path: root/doc/dict.n

'\"
'\" Copyright (c) 2003 Donal K. Fellows
'\"
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.TH dict n 8.5 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
dict \- Manipulate dictionaries
.SH SYNOPSIS
\fBdict \fIoption arg \fR?\fIarg ...\fR?
.BE
.SH DESCRIPTION
.PP
Performs one of several operations on dictionary values or variables
containing dictionary values (see the \fBDICTIONARY VALUES\fR section
below for a description), depending on \fIoption\fR.  The legal
\fIoption\fRs (which may be abbreviated) are:
.TP
\fBdict append \fIdictionaryVariable key \fR?\fIstring ...\fR?
This appends the given string (or strings) to the value that the given
key maps to in the dictionary value contained in the given variable,
writing the resulting dictionary value back to that variable.
Non-existent keys are treated as if they map to an empty string.
.TP
\fBdict create \fR?\fIkey value ...\fR?
Create a new dictionary that contains each of the key/value mappings
listed as arguments (keys and values alternating, with each key being
followed by its associated value.)
.TP
\fBdict exists \fIdictionaryValue key \fR?\fIkey ...\fR?
This returns a boolean value indicating whether the given key (or path
of keys through a set of nested dictionaries) exists in the given
dictionary value. This returns a true value exactly when \fBdict
get\fR on that path will succeed.
.TP
\fBdict filter \fIdictionaryValue filterType arg \fR?\fIarg ...\fR?
This takes a dictionary value and returns a new dictionary that
contains just those key/value pairs that match the specified filter
type (which may be abbreviated.)  Supported filter types are:
.RS
.TP
\fBdict filter \fIdictionaryValue \fBkey \fIglobPattern\fR
The key rule only matches those key/value pairs whose keys match the
given pattern (in the style of \fBstring match\fR.)
.TP
\fBdict filter \fIdictionaryValue \fBscript {\fIkeyVar valueVar\fB} \fIscript\fR
The script rule tests for matching by assigning the key to the
\fIkeyVar\fR and the value to the \fIvalueVar\fR, and then evaluating
the given script which should return a boolean value (with the
key/value pair only being included in the result of the \fBdict
filter\fR when a true value is returned.)  Note that the first
argument after the rule selection word is a two-element list.  If the
\fIscript\fR returns with a condition of \fBTCL_BREAK\fR, no further
key/value pairs are considered for inclusion in the resulting
dictionary, and a condition of \fBTCL_CONTINUE\fR is equivalent to a false
result. The key/value pairs are tested in the order in which the keys
were inserted into the dictionary.
.TP
\fBdict filter \fIdictionaryValue \fBvalue \fIglobPattern\fR
The value rule only matches those key/value pairs whose values match
the given pattern (in the style of \fBstring match\fR.)
.RE
.TP
\fBdict for {\fIkeyVar valueVar\fB} \fIdictionaryValue body\fR
This command takes three arguments, the first a two-element list of
variable names (for the key and value respectively of each mapping in
the dictionary), the second the dictionary value to iterate across,
and the third a script to be evaluated for each mapping with the key
and value variables set appropriately (in the manner of \fBforeach\fR.)
The result of the command is an empty string. If any evaluation of the
body generates a \fBTCL_BREAK\fR result, no further pairs from the
dictionary will be iterated over and the \fBdict for\fR command will
terminate successfully immediately. If any evaluation of the body
generates a \fBTCL_CONTINUE\fR result, this shall be treated exactly like a
normal \fBTCL_OK\fR result. The order of iteration is the order in
which the keys were inserted into the dictionary.
.TP
\fBdict get \fIdictionaryValue \fR?\fIkey ...\fR?
Given a dictionary value (first argument) and a key (second argument),
this will retrieve the value for that key. Where several keys are
supplied, the behaviour of the command shall be as if the result of
\fBdict get $dictVal $key\fR was passed as the first argument to
\fBdict get\fR with the remaining arguments as second (and possibly
subsequent) arguments. This facilitates lookups in nested
dictionaries. For example, the following two commands are equivalent:
.RS
.PP
.CS
dict get $dict foo bar spong
dict get [dict get [dict get $dict foo] bar] spong
.CE
.PP
If no keys are provided, \fBdict get\fR will return a list containing pairs of
elements in a manner similar to \fBarray get\fR. That is, the first
