LabeledSymbol - GTSAM Docs

A LabeledSymbol is a specialized version of gtsam.Symbol designed primarily for multi-robot applications or scenarios where an additional label is needed besides the type character and index. It encodes a type character (unsigned char), a label character (unsigned char), and an index (uint64_t) into a single 64-bit gtsam.Key.

import gtsam
from gtsam import LabeledSymbol

Initialization¶

A LabeledSymbol can be created by providing a type character, a label character, and an index. It can also be created by decoding an existing gtsam.Key (integer).

# Create LabeledSymbol 'x' from robot 'A' with index 7
# The underlying C++ expects chars, so we have to convert single-character strings with ord()
lsym1 = LabeledSymbol(ord('x'), ord('A'), 7)
print(f"LabeledSymbol from char/label/index: {lsym1}")

# Get the underlying integer key
key1 = lsym1.key()

# Reconstruct LabeledSymbol from the key
# Note: Decoding a key assumes it was encoded as a LabeledSymbol.
# If you decode a standard Symbol key, the label might be garbage.
x0_key = gtsam.Symbol('x', 0).key()
lsym2 = LabeledSymbol(x0_key)
print(f"LabeledSymbol from key {x0_key}: {lsym2}")

LabeledSymbol from char/label/index: : xA7

LabeledSymbol from key 8646911284551352320: : x

Properties and Usage¶

You can access the type character, label character, index, and underlying integer key.

robotB_landmark = LabeledSymbol(ord('l'), ord('B'), 3)

print(f"LabeledSymbol: {robotB_landmark}")
print(f"  Char (Type): {robotB_landmark.chr()}")
print(f"  Label (Robot): {robotB_landmark.label()}")
print(f"  Index: {robotB_landmark.index()}")
print(f"  Key: {robotB_landmark.key()}")

LabeledSymbol: : lB3

  Char (Type): 108
  Label (Robot): 66
  Index: 3
  Key: 7800797504559120387

Shorthand Function¶

GTSAM provides a convenient shorthand function gtsam.mrsymbol(c, label, j).

pc2_key = gtsam.mrsymbol(ord('p'), ord('C'), 2)

print(f"LabeledSymbol(ord('p'), ord('C'), 2).key() == gtsam.mrsymbol(ord('p'), ord('C'), 2): {LabeledSymbol(ord('p'), ord('C'), 2).key() == pc2_key}")

LabeledSymbol(ord('p'), ord('C'), 2).key() == gtsam.mrsymbol(ord('p'), ord('C'), 2): True