private String read_col(char[][] trans_block,int start_pos,int end_pos)
	{
		if(start_pos>end_pos)
			return "";
		StringBuilder sb=new StringBuilder();
		int row=trans_block.length;
		int col=trans_block[0].length;
		assert(end_pos<col);
		for(int i=1;i<row;i++)
		{
		 for(int j=start_pos;j<=end_pos;j++)
		 {
			 if(trans_block[i][j]!='\0')
			 {
			   sb.append(trans_block[i][j]);
		 
			 }
		 }
		}
			
		return sb.toString();
	}