Question

I downloaded Mnist dataset from Libsvm’s dataset page.
All samples are like the following:

5 153:3 154:18 155:18 156:18 157:126 ...

Does anyone knows what that means? 5 is the class label, but what is 153:3 pair for example? Also I couldn’t find the meaning from mnist’s own web page.

Answer 1

This is the way libsvm encodes (sparse) vectors. As you said 5 is the label, and the following pairs i:v say that the i-th entry of the vector is v. So you would encode a 3-dim vector (a,b,c) as

1:a 2:b 3:c

Which is inefficient for dense vectors but a good and established format for sparse data. As it is plain text, the storage space is not optimal, but good enough for most applications. Whereas the files are easy to write and to read.