Simple Example

This page provides a quick example of how to use Formulate to convert between different expression formats.

Basic Usage

The most basic usage involves calling from_$BACKEND and then to_$BACKEND, where $BACKEND is the format you’re converting from or to.

Converting from ROOT to numexpr

Here’s an example of converting a ROOT expression to numexpr:

import formulate

# TODO: this fails?
# Create an expression object from a ROOT expression
momentum = formulate.from_root('TMath::Sqrt(X_PX**2 + X_PY**2 + X_PZ**2)')

# Convert to numexpr format
numexpr_expression = momentum.to_numexpr()
print(numexpr_expression)
# Output: 'sqrt(((X_PX ** 2) + (X_PY ** 2) + (X_PZ ** 2)))'

# You can also convert back to ROOT format
root_expression = momentum.to_root()
print(root_expression)
# Output: 'TMath::Sqrt(((X_PX ** 2) + (X_PY ** 2) + (X_PZ ** 2)))'

Converting from numexpr to ROOT

Similarly, you can convert from numexpr to ROOT:

# TODO: this fails?
# Create an expression object from a numexpr expression
selection = formulate.from_numexpr('X_PT > 5 & (Mu_NHits > 3 | Mu_PT > 10)')

# Convert to ROOT format
root_expression = selection.to_root()
print(root_expression)
# Output: '(X_PT > 5) && ((Mu_NHits > 3) || (Mu_PT > 10))'

# You can also convert back to numexpr format
numexpr_expression = selection.to_numexpr()
print(numexpr_expression)
# Output: '(X_PT > 5) & ((Mu_NHits > 3) | (Mu_PT > 10))'

Using the Converted Expressions

Once you have converted an expression, you can use it with the appropriate backend:

With numexpr:

import numpy as np
import numexpr as ne

# Create some sample data
data = {
    'X_PT': np.array([3, 6, 9, 12]),
    'Mu_NHits': np.array([2, 4, 1, 5]),
    'Mu_PT': np.array([8, 5, 12, 7])
}

# Use the converted numexpr expression
selection = formulate.from_numexpr('X_PT > 5')  # TODO: remove, take from above
result = ne.evaluate(selection.to_numexpr(), local_dict=data)
print(result)
# Output: [False  True  True  True]

[False  True  True  True]

With ROOT (pseudo-code, as actual implementation depends on your ROOT setup):

# Assuming you have a ROOT TTree with branches X_PT, Mu_NHits, and Mu_PT
tree.Draw(">>eventList", selection.to_root())

# Now you can use the eventList to process selected events
# ...