element of each pair would be the key and the second element would be
the value for that key.
.PP
It is an error to attempt to retrieve a value for a key that is not
present in the dictionary.
.RE
.TP
\fBdict incr \fIdictionaryVariable key \fR?\fIincrement\fR?
This adds the given increment value (an integer that defaults to 1 if
not specified) to the value that the given key maps to in the
dictionary value contained in the given variable, writing the
resulting dictionary value back to that variable. Non-existent keys
are treated as if they map to 0. It is an error to increment a value
for an existing key if that value is not an integer.
.TP
\fBdict info \fIdictionaryValue\fR
This returns information (intended for display to people) about the
given dictionary though the format of this data is dependent on the
implementation of the dictionary. For dictionaries that are
implemented by hash tables, it is expected that this will return the
string produced by \fBTcl_HashStats\fR, similar to \fBarray statistics\fR.
.TP
\fBdict keys \fIdictionaryValue \fR?\fIglobPattern\fR?
Return a list of all keys in the given dictionary value. If a pattern
is supplied, only those keys that match it (according to the rules of
\fBstring match\fR) will be returned. The returned keys will be in the
order that they were inserted into the dictionary.
.TP
\fBdict lappend \fIdictionaryVariable key \fR?\fIvalue ...\fR?
This appends the given items to the list value that the given key maps
to in the dictionary value contained in the given variable, writing
the resulting dictionary value back to that variable. Non-existent
keys are treated as if they map to an empty list, and it is legal for
there to be no items to append to the list. It is an error for the
value that the key maps to to not be representable as a list.
.TP
\fBdict merge \fR?\fIdictionaryValue ...\fR?
Return a dictionary that contains the contents of each of the
\fIdictionaryValue\fR arguments.  Where two (or more) dictionaries
contain a mapping for the same key, the resulting dictionary maps that
key to the value according to the last dictionary on the command line
containing a mapping for that key.
.TP
\fBdict remove \fIdictionaryValue \fR?\fIkey ...\fR?
Return a new dictionary that is a copy of an old one passed in as
first argument except without mappings for each of the keys listed.
It is legal for there to be no keys to remove, and it also legal for
any of the keys to be removed to not be present in the input
dictionary in the first place.
.TP
\fBdict replace \fIdictionaryValue \fR?\fIkey value ...\fR?
Return a new dictionary that is a copy of an old one passed in as
first argument except with some values different or some extra
key/value pairs added. It is legal for this command to be called with
no key/value pairs, but illegal for this command to be called with a
key but no value.
.TP
\fBdict set \fIdictionaryVariable key \fR?\fIkey ...\fR? \fIvalue\fR
This operation takes the name of a variable containing a dictionary
value and places an updated dictionary value in that variable
containing a mapping from the given key to the given value. When
multiple keys are present, this operation creates or updates a chain
of nested dictionaries.
.TP
\fBdict size \fIdictionaryValue\fR
Return the number of key/value mappings in the given dictionary value.
.TP
\fBdict unset \fIdictionaryVariable key \fR?\fIkey ...\fR?
This operation (the companion to \fBdict set\fR) takes the name of a
variable containing a dictionary value and places an updated
dictionary value in that variable that does not contain a mapping for
the given key. Where multiple keys are present, this describes a path
through nested dictionaries to the mapping to remove. At least one key
must be specified, but the last key on the key-path need not exist.
All other components on the path must exist.
.TP
\fBdict update \fIdictionaryVariable key varName \fR?\fIkey varName ...\fR? \fIbody\fR
Execute the Tcl script in \fIbody\fR with the value for each \fIkey\fR
(as found by reading the dictionary value in \fIdictionaryVariable\fR)
mapped to the variable \fIvarName\fR. There may be multiple
\fIkey\fR/\fIvarName\fR pairs. If a \fIkey\fR does not have a mapping,
that corresponds to an unset \fIvarName\fR. When \fIbody\fR
terminates, any changes made to the \fIvarName\fRs is reflected back
to the dictionary within \fIdictionaryVariable\fR (unless
