1. Control variables

2. Example

3. Infinite loop

4. Trap user while invalid input1. Ask first, then loop

2. Hardcode bad, then loop

Review while loops

1

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

2

2. An example

Example: the loop control variable is x

x = 0;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!

3

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

4

Infinite loop, cont.

Why is this an infinite loop in MATLAB?

x = 11; % 1. initialization

while (x > 10) % 2. conditionfprintf(‘%02d\n’, x);x = x + 1; % 3. change loop

variableend

5

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:1. Ask first, then loop

2. Initialize the control variable to a bad value, and loop

6

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 distancedistance = input(‘Enter a distance (1-10 cm): ’);

%while this value was invalid, prompt againwhile (distance<1 || 10<distance)

distance = input(‘Error! Enter a value 1-10: ’);end

7

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 againwhile ~(1<= distance && distance<=10)

distance = input(‘Error! Enter a value 1-10: ’);end

8

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 oncedistance = -1; %BAD VALUE%prompt/repeat while invalidwhile (distance<1 || 10<distance)

distance = input(‘Enter a distance (1-10 cm): ’); end

This option does not allow an easy ERROR message.9

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 oncedistance = -1; %BAD VALUE%prompt/repeat while invalidwhile ~(1<= distance && distance<=10)

distance = input(‘Enter a distance (1-10 cm): ’); end

This option does not allow an easy ERROR message.10

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

11