10.1. Getting Started

This tutorial expects that you already have Larch installed and can run either the program larch, basic Larch command-line shell program or ‘Larch GUI`, the enhanced GUI interpreter:

C:> larch
Larch 0.9.46 (2019-Sep-12) M. Newville, M. Koker, B. Ravel, and others
Python 3.7.0 (default, Jun 28 2018, 07:39:16)
numpy 1.16.4, scipy 1.3.0, matplotlib 3.1.0, lmfit 0.9.14, wx 4.0.6

As of this writing, the Larch GUI is little more than an enhanced command-line shell, though it does include a “Data Browser” menu that allows you to view a tree-like display of all the data in a Larch session, and to show help on Larch functions.

10.1.1. Larch as a Basic Calculator

To start with, Larch can be used as a simple calculator:

larch> 1 + 2
larch> sqrt(4.e5)
larch> sin(pi/3)

You can create your own variables holding values, by assigning names to values, and then use these in calculations:

larch> hc = 12398.419
larch> d = 3.13556
larch> energy = (hc/(2*d)) / sin(10.0*pi/180)
larch> print(energy)
larch> angle = asin(hc/(10000*2*d))*180/pi
larch> print(angle)

Note that parentheses are used to group multiplication and division, and also to hold the arguments to functions like sin().

Variable names must start with a letter or underscore (_), followed by any number of letters, underscores, or numbers. You may notice that a dot (.) may appear to be in many variable names. This indicates an attribute of a variable – we’ll get to this in a later section.

If you’re familiar with other programming langauges, an important point for Larch (owing to its Python origins) is that variables are created dynamically, they are not pre-defined to have some particular data type. In fact, the a variable name (say, ‘angle’ above) can hold any type of data, and its type can be changed easily:

larch> angle = 'now I am a string'

Although the types of values for a variable can be changed dynamically, values in Larch (Python) do have a definite and clear type, and conversion between types is rigidly defined – you can add an integer and a real number to give a real number, but you cannot add a string and a real number. In fact, writing:

larch> angle = asin(hc/(10000*2*d))*180/pi

is usually described as “create a variable ‘angle’ and set its value to the result calculated (11.4…)”. For those used to working in C or Fortran, in which variables are static and must be of a pre-declared type, this description is a bit misleading. A better way to think about it is that the calculation on the right-hand-side of the equal sign results in a value (11.4…) and we’ve assigned the name ‘angle’ to hold this value. The distinction may seem subtle, but it can have some profound results, as we’ll see in the following section when discussing lists and other dynamic values.