TECO provides for unconditional branches, equivalent to GOTOs in other programming languages, using the O command, which has the following form:
Otag`
where tag is a location named elsewhere in the command string and ` signifies a <delim>. The text argument in the O command has the same format as the text arguments in search commands, which includes the use of string building characters such as ^EQq. Also, the O command may be modified with an at-sign, in which case the syntax of the command would be @O/tag/.
Note that leading and trailing spaces are ignored for tags specified in O commands. This is to improve readability. However, a consequence of this is that it is not possible to branch to a tag that has any spaces in it. This is one way that tags can be distinguished from comments.
When an O command is executed, the next command to be executed will be the tag referenced by the O command, followed by any subsequent commands.
Tags have the form:
!tag!
TECO performs no action on encountering a tag, and basically just treats them as no-ops, but can be divided into the following categories:
- Labels that are the targets of O commands.
- Comments that are ignored by TECO.
Labels may contain any number of ASCII characters, but are subject to the following restrictions:
- Labels must be unique (this is not the case for comments which are not the targets of O commands).
- Control characters are not allowed.
- Exclamation marks are not allowed, since they delimit tags.
- Commas are not allowed, since they delimit labels in computed GOTO commands).
The following conditions apply to the use to O commands:
- Branching out of a loop or conditional statement is allowed.
- Branching into a conditional statement is allowed.
- The tag branched to must be in the same command string, Q-register, or indirect command file as the O command.
- The tag must occur only once (this only applies to labels referenced by O commands; comments may be duplicated without causing an error).
- If an O command is inside a loop or nested loop, it cannot branch backward before the start of the outermost enclosing loop.
- Branches into the middle of a new loop, whether forward or backward, are not allowed.
| Command | Function |
|---|---|
| @Otag` | Causes TECO to branch to the first occurrence of the specified label tag in the current macro level. Command execution resumes at the start of the tag. |
| @O/tag/ | Equivalent to Otag`. |
| nOtag0,tag1, tag2,...` | Causes TECO to branch to the tag specified by the nth tag in the accompanying list. The string argument to this command consists of a sequence of tags separated by commas. If n is out of range, or if n selects a null tag (signified by two adjacent commas), then command execution continues with the first command following the <delim> that terminates this command. |
| nO/tag0,tag1, tag2,.../ | Same as the preceding command except that the list of tags is bracketed by a delimiter shown here as "/". The delimiter can be any character other than comma that does not appear within the list of tags. |