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In the expressions that determined the exit condition for our while loops
in the preceding section, we used logic elements (>=
and >
)
to compare two values. We'll also need to use logic elements in branching
constructs, so we'll list them all here in Table 6.1. Technically
speaking there are two types of logic elements, relational operators and
logical operators, but we won't worry about those distinctions here.
Table 6.1:
Logic elements in MATLAB used for comparing values and branching.
Relational operators 
Description 
< 
Less than 
<= 
Less than or equal 
> 
Greater than 
>= 
Greater than or equal 
== 
Equal to (Don't confuse with 

the assignment operator = ) 
= 
Not equal to 
Logic operators 
Description 
& 
AND 
 
OR 

NOT 
The purpose of branching in computer programming is to execute different
commands for different conditions. For example, consider the code below
% First example of branching
clear all; close all; clc;
for nums = 1:10 % Loop over all integers 1 thru 10 (by 1's)
remainder = mod(nums,2); % Calculate the remainder of nums/2
numstr = num2str(nums); % Convert the value of 'nums' into a string
% so it can be printed
if remainder == 0 % Note the use of a DOUBLE equal sign
disp([numstr ' is even']) % do this only if the remainder is zero
else
disp([numstr ' is odd']) % do this whenever remainder is NOT zero
end % Every ifelse block requires an end statement
end % This end statement marks the end end of the for loop
In this example, the remainder of is calculated for all integers
. If the remainder is zero, then we know that the integer
is even. The remainder is calculated with the mod
function. The
if
statement checks to see if the remainder is zero. If the
condition is true, the program branches to the first statement in
the ifelse block and prints the value of the number and the words
``is even''. On the other hand, if the condition is false (if the remainder
is not equal to zero), then the first branch of the ifelse block is
skipped and program execution proceeds to the else
statement. When
the else statement is executed, the statements in its block are
always executed. So, if the value of the variable remainder
is not zero, then the first branch of the ifelse statements will be
skipped and the second block (the else block) will be
executed. Every if
statement needs a corresponding end
statement to indicate the end of the branching blocks.
An if
statement can be used alone without an else
branch. Look at this small variation of the preceding example. A second
if
statement has been added that checks to see if an even number is
also divisible by 4. This example also shows how if
blocks can be
nested one inside the other.
% First example of branching
clear all; close all; clc;
for nums = 1:10 % Loop over all integers 1 thru 10 (by 1's)
remainder = mod(nums,2); % Calculate the remainder of nums/2
numstr = num2str(nums); % Convert the value of 'nums' into a string
% so it can be printed
if remainder == 0 % Note the use of a DOUBLE equal sign
disp([numstr ' is even']) % Do this only if the remainder is zero
if mod(nums,4) == 0 % See if nums/4 also has a zero remainder
disp('It is also divisible by 4')
end
else
disp([numstr ' is odd']) % Do this whenever remainder is NOT zero
end % Every ifelse block requires an end statement
end % This end statement marks the end of the for loop
One more branching construct you should be familiar with is
elseif
. These constructs are used in conjunction with an if
statement. Suppose we have scores from a test and we want to assign letter
grades based on the scores. If a score is 90% or above, we will assign an
A; otherwise if the score is 80% or above, we will assign a B; etc. The
following program illustrates how ifelseif blocks can be used to
assign the scores. Note that only one of the blocks will be executed
in each caseonce one block has been executed all of the others will be
skipped without checking if the conditional expressions are true or false.
% Example of elseif blocks
clc; clear all; close all; % clean up command window, memory, and desktop
scores = 15*randn(10,1)+75 % generate 10 bell curve scores; avg 75; sigma 15
for i = 1:length(scores) % length(scores) returns the number of elements in 'scores'
if scores(i) >= 90
disp('A')
elseif scores(i) >= 80
disp('B')
elseif scores(i) >= 70
disp('C')
elseif scores(i) >= 60
disp('D')
else
disp('F')
end
end
Using logical operators (see Table 6.1), we can also check for
multiple conditions. Let's change our first example from this section to
run over all the integers to 5. We'll assume that we only want to call
an integer even if it is divisible by 2 and it is not zero.
% Example of checking multiple conditions
clear all; close all; clc;
for nums = 5:5
remainder = mod(nums,2);
numstr = num2str(nums);
if (remainder == 0 & nums ~= 0) % Check for the truth of BOTH conditions
disp([numstr ' is even'])
elseif remainder ~=0 % Check for nonzero remainder
disp([numstr ' is odd'])
end
end
We made two changes to the original script. The
condition of the if
statement now includes two conditions
with the ``and'' operator &
between them. The statement block of the
if statement is executed only if both of the conditions are
true. Carefully note that we were forced to make a second change as
well. The original else
statement was changed to an elseif
statement with a condition testing the value of remainder for a nonzero
result. Without this change, the program will print ``0 is negative.'' Why?
What happens when remainder is zero and the value of nums
is
also zero? Explain your answer to the instructor before going on.
Next: 6.3 Modular programming
Up: 6. Real programming
Previous: 6.1 Loops
Contents
Gus Hart
20050128