Lists and Indexing

Almost every MTEX variable is a list of objects of one kind. An EBSD variable is a list of measurements, an grain2d variable is a list of grains, and grains.boundary is a list of grain boundary segments. An orientation variable commonly stores a list of orientations.

There is no separate type for one grain. A single grain is an grain2d list with length one.

Two consequences account for most list operations in MTEX.

The diagram compares the two main forms of indexing. Position indices pick specified list entries in the requested order. A logical condition keeps the entries whose mask value is true.

The sections that follow show the indexing itself on plain numbers, for clarity. The same two forms then apply unchanged to any MTEX list.

Make a list

Square brackets create a list by placing values next to one another.

x = [1,2,3,4,5,6]

x =
     1     2     3     4     5     6

The colon operator is shorthand for a regularly spaced range. With no step specified, it uses a step of one.

x = 1:10

x =
     1     2     3     4     5     6     7     8     9    10

Apply an operation to every element

MATLAB applies many arithmetic operations elementwise. The dot in .^ means that every element of x is squared independently, rather than computing a matrix power.

y = x.^2

y =
     1     4     9    16    25    36    49    64    81   100

Select by position

A list of positions selects those elements in the given order. Here, ind selects the first, third, and fifth squared values.

ind = [1,3,5]
y(ind)

ind =
     1     3     5
ans =
     1     9    25

Select by condition

A condition evaluated for the complete list returns one logical value per element. A logical value is either true or false. It can be used as an index to retain only the corresponding true entries.

cond = iseven(x)
y(cond)

cond =
  1×10 logical array
   0   1   0   1   0   1   0   1   0   1
ans =
     4    16    36    64   100

The condition tests which values in x are even. Using the same mask to index y returns their squares. This works because x and y have the same length and order.

Use the same pattern with MTEX objects

Nearly every MTEX selection has the same form. For example, grains(grains.area > 100) selects grains with area greater than 100, ebsd(ebsd.mad < 1) selects measurements with a mean angular deviation below 1, and gB(gB.misorientation.angle > 10*degree) selects grain boundary segments with a misorientation angle greater than 10 degrees.

In each expression, MTEX computes one property value per object. The comparison turns those values into a logical mask, and parentheses apply the mask to the original list.

MTEX also provides two domain-specific selectors. A phase name selects all measurements of that phase, as in ebsd('Forsterite'). For spatial data, a position selects the object found there, as in grains(x,y). See Selecting Grains for spatial grain selections.

References

Next

Continue with Configuration to learn how session preferences control display, computation, and plotting defaults.