std=gnu99 instead -std=c99 ; that enables many extensions while still using the C99 core language. But using options along those lines and ensuring there are no compilation warnings is good discipline. Using -Werror enforces the discipline; it converts any warning message from the compiler into an error, so the compilation fails.

Answer 2

您需要包括头文件stdlib.h 。