Insert/Delete Character

Inserting a character means creating a blank space in the middle of a line, and pushing the rest of the line rightward. The character in the rightmost column is lost.

Deleting a character means causing the character to disappear from the screen, closing up the gap by moving the rest of the line leftward. A blank space appears in the rightmost column.

Insertion and deletion of characters is useful in programs that maintain an updating display some parts of which may get longer or shorter. It is also useful in editors for redisplaying the results of editing within a line.

Many terminals provide commands to insert or delete a single character at the cursor position. Some provide the ability to insert or delete several characters with one command, using the number of characters to insert or delete as a parameter.

Many terminals provide an insert mode in which outputting a graphic character has the added effect of inserting a position for that character. A special command string is used to enter insert mode and another is used to exit it. The reason for designing a terminal with an insert mode rather than an insert command is that inserting character positions is usually followed by writing characters into them. With insert mode, this is as fast as simply writing the characters, except for the fixed overhead of entering and leaving insert mode. However, when the line speed is great enough, padding may be required for the graphic characters output in insert mode.

Some terminals require you to enter insert mode and then output a special command for each position to be inserted. Or they may require special commands to be output before or after each graphic character to be inserted.

Deletion of characters is usually accomplished by a straightforward command to delete one or several positions; but on some terminals, it is necessary to enter a special delete mode before using the delete command, and leave delete mode afterward. Sometimes delete mode and insert mode are the same mode.

Some terminals make a distinction between character positions in which a space character has been output and positions which have been cleared. On these terminals, the effect of insert or delete character runs to the first cleared position rather than to the end of the line. In fact, the effect may run to more than one line if there is no cleared position to stop the shift on the first line. These terminals are identified by the `in' flag capability.

On terminals with the `in' flag, the technique of skipping over characters that you know were cleared, and then outputting text later on in the same line, causes later insert and delete character operations on that line to do nonstandard things. A program that has any chance of doing this must check for the `in' flag and must be careful to write explicit space characters into the intermediate columns when `in' is present.

A plethora of terminal capabilities are needed to describe all of this complexity. Here is a list of them all. Following the list, we present an algorithm for programs to use to take proper account of all of these capabilities.

`im'

String of commands to enter insert mode. If the terminal has no special insert mode, but it can insert characters with a special command, `im' should be defined with a null value, because the `vi' editor assumes that insertion of a character is impossible if `im' is not provided. New programs should not act like `vi'. They should pay attention to `im' only if it is defined.

`ei'

String of commands to leave insert mode. This capability must be present if `im' is. On a few old terminals the same string is used to enter and exit insert mode. This string turns insert mode on if it was off, and off it it was on. You can tell these terminals because the `ei' string equals the `im' string. If you want to support these terminals, you must always remember accurately whether insert mode is in effect. However, these terminals are obsolete, and it is reasonable to refuse to support them. On all modern terminals, you can safely output `ei' at any time to ensure that insert mode is turned off.

`ic'

String of commands to insert one character position at the cursor. The cursor does not move. If outputting a graphic character while in insert mode is sufficient to insert the character, then the `ic' capability should be defined with a null value. If your terminal offers a choice of ways to insert--either use insert mode or use a special command--then define `im' and do not define `ic', since this gives the most efficient operation when several characters are to be inserted. Do not define both strings, for that means that both must be used each time insertion is done.

`ip'

String of commands to output following an inserted graphic character in insert mode. Often it is used just for a padding spec, when padding is needed after an inserted character (see section Padding).

`IC'

String of commands to insert n character positions at and after the cursor. It has the same effect as repeating the `ic' string and a space, n times. If `IC' is provided, application programs may use it without first entering insert mode.

`mi'

Flag whose presence means it is safe to move the cursor while in insert mode and assume the terminal remains in insert mode.

`in'

Flag whose presence means that the terminal distinguishes between character positions in which space characters have been output and positions which have been cleared.

An application program can assume that the terminal can do character insertion if any one of the capabilities `IC', `im', `ic' or `ip' is provided.

To insert n blank character positions, move the cursor to the place to insert them and follow this algorithm:

1. If an `IC' string is provided, output it with parameter n and you are finished. Otherwise (or if you don't want to bother to look for an `IC' string) follow the remaining steps.
2. Output the `im' string, if there is one, unless the terminal is already in insert mode.
3. Repeat steps 4 through 6, n times.
4. Output the `ic' string if any.
5. Output a space.
6. Output the `ip' string if any.
7. Output the `ei' string, eventually, to exit insert mode. There is no need to do this right away. If the `mi' flag is present, you can move the cursor and the cursor will remain in insert mode; then you can do more insertion elsewhere without reentering insert mode.

To insert n graphic characters, position the cursor and follow this algorithm:

1. If an `IC' string is provided, output it with parameter n, then output the graphic characters, and you are finished. Otherwise (or if you don't want to bother to look for an `IC' string) follow the remaining steps.
2. Output the `im' string, if there is one, unless the terminal is already in insert mode.
3. For each character to be output, repeat steps 4 through 6.
4. Output the `ic' string if any.
5. Output the next graphic character.
6. Output the `ip' string if any.
7. Output the `ei' string, eventually, to exit insert mode. There is no need to do this right away. If the `mi' flag is present, you can move the cursor and the cursor will remain in insert mode; then you can do more insertion elsewhere without reentering insert mode.

Note that this is not the same as the original Unix termcap specifications in one respect: it assumes that the `IC' string can be used without entering insert mode. This is true as far as I know, and it allows you be able to avoid entering and leaving insert mode, and also to be able to avoid the inserted-character padding after the characters that go into the inserted positions.

Deletion of characters is less complicated; deleting one column is done by outputting the `dc' string. However, there may be a delete mode that must be entered with `dm' in order to make `dc' work.

`dc'

String of commands to delete one character position at the cursor. If `dc' is not present, the terminal cannot delete characters.

`DC'

String of commands to delete n characters starting at the cursor. It has the same effect as repeating the `dc' string n times. Any terminal description that has `DC' also has `dc'.

`dm'

String of commands to enter delete mode. If not present, there is no delete mode, and `dc' can be used at any time (assuming there is a `dc').

`ed'

String of commands to exit delete mode. This must be present if `dm' is.

To delete n character positions, position the cursor and follow these steps:

1. If the `DC' string is present, output it with parameter n and you are finished. Otherwise, follow the remaining steps.
2. Output the `dm' string, unless you know the terminal is already in delete mode.
3. Output the `dc' string n times.
4. Output the `ed' string eventually. If the flag capability `mi' is present, you can move the cursor and do more deletion without leaving and reentering delete mode.

As with the `IC' string, we have departed from the original termcap specifications by assuming that `DC' works without entering delete mode even though `dc' would not.

If the `dm' and `im' capabilities are both present and have the same value, it means that the terminal has one mode for both insertion and deletion. It is useful for a program to know this, because then it can do insertions after deletions, or vice versa, without leaving insert/delete mode and reentering it.

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