Matlab 之 cannot be saved to a MAT-file whose version is older than 7.3

save mat文件的时候出现以下错误：

matlab错误提示：

Warning: Variable 'a' cannot be saved to a MAT-file whose version
is older than 7.3.
To save this variable, use the -v7.3 switch.

Skipping...

解决办法：

1、打开matlab，file/preferences，general选项卡-MAT-Files,选择第一个。

2、save -v7.3 TrainData.mat trainPatches