COP2271 Notes
while loops
1
while loops
The while loop allows your program to repeatedly execute a series of statements if a given condition
is true. Here is the syntax:
while condition
statements
end
The condition is any valid MATLAB expression that can be interpreted as true or false. The
condition for a while loop has the same behavior as a condition for an if statement. Replace
statement with the code you want repeatedly executed until the condition becomes false. Make
sure all code is listed before the end keyword. Here is our first example that prints the numbers
1-5 to the screen:
printMe = 1 ;
w h i l e printMe <= 5
d i s p ( printMe ) ;
printMe = printMe + 1 ;
end
First, the code assigns the value 1 to printMe. The condition for the while loop checks if
printMe is less than or equal to 5. The first time the program encounters the while loop,
printMe is 1 which is less than 5, making the condition true. The program then performs the
statements before the end keyword and then returns to top of the while loop to check the
condition again. This process is repeated until the condition is false. The table below
explains the loop in detail:
Iteration Condition Condition Result
First
1 <= 5
true
Second
2 <= 5
true
Third
3 <= 5
true
Fourth
4 <= 5
true
Fifth
5 <= 5
true
Sixth
6 <= 5
false
display
display
display
display
display
1
2
3
4
5
Statement
and increment printMe
and increment printMe
and increment printMe
and increment printMe
and increment printMe
exit loop, skipped
to
to
to
to
to
2
3
4
5
6
The key idea here is that after MATLAB reaches the end keyword, it jumps back to the while
keyword above. This action causes the code to loop and will repeat until the condition becomes
false! If your script ever gets stuck in an endless loop, you can hit CTRL + C to stop it from
running (CMD+C on OSX).
COP2271 Notes
while loops
2
Here is a more complex example:
clc ; clear ;
userNumber = i n p u t ( ' Try t o g u e s s my s e c r e t number :
numOfGuesses = 1 ;
w h i l e userNumber ˜= 42
i f userNumber < 42
d i s p ( ' Your g u e s s i s t o o low ! ' )
e l s e i f userNumber > 42
d i s p ( ' Your g u e s s i s t o o h i g h ! ' )
end
userNumber = i n p u t ( ' Next g u e s s :
numOfGuesses = numOfGuesses +1;
end
');
');
i f ( numOfGuesses == 1 )
d i s p ( ' You g o t i t on t h e f i r s t g u e s s ! ' )
else
f p r i n t f ( ' I t took you %d g u e s s e s ! \ n ' , numOfGuesses )
end
Jump statements: break and continue
The break statement allows us to interrupt the normal execution of loops (while and for). break
immediately stops the execution of the loop regardless of the condition. Note that the
break statements stops the closest loop only, regardless of how many loops are nested. In the
following example, we set the condition of the while loop to true (a MATLAB keyword), which
means this loop will keep running until a break is found:
printMe = 5 ;
while true
f p r i n t f ( '%d \n ' , printMe ) ;
printMe = printMe −1;
i f printMe <= 0
break ;
end
end
f p r i n t f ( ' Go ! \n ' ) ;
Notice that the loop is now controlled by the if statement. Essentially, break allows us to check
when to stop the loop using code inside of the loop itself! Break is often used in conjunction with
an if statement.
COP2271 Notes
while loops
3
Alternatively, the continue statement skips the rest of the current loop and begins the next
iteration. This allows us to skip lines of code inside a loop if needed. Try the following example
which only prints the positive numbers entered by the user.
u s e r V a l u e = −1;
while u s e r V a l u e ˜= 0
u s e r V a l u e = input ( ' P l e a s e e n t e r p o s i t i v e numbers ( 0 t o q u i t ) ! ' ) ;
i f userValue < 0
continue ;
end
f p r i n t f ( ' You p i c k e d p o s i t i v e number : %d \n ' , u s e r V a l u e ) ;
end