[R] simple question: reading lower triangular matrix

[Rogério Rosa da Silva]

Dear list,

I will like to learn how to read a lower triangular matrix in R. The
input file *.txt have the following format:

     A   B   C   D   E
A   0   
B   1    0
C   2    5    0   
D   3    6    8    0  
E   4    7    9   10    0


How this can be done?

Thanks in advance for your help

Rogério