Assigning distinct values to characters

1190 2007   May  9415
1191 2007   Aug 19065
1192 2016   Dec 19825
1193 2008   Nov 12805
1194 2009   Feb 20290
1195 2010   Mar 30140
1196 2016   Jul 14825
1197 2017   Sep 22520
1198 2018   Mar 26140

If i want to use a function so that i could assign a unique value to each month which one should I use ? And can I use a if else fucntion to assign?