public String getNanCols(){
		String ret;
		StringBuilder sb = new StringBuilder();
		int cnt = 0;
		int i,j,para;
		ret = "|----------------All NaN Features-----------------|\n";
		for(i=0;i<max_y;i++){
			para = 0;
			for(j=0;j<max_x;j++){
				if(ParaArray.isNaN(a[i][j]))
					para++;
			}
			if(para == max_x){
				sb.append(String.format("|%7d",i+1));
				if(cnt ++ %8 ==7){
					sb.append("|\n");
				}
			}
		}
		i=cnt%8;
		if(i>0){
			while(i<8){
				sb.append("|          ");
				if(i++ %8 == 7)	sb.append("\n");
			}
		}
		if(sb.length()>0)
			ret += sb.toString();
		else ret = "";
				
		return ret;
	}