lappend

lappend is a built-in command in the Tcl programming language used to append one or more values to the end of a list stored in a variable or an array element. If the destination does not yet exist, lappend creates it as a list containing the provided values. The operation preserves list structure and appends the new items in the given order.

Syntax and behavior

The command takes a destination name and one or more values: lappend destination value ?value ...?. The destination can be a regular variable or an array element, such as arr(index). If the destination already holds a non-list value, that value is treated as a single list element and the new values are appended, effectively converting the destination to a longer list. If destination refers to an array element that does not exist, it starts as a single-element list containing the appended values.

Examples

set x 1; lappend x 2; # x becomes "1 2"

lappend x 3 4; # x becomes "1 2 3 4"

lappend arr(i) a; # arr(i) becomes "a" (or "existing a" if arr(i) already had content)

lappend arr(i) b c; # arr(i) becomes "a b c" (or with existing contents)

Notes

lappend is often used to build or modify lists incrementally while ensuring the result remains a valid Tcl list. It interacts with other Tcl list commands (such as llength, lindex, and join) to manage list data.

See also

Tcl list operations, llength, lindex, join, split.