Dynamic Alloc

Lesson

코딩도장 핵심요약: 구조체 포인터 메모리 할당

We have used fixed sized 1-D or 2-D arrays

double a[4] = { 2, 2, 3, 4 };

How can the user set the size of the array i.e., change the value of m by n) during the run-time? Is it possible to declare arrays without knowing its size?

malloc()

void * malloc(size_t size);	

Characteristics of “void * ” type (void pointer type)

• void pointer type can be assigned arbitrary types

• void pointer type can perform pointer operations

• in order to use the void pointer type properly, casting is necessary

• returns a pointer to the allocated memory, or NULL if the request fails

Example:

//** 1D Array
(int *)malloc(sizeof(int) * (_row));

//** 2D Array
// 1. allocate row array first
matA = (int**)malloc(sizeof(int*) * (_row));
// 2. Then, allocate column 
for(int i = 0; i < _row; i++)	
    (matA)[i] = (int*)malloc(sizeof(int) * (_col));

Example Code

Example 1

Dynamic Allocation- 1D using functions

C_malloc1d_example.c

val = 1;
int *vecC;	
// Memory allocation
vecC = (int*)malloc(sizeof(vecC) * (_row));	

// Initialization with a value
for (i = 0; i < _row; i++)
    (vecC)[i] = val;

// Free allocated memory when program ends
free(vecC);

Example 2

Dynamic Allocation- 1D using functions

C_malloc1d_example2.c

int *vecD;
// Memory allocation
createVec(&vecD, _row);
// Initilization with values
initVec(vecD, _row,1);	
// Free allocated memory
free(vecD);

////////////////////////////////////////////////////

void createVec(int** _vec, int _row){		
	*_vec = (int*)malloc(sizeof(int) * (_row));
}
 
void initVec(int* _vec, int _row, int _val)
{
	int i;
	for(i = 0; i < _row; i++)
			(_vec)[i] = _val;	
}

Example 3

Dynamic Allocation- 2D using functions

C_malloc2d_example2.c

int **matB;
createMat(&matB,_row, _col);
initMat(matB,_row, _col,3);
printMat(matB, _row, _col);	
system("pause" );
free(matA);
free(matB);


////////////////////////////////////////////////////

void createMat(int*** _mat, int _row, int _col)
{
	int i;	
	*_mat = (int**)malloc(sizeof(int) * (_row));
	for(i = 0; i < _row; i++)
		(*_mat)[i] = (int*)malloc(sizeof(int) * (_col));		
}
 
void initMat(int** _mat, int _row, int _col, int _val)
{
	int i, j;
	for(i = 0; i < _row; i++)
	{
		for(j = 0; j < _col;j++)		
			(_mat)[i][j] = _val;			
	}
}

You are passing ‘int *mat’ to the function ‘create_mat( )’ without allocating size and memory of the 2-D array. Thus, you need to pass the address of ‘int **mat’ as ‘**&mat’ and the function receives it as ‘int* _mat’ (3 pointer notation)

createMat(&matB,_row, _col);
void createMat(int*** _mat, int _row, int _col)

Notice how different ‘malloc’ syntax is used in ‘Main()’ function and in ‘create_mat()’ function. Once, the memory of 2-D is allocated then you can pass the array to a function as

initMat(matB,_row, _col,3);
void initMat(int** _mat, int _row, int _col, int _val)

Example 4

Structure Dynamic Allocation- 2D Matrix

C_matrix_example.c

typedef struct {
	double** at;
	int rows;
	int cols;
}Matrix;


void main() {
	Matrix A, B, C;
	//You can make a matrix of the size you want
	A = createMat(3, 5);

}


//////////////////////////////////
Matrix createMat(int _rows, int _cols) {
	Matrix Out;
	// 1. allocate row first
	Out.at = (double**)malloc(sizeof(double*) * _rows);
	// 2. allocate column 
	for (int i = 0; i < _rows; i++)
		Out.at[i] = (double*)malloc(sizeof(double) * _cols);
	// 3. Initialize matrix with values
	Out.rows = _rows;
	Out.cols = _cols;
	for (int i = 0; i < _rows; i++)
		for (int j = 0; j < _cols; j++)
			Out.at[i][j] = 0;
	return Out;
}

Example 5

Structure Dynamic Allocation- 2D Matrix

myMatrix_ tutorial.h

myMatrix_ tutorial.c

C_matrix_example2.c

#include “myMatrix_tutorial.h"

int main()
{
	Matrix A, B, C;
	A = createMat(5, 3);
	initMat(A, 10);
	B = createMat(5, 3);
	initMat(B, 5);
	C = addMat(A, B);
	printMat(C);

	free(A);
	free(B);
	free(C);

	system("PAUSE");
	return 0;
}

//////////////////////////////
void initMat(Matrix _mat, double _val) {
	for (int i = 0; i < _mat.rows; i++)
		for (int j = 0; j < _mat.cols; j++)
			_mat.at[i][j] = _val;
}

Exercise

• Online C Compiler

• Exercise Code

• Exercise-Solution Code

Exercise 1

Download the following files

• myMatrix_ tutorial.h

• myMatrix_ tutorial.c

• myMatrix_exercise.c

Include “myMatrix_tutorial.h” and add following functions

Matrix subMat (Matrix _A, Matrix _b);

Add two matrices of 3x3 size.

• You can create any value 2D matrix of integer type

• Print the input matrix and output matrix

Create a function that subtracts two matrices