dynamic memory allocation

[ravi123]

I have an error saying expression must have a constant value could any one please help me out

void transpose_vec(double matrix[size_b_row1], double matrix_t[1][size_b_row1])
{
int size_b_row1;
using namespace std;
std::vector<int> temp(size_b_row1);
int i;
for(i=0;i<size_b_row1;i++)
{
matrix_t[1][i] = matrix[i];
}
}

[abhishekm71]

what is size_b_row1?

you have declared input matrices of size [size_b_row1]. This I assume should be a constant value. Then you have declared this same identifier as an int variable.

Also, without initializing this variable, you are using it in the for loop. I suggest you follow the matrix declaration as described in http://www.cplusplus.com/doc/tutorial/arrays/.

[ravi123]

void transpose_vec (double matrix[size_b_row1],
double matrix_t[1][size_b_row1])

{
int size_b_row1;
int i;
for(i=0;i<size_b_row1;i++)
{
matrix_t[1][i] = matrix[i];
}
}
this is actually wat the code was i tried a few things like vector got confused but i still get the same error expression must have a constant value could you please help me i have been debugging this error from 4 days

[keskiverto]

When you have int foo[N] in your code, the compiler must know at compile time what the value of N is.

The value of size_b_row1 must be set before/outside your function definition.

When you have

void foo( int bar ) {
  int bar;
}

You show two different variables with same name. The first is the parameter to the function, and the second is a local variable. If you then use bar, which one do you mean?

You do not set any value to your local variable size_b_row1. Then you use that variable anyway.

You declare that you have a double matrix_t[1][size_b_row1], but then you reference matrix_t[1][0]. That item is outside of your array.


Study the std::vector harder. It may look confusing at first, but it will make your life simpler.


PS. Despite your thread title, there is nearly no dynamic allocation here.

[Bourgond Aries]

Alternatively he could probably use a pointer instead.

[ravi123]

Thank you so much for the response , the error is expression must have a constant value for that we need to allocate memory dynamically so i wrote the topic as dynamic memory allocation

here is my code again if you want to
i will get an error on the first line itself

void transpose_vec(double matrix[size_b_row1],
double matrix_t[1][size_b_row1])
{
int size_b_row1;
int i;
for(i=0;i<size_b_row1;i++)
{
matrix_t[1][i] = matrix[i];
}
}

[keskiverto]

We have already seen that bit. Use of code tags would be nice.

How do you declare the size_b_row1 in the first place?

[ravi123]

my problem is that size_b_row1 is of type int
but it is not constant
i will follow it from the next time using code tags, thank you.

[abhishekm71]

I guess your problem is as follows:

You want to input matrices of length size_b_row1. This value of size_b_row1 varies in your calling function. What I suggest is as follows:

1. Don't use "dynamic memory allocation". If you must use it, then you have to use vectors. There must be some maximum limit of this size_b_row1. Lets say that limit is MAX_LIMIT;

Your code would then look like follows. (Ref: http://www.cplusplus.com/doc/tutorial/arrays/)

void transpose_vec (double matrix[], double matrix_t[][MAX_LIMIT], int size_b_row1)
{
  //int size_b_row1;
  int i;
  for(i=0;i<size_b_row1;i++)
  {
    matrix_t[0][i] = matrix[i];
  }
}

[ravi123]

oh my god!! it runs you are awesome!! thank you so much, but seriously my boss says he just wants dynamic memory allocation i did like this now i still get error
yeah size of row can maximum be 40 should i introduce a constant double some thing like this const double size_b_row1 =40; now i just get an error saying expression must have pointer to object type between the for loop(the error)


void transpose_vec()
{
std::vector<int> temp(size_b_row1);
using namespace std;
std::vector<double> matrix(size_b_row1);

std::vector<double> matrix_t(size_b_row1);

int i;
for(i=0;i<size_b_row1;i++)
{
matrix_t[1][i] = matrix[i];
}

}

[abhishekm71]

We can suggest a vector implementation code for you but it will not solve the purpose. What i instead propose is:

1. As pointed out earlier -
- put your code in code tags
- you are again trying to modify the value of matrix_t[1][i]. Please note that array indices start from 0.

2. From your code it seems there is a lot of confusion in function declaration, variables scope, vector implementation etc. You need to get these concepts clarified.

3. A slight hint, to implement matrices using vectors, you can maybe use nested vectors like the following:
std::vector<std::vector<double> > matrix_t (size_b_row1);

[ravi123]

heyy thank you soo much the nested std vector works, you really helped me great