A for loop is the right tool when you know in advance how many times to repeat something. Often, though, you want to repeat something until a condition changes, without knowing ahead of time how many passes that will take. A while loop repeats a block of code for as long as a condition remains true, checking that condition before every pass.

The MATLAB Help Center has documentation on while.

Syntax

While loops are created in MATLAB using this syntax:

while condition
    code to execute repeatedly ...
end

Before each pass through the loop, MATLAB evaluates condition. If it is true, the code inside the loop runs once, then MATLAB checks the condition again. As soon as the condition is false, the loop ends and execution continues after the end keyword. If the condition is false the very first time it’s checked, the code inside the loop never runs at all.

For example, this loop prints the powers of two that are less than 100:

value = 1;
while value < 100
    disp(value)
    value = value * 2;
end


     1

     2

     4

     8

    16

    32

    64

Notice that a for loop would be awkward here. We’d need to already know that 26=64 is the last power of two under 100 before writing for p = 0:6. With a while loop, we just state the condition we care about (value < 100) and let MATLAB handle the rest.

For Loops vs. While Loops

Both loops repeat code, but they answer different questions:

  • Use a for loop when you know the number of iterations ahead of time, such as looping over every element of an array.
  • Use a while loop when you’re repeating something until a condition is met, and you don’t know in advance how many passes that will take.

Example: ISS Reboost Interval

Question

The International Space Station’s orbit slowly decays due to drag from the thin upper atmosphere, so it must be periodically reboosted to a higher altitude. Suppose the station orbits at 420 km and loses 2 km of altitude per week to drag. Mission control wants to schedule a reboost before the altitude drops to 400 km.

Write a MATLAB script that determines how many full weeks the station can go before it must be reboosted, and what its altitude will be at that point.

Solution

We don’t know the number of weeks in advance. That’s exactly what we’re solving for, so a while loop is the right tool. The loop keeps subtracting the weekly altitude loss as long as the altitude is still above the reboost threshold, counting how many weeks that takes.

altitude = 420; % km, starting altitude
decay_per_week = 2; % km lost to atmospheric drag each week
reboost_threshold = 400; % km, minimum safe altitude

weeks = 0;
while altitude > reboost_threshold
    altitude = altitude - decay_per_week;
    weeks = weeks + 1;
end

disp('Weeks until reboost is needed:')
disp(weeks)
disp('Altitude at that point (km):')
disp(altitude)


Weeks until reboost is needed:
    10

Altitude at that point (km):
   400

Avoiding Infinite Loops

A while loop only stops when its condition becomes false. If nothing inside the loop ever changes the variables the condition depends on, the condition stays true forever and the loop never ends. This is called an infinite loop, and it’s one of the most common bugs when first learning while loops.

In the ISS example above, altitude decreases by decay_per_week on every pass - eventually making altitude > reboost_threshold false. If that update line were accidentally left out, the condition would never change and MATLAB would loop forever.

If you accidentally run an infinite loop in MATLAB, press Ctrl+C in the Command Window to stop execution. Before running a while loop for the first time, double check that something inside the loop moves the condition toward being false.

Reading Questions

  1. What is the main difference between a for loop and a while loop?
  2. When does MATLAB check a while loop’s condition - before each pass, after each pass, or both?
  3. What happens if a while loop’s condition is false the first time it is checked?
  4. What causes an infinite loop, and how do you stop one that is already running?
  5. Would a for loop or a while loop be better suited to summing numbers typed in by a user until they type 0? Why?

Practice Problem: Engine Spool-Up Time

Before a jet engine can produce useful thrust, its core has to spool up from a standstill to idle speed. How long that takes isn’t known ahead of time. It depends on how fast the core accelerates, which makes this a natural fit for a while loop.

In this practice problem, you’ll write a MATLAB script that finds how long a jet engine takes to reach idle.

Your Task

Write a script named engine_spoolup.m that starts from the given starting conditions:

n2 = 0; % percent, core speed at the start
spool_rate = 3.2; % percent per second
idle_threshold = 56; % percent

Using a while loop that increases n2 by spool_rate on each pass, compute:

  1. seconds - the number of seconds it takes for n2 to reach or exceed idle_threshold
  2. n2_final - the value of n2 once the loop stops

Your variable names for the two answers above must match exactly (seconds, n2_final) so that the checker below can find them.

Checking Your Work

Download check_engine_spoolup.m and save it in the same folder as your engine_spoolup.m script. Make sure that folder is your Current Folder in MATLAB, then run:

>> check_engine_spoolup

The checker runs your script and reports whether each of the two values is correct. This is practice, not a graded assignment. If something doesn’t pass, use the feedback to find and fix the issue, then run the checker again.