1. Review while loops Control variables Example Infinite loop
Trap user while invalid input
Ask first, then loop
Hardcode bad, then loop

2. 1. A control variable
Any loop has a starting point and an ending point. In programming, a “loop control variable” keeps track of where we are.
This variable has 3 phases:
Initialization of variable – starting point
Change of variable - incrementation
Condition to exit loop – ending point

3. 2. An example Example: the loop control variable is x
while (x < 10)  2. condition
x = x + 1;
fprintf(‘%02d\n’, x);
end
The loop starts at zero, and while x is strictly less than 10, x is incremented by 1 and displayed to the screen.
The loop stops when x actually reaches 10!

4. 3. Infinite loop
Infinite Loop  Loop control variable never meets the condition and the code runs and runs…….
Use CTRL+C in the command window to break it

5. Infinite loop, cont. Why is this an infinite loop in MATLAB?
x = 11; % 1. initialization
while (x > 10) % 2. condition
fprintf(‘%02d\n’, x);
x = x + 1; % 3. change loop variable
end

6. 4. Trap a user for a value
Ask a user for a decimal value between 1.0 to 10.0
What if they don’t give the proper value?
Two approaches:
Ask first, then loop
Initialize the control variable to a bad value, and loop

7. 4.1. Ask first, then loop (1)
Ask the use first for a value (Give the user a chance to NOT mess up!)
Then loop while it was a wrong value.
%ask user for a specific distance
distance = input(‘Enter a distance (1-10 cm): ’);
%while this value was invalid, prompt again
while (distance<1 || 10<distance)
distance = input(‘Error! Enter a value 1-10: ’);
end

8. 4.1. Ask first, then loop (2)
Ask the use first for a value (Give the user a chance to NOT mess up!)
Then loop while it was a wrong value.
%ask user for a specific distance
distance = input(‘Enter a distance (1-10cm): ’);
%while this value was invalid, prompt again
while ~(1<= distance && distance<=10)
distance = input(‘Error! Enter a value 1-10: ’);
end

9. 4.2. Make bad, then loop (1)
The trick is to make MATLAB think a bad value has been entered, without prompting the user!!  HARDCODE
%makes loop run at least once
distance = -1; %BAD VALUE
%prompt/repeat while invalid
while (distance<1 || 10<distance)
distance = input(‘Enter a distance (1-10 cm): ’);
end
This option does not allow an easy ERROR message.

10. 4.2. Make bad, then loop (2)
The trick is to make MATLAB think a bad value has been entered, without prompting the user!!  HARDCODE
%makes loop run at least once
distance = -1; %BAD VALUE
%prompt/repeat while invalid
while ~(1<= distance && distance<=10)
distance = input(‘Enter a distance (1-10 cm): ’);
end
This option does not allow an easy ERROR message.

11. Wrapping Up while loops are very common to validate user-inputs.
The block of code in a while loops only runs when the condition is true.
The code block must change the condition, or it is called an infinite loop.
A control-loop variable keeps track of whether the loop should execute or not