Introduction to Simulation: Beginner


For loops, while loops, and if statements

The if statement

MATLAB supports the variants of "if" construct:

  • if ... end
  • if ... else ... end
  • if .... elseif ... else ... end
It should be noted that the "end" keyword is required to terminate the if statement. Let's look at the basic syntax of the if statement. 

x = 5;
if x > 0
    disp('x is positive');
end


x is positive

We can also add an else statement to the if statement. 

x = -5;
if x > 0
    disp('x is positive');
else
    disp('x is negative');
end


x is negative

We can also add an elseif statement to the if statement. 

x = 0;
if x > 0
    disp('x is positive');
elseif x < 0
    disp('x is negative');
else
    disp('x is zero');
end


x is zero

A relational operator compares two numbers by determining whether a comparison is true or false. Here's a list of relational operators in MATLAB:
  • == (equal to)
  • ~= (not equal to)
  • < (less than)
  • > (greater than)
  • <= (less than or equal to)
  • >= (greater than or equal to)
  • & (AND operator)
  • | (OR operator)
  • ~ (NOT operator)
x <= 4 & x > 0 means x is less than or equal to 4 and x is greater than 0.

The for loop

Let's look at the basic syntax of the for loop. 


for i=[1 3 5 4 10 7]
    disp(i);
end


1
3
5
4
10
7

In the previous class, we considered creating ranges with the colon, which is probably the common use of the for loop. 

for i=1:10
    disp(i)
end


1
2
3
4
5
6
7
8
9
10

The while loop

The while loop is used to execute a block of code repeatedly as long as the condition is true. Let's look at the basic syntax of the while loop. 


i = 1;
while i <= 5
    disp(i);
    i = i + 1;
end


1
2
3
4
5

The break statement is used to terminate the loop immediately. Let's look at the basic syntax of the break statement. 

i = 1;
while i <= 5
    disp(i);
    if i == 3
        break;
    end
    i = i + 1;
end


1
2
3

The continue statement is used to skip the current iteration and continue with the next iteration. Let's look at the basic syntax of the continue statement. 

i = 1;
while i <= 5
    i = i + 1;
    if i == 3
        continue;
    end
    disp(i);
end


2
4
5
6

Examples

Let's consider an even problem. 


X = 1:10 
Y = zeros(1,10)
for i=1:10
    if mod(X(i),2) == 0
        Y(i) = 1;
    else
        Y(i) = 0;
    end
end
Y


X =
    1    2    3    4    5    6    7    8    9   10

Y =
  0  0  0  0  0  0  0  0  0  0

Y =
   0   1   0   1   0   1   0   1   0   1

Now let's do a similar example. The problem is the following. Let's say we want to sum all the elements of an array that are divisible by 3. We'll use the same X as before. 

S = 0;
for i=1:10
    if mod(X(i),3) == 0
        S = S + X(i);
    end
end
S


S = 18

There is also another way to do this. 

S = 0;
for x=X
    if mod(x,3) == 0
        S = S + x;
    end
end


S = 18

Now let's look on another problem. Suppose we want to check if the matrix contains a specific element. 

X
found = 0;
i = 0;
while ~found
    i = i + 1
    if X(i) == 8
        disp('I found it!');
        found = 1;
    end
end


X =
    1    2    3    4    5    6    7    8    9   10

i = 1
i = 2
i = 3
i = 4
i = 5
i = 6
i = 7
i = 8
I found it!

It will continue to go while it's true and stop when it's false. We also can serve the same purpose with for loop (try yourself): 

for i=1:10
    fprintf('i = %d\n', i);
    if X(i) == 8
        disp('I found it!');
    end
end


i = 1
i = 2
i = 3
i = 4
i = 5
i = 6
i = 7
i = 8
I found it!
i = 9
i = 10

It is inefficient because it is doing extra steps. We can use the Break statement to cut the loop. 

for i=1:10
    fprintf('i = %d\n', i);
    if X(i) == 8
        disp('I found it!');
        break;
    end
end


i = 1
i = 2
i = 3
i = 4
i = 5
i = 6
i = 7
i = 8
I found it!

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