A recent query on comp.lang.tcl asked how one might find the sum of two binary numbers, without converting them to decimal returning the result as binary.

[Glen Jackson] wrote the following code:

======
if {[string match 8.4* [package require Tcl]]} { 
    package require struct::list 
    interp alias {} list_reverse {} struct::list reverse 
} else { 
    interp alias {} list_reverse {} lreverse 
} 


proc add_binary_strings {n1 n2} { 
    set answer [list] 
    set carry 0 
    foreach d1 [list_reverse [split $n1 ""]] \ 
            d2 [list_reverse [split $n2 ""]] \ 
    { 
        if {$d1 eq ""} {set d1 0} 
        if {$d2 eq ""} {set d2 0} 
        switch -exact [set sum [expr {$d1 + $d2 + $carry}]] { 
            0 - 
            1 { set carry 0 } 
            2 { set sum 0; set carry 1 } 
            3 { set sum 1; set carry 1 } 
        } 
        lappend answer $sum 
    } 
    lappend answer $carry 
    return [string trimleft [join [list_reverse $answer] ""] 0] 
} 


set n1 100010111 
set n2   1011011 


puts "$n1 + $n2 = [add_binary_strings $n1 $n2]" 
======

[Lars H]: Ah, but it's slightly neater to do it without [expr]:

======
proc add_binary_strings {n1 n2} { 
    set sum {}
    set carry 0 
    foreach d1 [list_reverse [split $n1 ""]] d2 [list_reverse [split $n2 ""]] { 
        switch -- [string map {0 ""} "$d1$d2$carry"] { 
            ""  { lappend sum 0; set carry 0}
            1   { lappend sum 1; set carry 0}
            11  { lappend sum 0; set carry 1}
            111 { lappend sum 1; set carry 1}
        } 
    } 
    lappend sum $carry 
    return [string trimleft [join [list_reverse $sum] ""] 0] 
} 
======

[vh] 17 Apr 2017 - How can I manipulate the output so that I can use the [format %d $output] command to return the decimal value?  This command assumes that the output string is already a decimal value. I was trying to use this function to help me with the +1 part of the 2's complement calculation.  Am I doing to much work?  Is there another, easier way? (Ok, that was probably a dumb question - I've added a bin2dec function to by library...)

** See Also **

   [Binary representation of numbers]:   

<<categories>> Mathematics | Binary Data