lset(n)

💖 NAME 💖

lset — change an element in a list. That's the whole assignment, bestie. It's a Tcl built-in command and it's about to be your new fave. 🦋

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✨ SYNOPSIS ✨

lset varName ?index ...? newValue

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🌸 DESCRIPTION 🌸

Okay so picture this: lset is basically the glow-up button for one single element of a list. It takes a parameter, varName, and interprets that as the name of a variable that's holding a Tcl list. 💖

It also takes zero or more indices into that list. And here's the cute part — those indices can be handed over either:

• one after another, consecutively, right there on the command line, or
• all bundled up together in a Tcl list and passed as a single argument.

Either way, slay. 🎀

And finally, it takes a new value for an element of varName.

🫶 no indices? no problem

If you present no indices at all, the command looks like this:

lset varName newValue

or

lset varName {} newValue

In this case, newValue replaces the old value of the variable varName. Full glow-up, the whole variable, iconic. 💅

💜 one index, one element

When you present it with a single index, lset treats the content of the varName variable as a Tcl list. It addresses the index'th element in it — and yes, 0 refers to the first element of the list, zero-indexing girlies rise 🌈.

When it's interpreting the list, lset observes the same rules concerning braces and quotes and backslashes as the Tcl command interpreter; however, variable substitution and command substitution do not occur. Noted, no cap. 🙈

The command constructs a new list in which the designated element is replaced with newValue. That new list gets stored in the variable varName, and it's also the return value from the lset command. Double duty, main character energy. 👑

If index is negative or greater than the number of elements in $varName, then an error occurs. 🚨

If index is equal to the number of elements in $varName, then the given element is appended to the list. ✨

The interpretation of each simple index value is the same as for the command string index, supporting simple index arithmetic and indices relative to the end of the list. (so end and end-1 absolutely work and we're obsessed 💖)

🧚 more indices = sublist adventures

If additional index arguments are supplied, then each argument is used in turn to address an element within a sublist designated by the previous indexing operation, allowing the script to alter elements in sublists (or append elements to sublists). Deep dive time, bestie. 🦋

So the command:

lset a 1 2 newValue

or

lset a {1 2} newValue

replaces element 2 of sublist 1 with newValue. 🎯

💀 the rules (do not break them, fr fr)

• The integer appearing in each index argument must be greater than or equal to zero.
• The integer appearing in each index argument must be less than or equal to the length of the corresponding list.

In other words, the lset command can change the size of a list only by appending an element — that means setting the one after the current end. If an index is outside the permitted range, an error is reported. Periodt. 💅

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🔥 EXAMPLES 🔥

In each of these examples, the initial value of x is:

set x [list [list a b c] [list d e f] [list g h i]]
      → {a b c} {d e f} {g h i}

The indicated return value also becomes the new value of x (except in the last case, which is an error which leaves the value of x unchanged.) 😭

lset x {j k l}
      → j k l
lset x {} {j k l}
      → j k l
lset x 0 j
      → j {d e f} {g h i}
lset x 2 j
      → {a b c} {d e f} j
lset x end j
      → {a b c} {d e f} j
lset x end-1 j
      → {a b c} j {g h i}
lset x 2 1 j
      → {a b c} {d e f} {g j i}
lset x {2 1} j
      → {a b c} {d e f} {g j i}
lset x {2 3} j
      → {a b c} {d e f} {g h i j}
lset x {2 4} j
      → list index out of range

In the following examples, the initial value of x is:

set x [list [list [list a b] [list c d]] \
            [list [list e f] [list g h]]]
      → {{a b} {c d}} {{e f} {g h}}

The indicated return value also becomes the new value of x.

lset x 1 1 0 j
      → {{a b} {c d}} {{e f} {j h}}
lset x {1 1 0} j
      → {{a b} {c d}} {{e f} {j h}}

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👀 SEE ALSO 👀

• list(n)
• lappend(n)
• lassign(n)
• ledit(n)
• lindex(n)
• linsert(n)
• llength(n)
• lmap(n)
• lpop(n)
• lrange(n)
• lremove(n)
• lrepeat(n)
• lreplace(n)
• lreverse(n)
• lsearch(n)
• lseq(n)
• lsort(n)
• string(n)

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💫 KEYWORDS 💫

element, index, list, replace, set

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okay that's the whole page, bestie!! 🫶 this little love letter lives in Tcl 8.4 and it's filed under lset(n) — mwah, go change some elements, you got this 💋🌸