\fIdictionaryVariable\fR itself becomes unreadable, when all updates
are silently discarded), even if the result of \fIbody\fR is an error
or some other kind of exceptional exit. The result of \fBdict
update\fR is (unless some kind of error occurs) the result of the
evaluation of \fIbody\fR.
.RS
.PP
Each \fIvarName\fR is mapped in the scope enclosing the \fBdict update\fR;
it is recommended that this command only be used in a local scope
(\fBproc\fRedure or lambda term for \fBapply\fR). Because of
this, the variables set by \fBdict update\fR will continue to
exist after the command finishes (unless explicitly \fBunset\fR).
Note that the mapping of values to variables
does not use traces; changes to the \fIdictionaryVariable\fR's
contents only happen when \fIbody\fR terminates.
.RE
.TP
\fBdict values \fIdictionaryValue \fR?\fIglobPattern\fR?
Return a list of all values in the given dictionary value. If a
pattern is supplied, only those values that match it (according to the
rules of \fBstring match\fR) will be returned. The returned values
will be in the order of that the keys associated with those values
were inserted into the dictionary.
.TP
\fBdict with \fIdictionaryVariable \fR?\fIkey ...\fR? \fIbody\fR
Execute the Tcl script in \fIbody\fR with the value for each key in
\fIdictionaryVariable\fR mapped (in a manner similarly to \fBdict
update\fR) to a variable with the same name. Where one or more
\fIkey\fRs are available, these indicate a chain of nested
dictionaries, with the innermost dictionary being the one opened out
for the execution of \fIbody\fR. As with \fBdict update\fR, making
\fIdictionaryVariable\fR unreadable will make the updates to the
dictionary be discarded, and this also happens if the contents of
\fIdictionaryVariable\fR are adjusted so that the chain of
dictionaries no longer exists. The result of \fBdict with\fR is
(unless some kind of error occurs) the result of the evaluation of
\fIbody\fR.
.RS
.PP
The variables are mapped in the scope enclosing the \fBdict with\fR;
it is recommended that this command only be used in a local scope
(\fBproc\fRedure or lambda term for \fBapply\fR). Because of
this, the variables set by \fBdict with\fR will continue to
exist after the command finishes (unless explicitly \fBunset\fR).
Note that the mapping of values to variables does not use
traces; changes to the \fIdictionaryVariable\fR's contents only happen
when \fIbody\fR terminates.
.PP
If the \fIdictionaryVariable\fR contains a value that is not a dictionary at
the point when the \fIbody\fR terminates (which can easily happen if the name
is the same as any of the keys in dictionary) then an error occurs at that
point. This command is thus not recommended for use when the keys in the
dictionary are expected to clash with the \fIdictionaryVariable\fR name
itself. Where the contained key does map to a dictionary, the net effect is to
combine that inner dictionary into the outer dictionary; see the
\fBEXAMPLES\fR below for an illustration of this.
.RE
.SH "DICTIONARY VALUES"
.PP
Dictionaries are values that contain an efficient, order-preserving
mapping from arbitrary keys to arbitrary values.
Each key in the dictionary maps to a single value.
They have a textual format that is exactly that of any list with an
even number of elements, with each mapping in the dictionary being
represented as two items in the list. When a command takes a
dictionary and produces a new dictionary based on it (either returning
it or writing it back into the variable that the starting dictionary
was read from) the new dictionary will have the same order of keys,
modulo any deleted keys and with new keys added on to the end.
When a string is interpreted as a dictionary and it would otherwise
have duplicate keys, only the last value for a particular key is used;
the others are ignored, meaning that,
.QW "apple banana"
and
.QW "apple carrot apple banana"
are equivalent dictionaries (with different string representations).
.PP
Operations that derive a new dictionary from an old one (e.g., updates
like \fBdict set\fR and \fBdict unset\fR) preserve the order of keys
in the dictionary. The exceptions to this are for any new keys they
add, which are appended to the sequence, and any keys that are
removed, which are excised from the order.
.SH EXAMPLES
.PP
Basic dictionary usage:
.PP
.CS
# Make a dictionary to map extensions to descriptions
set filetypes [\fBdict create\fR .txt "Text File" .tcl "Tcl File"]

