Hi there! I was just trying to compare two strings with expr, but [expr $s1==$s2] won't even create. Why? I'm familiar with using floats on expr, but not symbols

[expr] can't compare symbols. Use [select symbol] for that.

thanks, I still wonder in which context I could use symbols and expr, I guess there must be a full load of useful uses

I believe they are used to refer to a table, like [expr size("$s1")], and to refer to variables named in [v foo], as in [expr foo%3]

See 5.reference/all_about_expr_functions.pd for an exhaustive list.