Writing to the Screen

Moving Around the Screen

First, let us look how to move the cursor. There are two main functions that do this: mv and mv_to. Both functions accept two arguments (though the arguments can be passed as a single vector of length 2): a row value and a column value. Row values are always listed before column values and the top left corner of the screen has a coordinate of (1,1). Notation is similar to matrix notation.

The picture below represents the coordinates along a 8x16 screen. The cursor is represented by the "X" and is located at (1,3). The "Z" block is located at (6,9).

## +----------------+
## |  X             |
## |                |
## |                |
## |                |
## |                |
## |        Z       |
## |                |
## |                |
## +----------------+

The simplest way to move the cursor to the Z block is to use mv_to(row, col) to move to an absolute position.

library(cursr)
mv_to(6, 9)

# OR ...

new_location <- c(6,9)
mv_to(new_location)

Alternatively, we could move the cursor relative to its current position. This would require moving down 5 rows and over 6 columns. The mv(row, col) command moves the cursor relative to its current position. If a value is omitted, it is treated as 0.

mv(5, 6)

We have to be careful with mv. When entered directly into the console, the cursor immediately move down one row and to the first column. So, the new position would be (7,2) instead of (6,4). Furthermore, the cursor will be moved to the right two places as soon as the prompt > appears.

Note, that mv should work correctly when used within the body of a function instead of being entered directly into the prompt. Also, term_dim() can be used to determine the size of the screen.

Writing to the Screen

The two main functions to write to the screen are wr and wrat. wr echoes the character string at the current cursor position. wrat writes the character string at a particular position, using the same coordinate style mentioned previously.

mv_to(1,3)
wr("Hello")

wrat(6,9, "World")

## +----------------+
## |  Hello         |
## |                |
## |                |
## |                |
## |                |
## |        World   |
## |                |
## |                |
## +----------------+

The cursr package contains a variety of functions for drawing lines, boxes, arcs, circles, and curves to the screen as well. The functions are prefixed with draw_. See the cursr manual for more details.

Color and Attributes

All of the writing and drawing functions in the cursr package accept three different parameters to influence their style: foreground color "fg", background color "bg", and other attributes "attr".

Colors can be specified in a number of different ways. The color name can be used for basic colors. Hexademical color codes, 256 color codes, and RGB values are also accepted. For example, we can change the foreground color to be red with any of the following commands.

# Change foreground color to be "red" with any of the following
fg_on("red")
fg_on("#FF0000")
fg_on(9)
fg_on(255, 0, 0)

# Return foreground color to default
fg_off()

Attributes are specified using 2-letter character values similar to their LaTeX abbreviations: "bf" for bold, "it" for italics, etc.

Each of these can be used separately via functions such as bg_on() and attr_on(), or they can be combined with other functions such as below.

# Write at cursor in bold, blue on red background
wr("Hello World!", bg="red", fg="blue", attr="bf")