Control Flow

## Learning Objectives - Use conditional statements (if-else, switch) - Implement loops (for, while) - Control loop execution (break, continue) - Handle errors appropriately ## Conditional Statements ### if Statement ```matlab x = 10; if x > 0 disp('x is positive'); end ``` ### if-else Statement ```matlab x = -5; if x >= 0 disp('x is non-negative'); else disp('x is negative'); end ``` ### if-elseif-else Statement ```matlab score = 85; if score >= 90 grade = 'A'; elseif score >= 80 grade = 'B'; elseif score >= 70 grade = 'C'; elseif score >= 60 grade = 'D'; else grade = 'F'; end disp(grade); % 'B' ``` ### Nested if Statements ```matlab x = 10; y = 20; if x > 0 if y > 0 disp('Both x and y are positive'); else disp('x positive, y non-positive'); end else disp('x is not positive'); end ``` ## switch Statement ### Basic switch ```matlab day = 'Monday'; switch day case 'Monday' disp('Start of work week'); case 'Friday' disp('End of work week'); case {'Saturday', 'Sunday'} disp('Weekend!'); otherwise disp('Regular day'); end ``` ### Multiple Values per Case ```matlab month = 2; switch month case {1, 3, 5, 7, 8, 10, 12} days = 31; case {4, 6, 9, 11} days = 30; case 2 days = 28; end ``` ### switch vs if-else ```matlab % switch is often cleaner for discrete values % if-else is better for ranges or complex conditions % Example with ranges (if-else is better) value = 75; if value >= 90 result = 'A'; elseif value >= 80 result = 'B'; else result = 'C'; end ``` ## for Loops ### Basic for Loop ```matlab % Loop through a range for i = 1:5 fprintf('i = %d\n', i); end ``` ### Loop Through Array ```matlab arr = [10, 20, 30, 40, 50]; sum = 0; for val = arr sum = sum + val; end disp(sum); % 150 ``` ### Nested for Loops ```matlab % Print multiplication table for i = 1:5 row = ''; for j = 1:5 row = [row, sprintf('%4d', i*j)]; end disp(row); end ``` ### Loop with Step ```matlab % Even numbers from 2 to 10 for i = 2:2:10 disp(i); % 2, 4, 6, 8, 10 end % Backwards for i = 10:-1:1 disp(i); % 10, 9, 8, ..., 1 end ``` ### Loop Index Variations ```matlab A = [1, 2, 3; 4, 5, 6]; % Loop through row and column indices [m, n] = size(A); for i = 1:m for j = 1:n fprintf('A(%d,%d) = %d\n', i, j, A(i,j)); end end ``` ## while Loops ### Basic while Loop ```matlab count = 0; while count < 5 disp(count); count = count + 1; end ``` ### while with Condition ```matlab % Find first power of 2 greater than 1000 value = 1; while value <= 1000 value = value * 2; end disp(value); % 1024 ``` ### Infinite Loop Prevention ```matlab % Always ensure loop condition will eventually be false maxIterations = 100; count = 0; x = 1; while abs(x) > 1e-10 && count < maxIterations x = x / 2; count = count + 1; end if count == maxIterations disp('Max iterations reached'); end ``` ## break and continue ### break Statement ```matlab % Find first even number in array arr = [1, 3, 5, 7, 8, 9, 10]; for val = arr if mod(val, 2) == 0 disp(['First even: ', num2str(val)]); break; end end ``` ### continue Statement ```matlab % Print odd numbers only for i = 1:10 if mod(i, 2) == 0 continue; % Skip even numbers end disp(i); % 1, 3, 5, 7, 9 end ``` ### break vs continue ```matlab % break - exits the loop entirely % continue - skips to next iteration ``` ## Loop Control Examples ### Sum Until Condition ```matlab % Sum array elements until sum exceeds 100 arr = [10, 20, 30, 40, 50, 60]; sum = 0; for i = 1:length(arr) sum = sum + arr(i); if sum > 100 break; end end fprintf('Sum: %d, Elements used: %d\n', sum, i); ``` ### Skip Invalid Values ```matlab % Process only positive values data = [-5, 10, -3, 20, -1, 30]; validSum = 0; for val = data if val <= 0 continue; end validSum = validSum + val; end disp(validSum); % 60 ``` ## Vectorization (Alternative to Loops) ### Replace Loops with Vector Operations ```matlab % Slow: Loop approach n = 1000; result = zeros(1, n); for i = 1:n result(i) = i^2; end % Fast: Vectorized approach result = (1:n).^2; ``` ### Logical Indexing ```matlab % Find elements greater than 5 arr = [1, 6, 3, 8, 2, 9]; % Loop approach large = []; for val = arr if val > 5 large = [large, val]; end end % Vectorized approach large = arr(arr > 5); ``` ## Error Handling ### try-catch ```matlab try result = 10 / 0; disp('Division succeeded'); catch ME disp('Error occurred:'); disp(ME.message); end ``` ### Multiple catch Blocks ```matlab try x = notAFunction(); catch ME if strcmp(ME.identifier, 'MATLAB:UndefinedFunction') disp('Function not found'); else disp(['Error: ', ME.message]); end end ``` ## Summary - Use `if-elseif-else` for conditional logic - Use `switch` for discrete value matching - Use `for` loops when iteration count is known - Use `while` loops when condition-based iteration needed - Use `break` to exit loops early - Use `continue` to skip iterations - Prefer vectorization over loops for performance - Use `try-catch` for error handling