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What are Loops?. Simply put, loops allow the same segment of code to be run many times Whether to execute the segment again or not is determined by some truth conditionThis condition may be hidden (we will see this in for" loops). First, an interlude Debugging. MATLAB allows for breakpointsP
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1. CS 101 – 010
Lecturer: Daniel J. Boston
GITC 2315-C
wednesday friday
1:00-2:30pm 2:30-4pm Computer Programming and Problem Solving Loops (and more)
2. What are Loops? Simply put, loops allow the same segment of code to be run many times
Whether to execute the segment again or not is determined by some truth condition
This condition may be hidden (we will see this in “for” loops)
3. First, an interlude – Debugging MATLAB allows for breakpoints
Provides a way to “pause” execution and check the current state of your program’s variables
Define each breakpoint by right-clicking a line of code and choosing “Set/Clear Breakpoint” from the menu
A red circle will appear besides the breakpoint lines
Start your program using “F5” or the Run button
Execution will pause at each breakpoint
“Step” through the program by pressing “F10”
This executes one line at a time
4. Debugging MATLAB also allows for conditional breakpoints
Right-click and choose “Set/Modify Conditional Breakpoint” to set one
Enter a “truth statement” or expression involving the variables used and defined inside your program up to that point
Can also set MATLAB to pause on errors
Debug menu -- “Stop if Errors/Warning” option
5. Debugging A final MATLAB resource is the Code Analyzer
This was formerly M-Lint
Analyzes your program and tries to identify potential errors, poor usage, or obsolete features
Click the “Tools” – “Code Analyzer” – “Show Code Analyzer Report” button to access
6. Introducing: while Loop While loops have the following form:
while expression
...
... Code Block
...
end
If expression evaluates to true, the Code Block is re-executed
If expression evaluates to false, execution continues after end (Code Block is not re-executed)
7. Introducing: for Loop For loops have the following form:
for index_variable = array_expression
...
... Code Block
...
end
array_expression should be an array
One at a time, index_variable is set to columns drawn from array_expression
When no more columns exist, program execution continues after end
8. Introducing: for Loop The execution of a for loop follows this pattern:
MATLAB generates an array using array_expression
Before executing Code Block, index_variable is set to (roughly) array_expression(:,1) or the first column of array_expression
After Code Block has been executed, index_variable becomes the next column of array_expression
Step 3 is repeated until no columns remain in array_expression
9. Some notes on for loops Similar to if-end statements, it is a good idea to indent the Code Block of loops
Do not modify index_variable inside the for loop!
If Code Block uses an array, best to pre-allocate it
E.g. my_array = zeros(1,200);
It is almost always faster to perform vector calculations using vector operations instead of loops
This is known as “vectorization” and is one of MATLAB’s greatest features
10. Introducing: Loop Controls break;
“Breaks out” of the loop, regardless of terminating condition’s status
Allows you to exit a loop early based on some specific condition
continue;
Skips the remaining lines of Code Block and starts again at the top of Code Block
In for loops, causes index_variable to immediately assume the next column value of array_expression
11. Nested Loops Similar to if-end, you can nest loops
for index_var1 = array_expr1 ...
for index_var2 = array_expr2
... Inner Outer
... Code Code
... Block Block
end % “end” of inner loop
...
end % “end” of outer loop
12. Putting it together: Redux y = zeros(20,20);
for i = 1:size(y,1) % traverse rows
for j = 1:size(y,2) % traverse columns
if j < i
continue; % “skip” low order columns
end
y(i, j) = i + j;
end
end % What does this produce?
13. Putting it together: Redux y = zeros(20,20); i = 10; j = 10;
while sum( sum( y ) ) < 200 % what’s this?!
y(i, j) = 1; i = i + round( rand * 2 - 1 ); j = j + round( rand * 2 - 1 ); % fancy!
if ( i < 1 || j < 1 || i > size(y,1) || j > size(y,2) )
break;
end
end
for l = 1:size(y,1) % What kind of result do we get?
for m = 1:size(y,2)
fprintf('%.0f', y(l, m) );
end
fprintf('\n');
end
fprintf('\n');
14. Vectorization Part of the power of MATLAB is the ability to vectorize
This is process of using arrays and array math instead of loops
It takes some practice, but we were vectorizing before we learned loops!
One powerful technique in vectorization is the logical array mask
It is an array of ones and zeros used to selectively choose elements in another array
15. Vectorization % We want to get the square root of
% numbers larger than 5.
A = [1 2 3; 4 5 6; 7 8 9];
B = A > 5;
> 5 =
16. Achieving our problem statement with loops for i = 1:size(A,1)
for j = 1:size(A,2)
if A(i, j) > 5
A(i, j) = sqrt( A(i, j) );
end
end
end
17. Achieving with vectorization A(B) = sqrt( A(B) );
A B
B acts as an operation mask on A
