R :: data.table: Generate a running balance by group using previous balance and row-wise iteration

I have following DT (data.table) in R.

dt <- fread("
id| rowids | charge | payment | balance
a |   1    |  7.1   |   0     |     
a |   2    |  1.2   |   3     |   
a |   3    |  1.7   |   1     |   
b |   1    |  8.1   |   0     |   
b |   2    |  2.5   |   4     |   
b |   3    |  2.3   |   2     |   
b |   4    |  3.2   |   1     |   
            ", 
            sep = "|",
            colClasses = c("character", "numeric", "numeric", "numeric", 
"numeric"))

The "balance" is should be computed, within each id group, as "balance <- previous.row.balance + charge - payment", where the "previous.row.balance" is the previous row entry of "balance".

I initially underestimate the difficulty to compute the running balance. I was thinking about dt[,previous.row.balance := (shift(balance,1),by=id]. But R does vectorized computation. I did not have values in "balance" available for me to perform shift() since "balance" will be computed through row-by-row iteration.

I searched on StackOverflow and found a similar question and its first answer greatly helped me to think through the whole process. I adapted the code in the first answer to my problem and got the following code working wonderfully to generate the running balance by group.

dt[rowids == 1, balance := charge, by=.(id)]
dt[rowids != 1, balance :=
    dt[,
        {
            balance1 <- balance[1L]
            .SD[rowids != 1,
                {balance1 <-  balance1 + charge - payment
                    .(balance1)
                },
                by=.(rowids)]
        },
        by=.(id)][, -1L:-2L]
]

Here are my questions.

1. I still cannot understand how by=.(id)][, -1L:-2L], the chained brackets worked the iteration out. Since the code does not employ shift() by = group, I guess [, -1L:-2L] does the trick here to perform the iteration. But how? What does [, -1L:-2L] actually do here?

Sorry that I have to ask this question here, instead of commenting or asking under that question . The reason is that I am brand new to StackOverflow with only 1 point of reputation. I am not allowed to comment on the original answer to that question. I also would like to vote up for that answer. Before I can do that, I have to earn more points.

2. Is there any other way, using data.table and R vectorizing computation to achieve this running balance goal, without wrapping any loop for row iteration?

Any insight or thought is appreciated!