MATLAB避坑指南

Indexing

• - 1-based indexing — first element is A(1), not INLINECODE1
• INLINECODE2 keyword for last index — A(end), A(end-1), works in any dimension
• Linear indexing on matrices — A(5) accesses 5th element column-major order
• Logical indexing returns vector — A(A > 0) gives 1D result regardless of A's shape

Matrix vs Element-wise

• - * is matrix multiplication — .* for element-wise
• INLINECODE9 solves A*x = B — ./ for element-wise division
• INLINECODE12 is matrix power — .^ for element-wise power
• Forgetting the dot is silent bug — dimensions might accidentally match

Vector Shape Matters

• - Row vector: [1 2 3] or [1, 2, 3] — shape is 1×3
• Column vector: [1; 2; 3] — shape is 3×1
• Transpose with ' (conjugate) or .' (non-conjugate) — for complex, they differ
• INLINECODE19 between row and column gives scalar or matrix — depending on order

Array Preallocation

• - Growing arrays in loops is slow — preallocate: INLINECODE20
• INLINECODE21, ones, nan for preallocation — specify size upfront
• Cell arrays: cell(n, m) — preallocate cells too

Broadcasting

• - Implicit expansion since R2016b — A + b works if dimensions compatible
• Singleton dimensions expand — [1;2;3] + [10 20] gives 3×2
• Before R2016b needed bsxfun — legacy code may still use it

NaN Handling

• - NaN ~= NaN is true — use isnan() to check
• Most operations propagate NaN — sum([1 NaN 3]) is NaN
• Use 'omitnan' flag — sum(A, 'omitnan'), INLINECODE33

Cell Arrays vs Matrices

• - {} for cell arrays — hold mixed types, different sizes
• INLINECODE35 indexing returns cell — C(1) is 1×1 cell
• INLINECODE37 indexing extracts content — C{1} is the actual value
• Comma-separated list from C{:} — useful for function arguments

Common Mistakes

• - = for assignment, == for comparison — if x = 5 is error in MATLAB
• Semicolon suppresses output — forget it and flood command window
• INLINECODE43 removes all variables — use clearvars for selective, close all for figures
• INLINECODE46 and j are imaginary unit — don't use as loop variables, or reassign explicitly
• String vs char: "text" vs 'text' — double quotes are string arrays (R2017a+)

Functions

• - Anonymous functions: f = @(x) x^2 — quick inline functions
• Multiple outputs: [a, b] = func() — must capture or use ~ to ignore
• INLINECODE53/nargout for optional args — check how many inputs/outputs provided
• INLINECODE55/varargout for variable args — cell array of extra arguments

Debugging

• - dbstop if error — breakpoint on any error
• INLINECODE58 in code pauses execution — enter debug mode at that line
• INLINECODE59 shows variable sizes — size(A) for specific variable