matlab中princomp和pcacov的区别在哪？

SVD (used in PRINCOMP) and EIG (used in PCACOV) just happen to pick opposite
signs for the singular/eigen vectors in this case. It's actually a little
surprising that they pick opposite sign for all of the PCs, but there you go.
BIPLOT enforces a sign convention (lines 99-104 of the current version), but
that's just to make a nicer-looking picture. In real applications that I'm
familiar with, the sign of the PC loadings shouldn't matter.