c++ - Exporting a map with changing number of elements to CSV -

i need export data 3 maps preferably single csv , able without making column every possible key (there may 65024 of them).

the output csv containing value @ each of keys @ each timestep (may several hundred thousand).

anyone got ideas?

reduce granularity categorizing keys groups , store them 1 timestep per row. can plot 1 datapoint per line.

let me know if need clarification, i'd need more info.