hi everyone

I have a PLINK mapfile whit for columns including chromosome name, snp id, Genetic distance (morgans) (All column is zero), Base-pair position.

NC1.1 rs1 0 145
NC1.1 rs2 0 201
NC2.1 rs3 0 208
.
.
NCn.1 rsn 0 509

I want to split file by chromosome so after this, I will have files like

NC1.1 rs1 0 145
NC1.1 rs2 0 201

and

NC2.1 rs3 0 208
.
.

...

anybody can help me to do that?