algebra: variables

creating

In contrast, you can create and (in most cases) modify Global variables. There are many methods to create them:

• by applying arithmetic to existing variables;
• by using the Global command to create variables of the types Logical; Matrix, Number, String,
• by using the Vector command or functions like ROW, ONES and ZEROES to create a 1-dimensional Matrix or the BOOK_MATRIX procedure to create Matrices of any shape;
• by using the concatenation operator & to create a Matrix from one or more Numbers;
• by calling BOOK_HISTOGRAM to make an Histogram;
• by invoking commands that store part of their output in globals, e.g. the KEEP-RESULTS option of the ARRIVAL command.

Also some global variables are defined by the program:

• some globals are predefined;
• loop variables of Do loops and the arguments of Call statements are automatically declared as globals.

naming

• length up to 10 characters;
• first character is upper case alphabetic;
• none of the following operators:
  • numeric operators: + - * /
  • logical operators: & |
  • negation: ^
  • comparison operators: < = # >
• no punctuation: , . : ;
• no brackets: ( [ { ) ] }
• no quotes: ' " `
• no blanks.

Examples:

Global a_1 = 5
Global abc = `This is a string`
Global v=row(20)

Although permitted, it is not advisable to force variable names to contain lower case characters as in the following:

// Not recommended
Global "Abc" 123

substitution

If the formula evaluates to a Number, Logical or String, then the value is simply substituted. If the result is an Histogram, then the string "Histogram" is substituted for the formula. For a 1-dimensional Matrix, the first portion of the matrix is substituted but for higher dimensional matrices, the string "n-Matrix" will appear, where n is the dimension of the matrix.

Curly brackets { } are very often misused, in particular they should never be used in Do loops, in the conditions of If_lines and If_blocks, in the arguments of procedure Calls, in Parse and Global statements.

The contents of { } is evaluated and substituted before the statement is interpreted, while you would probably expect evaluation to happen only when the control statements are executed. For an overview of the use of round, square and curly brackets.

Format:

any text {formula} more text

Example:

Global a=60
Say "Tangent of {a} degrees is {tan(pi*a/180)}."

The global variable A is assigned the value 60. In the following SAY statement, A is converted from degrees to radians and its tangent is displayed.

Further examples of this use of global variables can be found in the examples given for many other commands.

// What one should not do
Global n=5               For i From 1 To 5 Do
For i From 1 To {n} Do      For j From 1 To {i} Do
    ...                        Say "j={j}"
Enddo                       Enddo
                         Enddo

The example on the left would still work if the value of n doesn't change during execution, the example on the right would fail. A priori, I is not defined at the time the loop on J is read. If it is, then the loop on J would be executed a fixed number of times that has no relation with the value of I in the outer loop.

types

Garfield variables know their own type. Garfield takes care that a version of the operators and the functions appropriate for the given data type or combination of data types is called. The type of the argument propagates to the result for most operators and functions.

Global variables come in the following types:

The mode is a numeric equivalent of the type, the mode is used only internally for quick reference.

Additional information on:

storage

The value of Numbers and Logicals is stored directly in R, while for Histograms, Matrices and Strings a reference number is kept in R.

Each element of R has type information attached to it.

All R's from R(0) downwards are assumed to be constants that are not affected by the execution of the instruction list.

constants

Although changing the values of the registers R(0) to R(-6) is not by itself prevented, the "jump always" code will for instance not work if R(-1) is set to something else than the value\&nbsp;1. Similar trouble can arise from changing the other constants in this range.

The space below R(-7) can be used for constants present in user entry points. These constants are deleted when the list is deleted with which they are associated.

The values of the constants which can be used in all lists are:

Notes:

1. The values for False and True are internally represented as 0 and 1 respectively, but arithmetic between Logicals and Numbers is not encouraged.

predefined

Additional information on:

Formatted on 21/01/18 at 16:55.