Converting small dataset to big/main dataset with for loop

Hi guys, I want to convert data (dates and returns) to my big/main dataset (dataset_qmj). The small data are annualy, the big ones monthly. Further, the company codes have to match. Therefore I constructed such a codes as follows, but it takes so much time... Thank you so for your help and merry christmas!!! datadate_qmj_month=month(datadate_main); datadate_qmj_year=year(datadate_main); datadate_z_quality_month=month(datadate_small); datadate_z_quality_year=year(datadate_small); z_quality_adj=NaN(size(datadate_main,1),1); for row=1:size(datadate_main,1) % convert z_quality to big dataset index_datadate_year=datadate_qmj_year(row)==datadate_z_quality_year; index_datadate_month= datadate_qmj_month(row)==datadate_z_quality_month; index_datadate_adj=index_datadate_year & index_datadate_month; index_LPERMNO=LPERMNO_main(row)==LPERMNO; index_z_quality=index_datadate_adj & index_LPERMNO; if sum(index_z_quality)==1 z_quality_adj(row)=z_quality(index_z_quality); end end