Finding out character 'a' from a file then count the lines in which the character is appeared [duplicate]

Question

Dont know how to do this. Tried something like below.Want more optimisation in code.
Everyting should be in one function only,
guide me how to open close a file,
how to find character in each line,
increase the counter.

void simpleFileIn(void) {

    string line;
    ifstream myfile("example.txt");
    if (myfile.is_open()) {
        while (getline(myfile, line)) {
            //found(line);
            size_t size = strlen(line);
            cout << line << '\n';
        }
        myfile.close();
    }
    else
        cout << "Unable to open file";
}

the function simpleFileIn() should work, to open a file then close after work is done.
find out character a and count the integers. want to close/ delete this question as i am in ban asking more help me. situation getting worse day after day

Answer 1

You need to allocate rows in a loop as you go:

int** pptr = new int* [rows]; // <<== rows, not cols
for(int i=0;i<rows;i++){
    pptr[i] = new int[cols]; // <<== Add this line
    for(int j=0;j<cols;j++){
        cout<<"Enter value at "<<i<<j<<endl;
        cin>>pptr[i][j];
        cout<<"Value is "<<pptr[i][j]<<endl;
    }
}

You also need to delete individual rows before deleting the array of pointers to them. Use delete[] operator with square brackets:

for(int i=0;i<rows;i++){
    delete[] pptr[i];
}
delete[] pptr;

You do not need to assign NULLs to deleted pointers, unless you plan to reuse the pointer for something else later on.

Answer 2

You are allocing array of pointers wrong.

First you have to allocate enough space for row pointers

int** pptr = new int* [rows];

For every pointer enough space for col integers

for (int i = 0; i < cols; i++)
{
    pptr[i] = new int [cols];
}

To delete arrays use delete[] instead of delete.

Delete every individual row

for (int i = 0; i < rows; i++)
{
    delete [] pptr[i];
}

Then delte array of pointers

delete [] pptr;

There is no need to assigning NULL to deleted pointer since you wont use them again. Also in c++ you should use nullptr instead of NULL.

Here is the correct using of array of pointers.

---

Your mistakes

int* ptr = new int [rows];
int** pptr = new int* [cols];
*pptr=ptr;

• Swapped rows & cols
• Allocated memory only for first pointer/row, others were uninitialized -> UB
• Used delete instead of delete[]

Answer 3

So there seems to be some confusion with allocation. From your code

int* ptr = new int [rows];
int** pptr = new int* [cols];
*pptr=ptr;

you have now created to 1-dimensional arrays. You then dereference the pptr and assign to it ptr this is the same as

pptr[0] = ptr;

So you are only initializing the very first column. You want to change this code to be

int** pptr = new int* [cols];
for (int i = 0; i < cols; ++i) {
    pptr[i] = new int [rows];
}

This will allocate the memory properly

Answer 4

You can create a constructor of sorts for your 2D array so that you have one-line bookkeeping:

#include <iostream>

template <typename T>
T** new_( std::size_t rows, std::size_t columns )
{
  auto dsize = rows    * sizeof(T*);
  auto rsize = columns * sizeof(T);
  auto tsize = rows    * rsize;

  unsigned char* data = new unsigned char[ dsize + tsize ];
  T** result = (T**)data;
  T*  table  = (T*)(data + dsize);

  while (rows--) 
    result[ rows ] = table + rows * columns;

  return result;
}

int main()
{
  int m; std::cout << "m? ";  std::cin >> m;
  int n; std::cout << "n? ";  std::cin >> n;

  // Create the new matrix
  int** a = new_<int>( m, n );

  // Do stuff with a[ r ][ c ] here.
  // It looks and behaves JUST LIKE a normal 2D C array
  // in all respects EXCEPT one: &a != &(a[0][0]).
  // Use the latter when passing to a flat function!

  // Delete it
  delete [] a;
}

Enjoy the weirdness.