Loops in matlab

MATLAB control structures continued

CIV1900: Engineering Skills

Control structures determine what gets executed

• control flow is determined by control structures
• MATLAB has four control structures

• two for deciding between alternatives:
• if statements
• switch statements

• two for repeating (a.k.a. looping or iteration):
• while loops
• for loops

• MATLAB also has implicit loops over arrays

CIV1900 MATLAB control structures 3

An if statement runs the body when the condition is true

• if statements are of the form:
if <condition>
<body>
end

• if and end are keywords (can't be used as variables)
• <condition> is a logical expression which can be evaluated
to true or false
• <body> is the body of the if statement
• one or more statements
• only executed when the condition evaluates to true


You can explore the conditions on their own

• try all of these conditional operators:

Operator Description Operator Description
x > 4 greater than x < 4 less than
x >= 4 greater than or equal to x <= 4 less than or equal to
x == 4 equal to x ~= 4 not equal to

• for example:
• >> x = 4
>> x == 4
ans = 1
• equality is == to distinguish from assignment
• remember ans is a logical not a number


What about if the condition is false?

• an else clause can be run if the condition is false:
if <condition>
<true-body>
else
<false-body>
end

• else is also a keyword
• <false-body> is run when the condition is false


Else body only runs if none of the conditions are true

• the general scheme looks like this:
if <condition1>
<true-body1>
elseif <condition2>
<true-body2>
…
else
<false-body>
end

• where the elseif and else clauses are optional
• and the elseif clause can be repeated for more conditions


while statements are like repeating if statements

• a while loop repeats the body while the condition stays true
• the general scheme looks very similar to an if statement:
while <condition>
<body>
end

• while and end are keywords

• when <condition> is false the loop will not execute again


The condition is evaluated before the body is executed

• a while loop repeats the following steps
• first, evaluating the conditional expression
• if the condition is true, run the body
• if the condition is false, jump to the statement after the body

• an iteration is a single execution of the body
• the condition is evaluated before each iteration begins

• sometimes the body may never get executed


A very common idiom for while loops

x = 1;
while x <= 3
fprintf('In the loop, x = %d n',x);
x = x + 1;
end
fprintf('After the loop, x = %d n',x);

• the output will look like this:
In the loop, x = 1
In the loop, x = 2
In the loop, x = 3
After the loop, x = 4


Not updating the loop variable is a common mistake

• what happens when we run this program?
x = 1;
while x <= 3
fprintf('In the loop, x = %d n',x);
end
fprintf('After the loop, x = %d n',x);

• you can stop the infinite loop by pressing Control-C
• x is never updated in the body, so it stays at 1
• the condition is never false
• the loop never stops!


Looping over array indices is common

values = [1 5 2 8 -3 4];
i = 1;
total = 0;
while i <= length(values)
total = total + values(i);
i = i + 1;
end
fprintf('The total is %d n',total);

• the length function returns the length of a vector
• the loop variable i ranges from 1 up to 6
• single letter loop variables, especially i, j, and k are common


for loops simplify looping over arrays

• for loops iterate over each element of an array or range

• they have the general form
for <variable> = <array>
<body>
end

• <body> is run once for each element of the array

for x = 1:3
fprintf(‘x is now %d n’,x);
end


for loops really simplify our previous example

marks = [75, 80, 71, 82, 86];

total = 0;
for m = marks
total = total + m;
end

avg = total/length(marks);
fprintf('Average mark %d n', avg);

• variable m is assigned the value 75, then 80, then 71, …
• after each time m is assigned, the for loop body is run


for loops can be used with ranges

for i = 1:2:8
fprintf('i = %d n', i);
end
fprintf('after the loop i = %d n', i);

• i will be assigned 1, then 3, then 5, …
• so the output is
i = 1 Note:
i = 3 That the increment can
also be negative e.g.
i = 5
i = 7 8:-2:1
after the loop i = 7


When we need the index, loop over 1:length(A)

• for example, to find the index of the maximum in values:
maxval = values(1);
maxindex = 1;
for i = 1:length(values)
if values(i) > maxval
maxval = values(i);
maxindex = i;
end
end
fprintf('max at index %d n', maxindex]);


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