Guide to LaTeX (4th Edition) (Tools and Techniques

C.2. Some sample errors 413
message once again, this time stating that it cannot find the file x.ext.
Some installations provide a dummy file named null.tex, while one of
the extension packages available (Section B.5.4) offers a set of files named
after the standard response letters x.tex, e.tex, r.tex, h.tex, s.tex
to emulate the responses to normal error messages.
Emergency stop: sometimes one reaches a situation where the program
cannot be halted after an error, even with I\stop or X. In such a case, one
has to apply the operating system’s program interrupt, usually CTRL Z.
C.2.3 Mathematical errors
Surprisingly few errors occur as a result of incorrect application of mathematical
formula commands themselves, even for a user with only a little
experience. More often, the errors in mathematical formulas are fiddly
ones such as forgetting a closing brace } or neglecting to switch back
to text mode. Another type of common error is to use a symbol in text
mode that may only appear in math mode. We will point out a few typical
examples here.
One wants to produce the text: ‘The price is $3.50 and the order
number is type sample’, and types the input text:
The price is $3.50 and the order number is type_sample.
This text contains two errors, such that the first cancels the second one:
the $ sign is the math mode switch for generating formulas within text
(Section 5.1). The proper way to write a true dollar sign in text is as \$.
Instead, in the sample text, the $ alone switches to math mode, setting
everything that follows as a formula. However, the closing switch-back $
is missing, something that T E X will first notice at the end of the current
paragraph.
! Missing $ inserted.
$
l.4
?
If 〈return〉 is simply pressed as a response, T E X inserts a $ at this point.
For our sample text, this would be at the end of the text before the blank
line. This means that the text from the first $ to the end of the paragraph
will be set as a text formula:
The price is 3.50andtheor der number istypesample.
This output should show the user right away what went wrong. Near
the end of the line, there is a letter s printed as an index. This is the
second error in the example. The underbar character _ is only permitted