fix bug

        cur_expr <- expr_mat[,seurat_obj@meta.data[[group.by]] == cur_group]

        print(dim(cur_expr))

        gene_filter <- rowSums(as.matrix(cur_expr)) >= round(fraction*ncol(cur_expr));

        gene_filter <- Matrix::rowSums(cur_expr) >= round(fraction*ncol(cur_expr));

        group_gene_list[[cur_group]] <- rownames(seurat_obj)[gene_filter]

      }

      gene_list <- unique(unlist(group_gene_list))

    } else{

      # identify genes that are expressed in at least some fraction of cells

      gene_filter <- rowSums(expr_mat) >= round(fraction*ncol(seurat_obj));

      gene_filter <- Matrix::rowSums(expr_mat) >= round(fraction*ncol(seurat_obj));

      gene_list <- rownames(seurat_obj)[gene_filter]

    }