Substitution in an ‘if’ statement in tcl

I have 2 expressions, one that works and one that doesn’t. Any idea why the second test doesn’t work? The Day variable represents Sun Mon Tue…. I loop through the hours for each day Sun(0) Sun(1)….Sat(23). The if statement determine the beginning or end of a gap.

This works:

if {[set ${Day}($i)] != 0 && [expr [set ${Day}([expr $i+1])] + [set ${Day}([expr $i+2])] + [set ${Day}([expr $i+3])]] == 0} {

This doesn’t

if {$i > 2 && [set ${Day}($i)] != 0 && [expr [set ${Day}([expr $i-1])] + [set ${Day}([expr $i-2])] + [set ${Day}([expr $i-3])]] == 0} {