# Add/update the dictionary
\fBdict set\fR filetypes .tcl "Tcl Script"
\fBdict set\fR filetypes .tm  "Tcl Module"
\fBdict set\fR filetypes .gif "GIF Image"
\fBdict set\fR filetypes .png "PNG Image"

# Simple read from the dictionary
set ext ".tcl"
set desc [\fBdict get\fR $filetypes $ext]
puts "$ext is for a $desc"

# Somewhat more complex, with existence test
foreach filename [glob *] {
    set ext [file extension $filename]
    if {[\fBdict exists\fR $filetypes $ext]} {
        puts "$filename is a [\fBdict get\fR $filetypes $ext]"
    }
}
.CE
.PP
Constructing and using nested dictionaries:
.PP
.CS
# Data for one employee
\fBdict set\fR employeeInfo 12345-A forenames "Joe"
\fBdict set\fR employeeInfo 12345-A surname   "Schmoe"
\fBdict set\fR employeeInfo 12345-A street "147 Short Street"
\fBdict set\fR employeeInfo 12345-A city   "Springfield"
\fBdict set\fR employeeInfo 12345-A phone  "555-1234"
# Data for another employee
\fBdict set\fR employeeInfo 98372-J forenames "Anne"
\fBdict set\fR employeeInfo 98372-J surname   "Other"
\fBdict set\fR employeeInfo 98372-J street "32995 Oakdale Way"
\fBdict set\fR employeeInfo 98372-J city   "Springfield"
\fBdict set\fR employeeInfo 98372-J phone  "555-8765"
# The above data probably ought to come from a database...

# Print out some employee info
set i 0
puts "There are [\fBdict size\fR $employeeInfo] employees"
\fBdict for\fR {id info} $employeeInfo {
   puts "Employee #[incr i]: $id"
   \fBdict with\fR info {
      puts "   Name: $forenames $surname"
      puts "   Address: $street, $city"
      puts "   Telephone: $phone"
   }
}
# Another way to iterate and pick out names...
foreach id [\fBdict keys\fR $employeeInfo] {
   puts "Hello, [\fBdict get\fR $employeeInfo $id forenames]!"
}
.CE
.PP
A localizable version of \fBstring toupper\fR:
.PP
.CS
# Set up the basic C locale
set capital [\fBdict create\fR C [\fBdict create\fR]]
foreach c [split {abcdefghijklmnopqrstuvwxyz} ""] {
   \fBdict set\fR capital C $c [string toupper $c]
}

# English locales can luckily share the "C" locale
\fBdict set\fR capital en [\fBdict get\fR $capital C]
\fBdict set\fR capital en_US [\fBdict get\fR $capital C]
\fBdict set\fR capital en_GB [\fBdict get\fR $capital C]

# ... and so on for other supported languages ...

# Now get the mapping for the current locale and use it.
set upperCaseMap [\fBdict get\fR $capital $env(LANG)]
set upperCase [string map $upperCaseMap $string]
.CE
.PP
Showing the detail of \fBdict with\fR:
.PP
.CS
proc sumDictionary {varName} {
   upvar 1 $varName vbl
   foreach key [\fBdict keys\fR $vbl] {
      # Manufacture an entry in the subdictionary
      \fBdict set\fR vbl $key total 0
      # Add the values and remove the old
      \fBdict with\fR vbl $key {
         set total [expr {$x + $y + $z}]
         unset x y z
      }
   }
   puts "last total was $total, for key $key"
}

set myDict {
   a {x 1 y 2 z 3}
   b {x 6 y 5 z 4}
}

sumDictionary myDict
#    prints: \fIlast total was 15, for key b\fR

puts "dictionary is now \\"$myDict\\""
#    prints: \fIdictionary is now "a {total 6} b {total 15}"\fR
.CE
.PP
When \fBdict with\fR is used with a key that clashes with the name of the
dictionary variable:
.PP
.CS
set foo {foo {a b} bar 2 baz 3}
\fBdict with\fR foo {}
puts $foo
#    prints: \fIa b foo {a b} bar 2 baz 3\fR
.CE
.SH "SEE ALSO"
append(n), array(n), foreach(n), incr(n), list(n), lappend(n), set(n)
.SH KEYWORDS
dictionary, create, update, lookup, iterate, filter