Page 51 - Basics of MATLAB and Beyond

X = QUADRATIC(A,B,C) returns the two roots of the
quadratic equation
y = A*x^2 + B*x + C.
The roots are contained in X = [X1 X2].
matlab only echoes the comment lines that are contiguous; the first non-
comment line, in this case the blank line before the signature, tells mat-
lab that the help comments have ended. The first line of the help com-
ments is searched and, if successful, displayed when you type a lookfor
command.
Comment lines can appear anywhere in the body of an m-file. Com-
ments can be put at the end of a line of code:
rootdisc = sqrt(b.^2 - delta); % Root of the discriminant
Blank lines can appear anywhere in the body of an m-file. Apart from
ending the help comment lines in a function, blank lines are ignored.

8.3 Flow Control
matlab has four kinds of statements you can use to control the flow
through your code:
if, else and elseif execute statements based on a logical test
switch, case and otherwise execute groups of statements based on
a logical test
while and end execute statements an indefinite number of times,
based on a logical test
for and end execute statements a fixed number of times

If, Else, Elseif
The basic form of an if statement is:
if test
statements
end
The test is an expression that is either 1 (true) or 0 (false). The
statements between the if and end statements are executed if the
test is true. If the test is false the statements will be ignored and
execution will resume at the line after the end statement. The test
expression can be a vector or matrix, in which case all the elements
must be equal to 1 for the statements to be executed. Further tests
can be made using the elseif and else statements.
Exercise 2 Write a function m-file that takes a vector input and
returns 1 if all of the elements are positive, −1 if they are all neg-
ative, and zero for all other cases. Hint: Type help all. (Answer
on page 183.)

c 2000 by CRC Press LLC