1. Week 7: Computer Tools for Problem Solving and Critical Thinking
For Loops
Week 7: Computer Tools for Problem Solving and Critical Thinking

2. Iterations
Sometimes we want to repeat a task over and over until some condition is met. For example-
For every student on my class list, display their student number
For a list of numbers, I want to extract all the even numbers from the list
For a list of numbers I need to display the square of each number
These are what are called iteration tasks- or something repeated over and over a certain number of times

3. Iteration Tasks Iteration tasks have five basic components- A counter
A start value for the counter
A step size for the counter
A finishing/stopping value for the counter
Instructions to follow each iteration (or value of) of the counter

4. Click through for instructions
Iteration Flow Chart
Start
Let’s go back to our example: For a list of numbers, x=[ ] we want to display the square of each number. What do we need?
Our list of numbers- a vector, x, of values
Our counter variable, i- if we want to extract values from vector x one value at a time, starting at the first value, we need to start with a value of i=1.
Now let’s think what we want the program to do. We want to go through each element of the vector x and display x2 one element at a time.
So our step size is 1 (one element at a time)
Once we get past the last element, we want the program to stop. For example for vector x=[ ] once our counter gets to 5, x(5) exceeds vector dimensions.
Our stopping value is the length of the vector, in this case 4
Now that we have all 5 of our iteration components we can think about how MATLAB is going to execute the instructions of disp(x(i)^2) within the range of vector x length one element at a time.
This is a looping structure. Because we know the start and stop values of our counter (i.e. we know how many times we want to loop) we can use a For loop
Input vector x
set counter start=1 No
is i<= length(x)?
Yes
Stop
LOOP
disp(x(i)^2)
set i=i+1

5. For Loops in MATLAB BASIC SYNTAX with our FIVE components:
for counter = start_value:step_size:stop_value instructions end
*Read through the Mathwors documentation on For Loops:

6. Click through for instructions
For Loops in MATLAB
Click through for instructions
for counter = start_value:step_size:stop_value instructions end
How will we code this? What was first on our flow chart?
x= input (‘Please insert a vector:’)
Then we have to set our counter, i, value to the start value 1:
start_value= 1;
On our flow chart we now have to ask if our counter, i, is bigger than the length of x.
We should create a variable with this value. This value is also our stop value.
stop_value= length(x);
Then we can also create a variable for our step size:
step_size= 1;

7. Click through for instructions
For Loops in MATLAB
Click through for instructions
Now we can start to code our looping structure:
for i = start_value:step_size:stop_value instructions end
What about the instructions? We want to display the square of each element of x. The line of code we need inside the for loop is thus:
disp((x(i))^2);

8. For Loops in MATLAB
What will this all look like together? Create an .m file ForLoopsDemo.m and try the code below:
%This program takes a vector input and displays the squares of %each element
x= input (‘Please insert a vector:’)
start_value=1;
stop_value= length(x);
step_size=1;
for i = start_value:step_size:stop_value
disp((x(i))^2);
end

9. Exercises
Starting from the code above, edit the script to display only the even numbered elements. Make a vector that has at least 10 entries. Hint-you won’t start at 1 and you won’t step by 1.
Starting from the code above, edit the script to now store all the even numbered elements in a vector, y, and display y at the end. Hint- your instructions in the loop will be to create y, your disp command will be used outside the looping structure.
Display a count down from 10 to 0. Hint-your step size doesn’t have to be positive.
Compute the sum of the numbers 1 to 10 without using any built-in MATLAB commands for sum. For simplicity you can do this first without creating a vector of the values, but for next exercise you will need to.
Compute the sum of all the elements in a vector without using any built-in MATLAB commands for sum . Challenge-create a vector that should sum to 0.
Using the code above in 4, compute the average of your vector without using any built-in MATLAB commands for average.
Write a program that will display the cube of a number up to a given integer. Hint- you will need to refresh on fprintf. The display should look something like this: The number is : 1 and cube of 1 is :1 The number is : 2 and cube of 2 is :8 The number is : 3 and cube of 3 is :27 The number is : 4 and cube of 4 is :64