# MATLAB Quick Reference

*Arrays, matrices, plotting, file I/O, control flow*

> Source: MATLAB Documentation (mathworks.com/help/matlab) · MIT

## Basics

### Command Window

```
x = 5;           % assign (semicolon suppresses output)
x = 5            % assign and display result
disp('Hello')    % print to console
clc              % clear command window
clear            % clear all variables
```

### Help & Info

```
help sin          % quick help for function
doc sin           % open documentation
who               % list variables in workspace
whos              % list with details (size, type)
```

### Operators

| Command | Description |
|---------|-------------|
| `+ - * / ^` | Arithmetic (matrix operations) |
| `.* ./ .^` | Element-wise operations |
| `== ~= < > <= >=` | Comparison operators |
| `&& \|\| ~` | Logical AND, OR, NOT (scalars) |
| `& \| ~` | Element-wise logical (arrays) |

## Variables & Types

### Numeric Types

```
x = 3.14;             % double (default)
n = int32(42);        % 32-bit integer
z = 2 + 3i;           % complex number
tf = true;            % logical
```

### Type Checking

| Command | Description |
|---------|-------------|
| `class(x)` | Return type name as string |
| `isa(x, 'double')` | Check if x is specific type |
| `isnumeric(x)` | True if numeric type |
| `ischar(x)` | True if character array |
| `islogical(x)` | True if logical type |

### Special Constants

| Command | Description |
|---------|-------------|
| `pi` | 3.14159... |
| `Inf / -Inf` | Infinity |
| `NaN` | Not a Number |
| `eps` | Machine epsilon (~2.2e-16) |
| `i / j` | Imaginary unit |

## Arrays & Matrices

### Creating Arrays

```
v = [1 2 3 4 5];         % row vector
v = [1; 2; 3];           % column vector
A = [1 2; 3 4];          % 2x2 matrix
r = 1:5;                 % [1 2 3 4 5]
r = 0:0.5:2;             % [0 0.5 1 1.5 2]
```

### Built-in Constructors

```
zeros(3)        % 3x3 of zeros
ones(2, 4)      % 2x4 of ones
eye(3)          % 3x3 identity
rand(2, 3)      % 2x3 uniform random
linspace(0,1,5) % 5 evenly spaced [0..1]
```

### Indexing & Slicing

```
A(2, 3)          % row 2, col 3
A(1, :)          % entire first row
A(:, 2)          % entire second column
A(1:2, 1:2)      % submatrix
A(end, :)        % last row
```

### Matrix Operations

| Command | Description |
|---------|-------------|
| `A'` | Transpose (conjugate) |
| `A.'` | Transpose (no conjugate) |
| `inv(A)` | Matrix inverse |
| `det(A)` | Determinant |
| `eig(A)` | Eigenvalues and eigenvectors |
| `A \ b` | Solve Ax = b |
| `size(A)` | Dimensions [rows cols] |
| `numel(A)` | Total number of elements |

## Control Flow

### if / elseif / else

```
if x > 0
    disp('positive')
elseif x == 0
    disp('zero')
else
    disp('negative')
end
```

### for & while

```
for i = 1:10
    fprintf('i = %d\n', i);
end
while x > 0
    x = x - 1;
end
```

### switch

```
switch grade
    case 'A'
        disp('Excellent')
    case {'B', 'C'}
        disp('Good')
    otherwise
        disp('Try harder')
end
```

### Loop Control

| Command | Description |
|---------|-------------|
| `break` | Exit the innermost loop |
| `continue` | Skip to next iteration |
| `return` | Exit function immediately |

## Functions

### Function File

```
% Save as myfunc.m
function result = myfunc(x, y)
    result = x.^2 + y.^2;
end
```

### Multiple Outputs

```
function [mn, mx] = minmax(v)
    mn = min(v);
    mx = max(v);
end
[lo, hi] = minmax([3 1 4 1 5]);
```

### Anonymous Functions

```
f = @(x) x.^2 + 1;
f(3)                    % returns 10
g = @(x,y) x + y;
arrayfun(f, [1 2 3])    % apply to each element
```

### Useful Built-ins

| Command | Description |
|---------|-------------|
| `sum(v)` | Sum of elements |
| `mean(v)` | Mean value |
| `max(v) / min(v)` | Maximum / minimum |
| `sort(v)` | Sort ascending |
| `find(v > 3)` | Indices where condition is true |
| `length(v)` | Length of vector |

## Plotting

### 2D Plots

```
x = 0:0.1:2*pi;
plot(x, sin(x), 'r-', 'LineWidth', 2)
xlabel('x'); ylabel('sin(x)')
title('Sine Wave'); grid on
legend('sin(x)')
```

### Multiple Plots

```
hold on
plot(x, sin(x), 'b-')
plot(x, cos(x), 'r--')
hold off
subplot(1,2,1); plot(x, sin(x))
subplot(1,2,2); plot(x, cos(x))
```

### Other Plot Types

| Command | Description |
|---------|-------------|
| `bar(x, y)` | Bar chart |
| `histogram(data)` | Histogram |
| `scatter(x, y)` | Scatter plot |
| `pie(data)` | Pie chart |
| `surf(X, Y, Z)` | 3D surface plot |
| `imagesc(A)` | Display matrix as image |

### Save Figure

```
saveas(gcf, 'plot.png')
exportgraphics(gcf, 'plot.pdf')
```

## File I/O

### Text Files

```
data = readmatrix('data.csv');
writematrix(A, 'output.csv')
T = readtable('data.csv');
writetable(T, 'output.csv')
```

### MAT Files

```
save('workspace.mat')           % save all variables
save('data.mat', 'x', 'y')     % save specific vars
load('data.mat')                % load into workspace
S = load('data.mat');           % load into struct
```

### Low-Level File I/O

```
fid = fopen('log.txt', 'w');
fprintf(fid, 'Value: %f\n', 3.14);
fclose(fid);
lines = readlines('log.txt');
```

## String Operations

### String vs Char Array

```
s = "Hello";        % string (double quotes)
c = 'Hello';        % char array (single quotes)
s + " World"        % "Hello World" (string)
[c, ' World']       % 'Hello World' (char concat)
```

### String Functions

| Command | Description |
|---------|-------------|
| `strlength(s)` | Length of string |
| `upper(s) / lower(s)` | Case conversion |
| `contains(s, pat)` | True if pattern found |
| `replace(s, old, new)` | Replace substring |
| `split(s, delim)` | Split into array |
| `join(arr, delim)` | Join string array |
| `strip(s)` | Remove leading/trailing whitespace |

### Formatting

```
sprintf('x = %.2f', 3.14159)   % "x = 3.14"
fprintf('i = %d\n', 42)         % print to console
num2str(3.14)                   % number to string
str2double("3.14")              % string to number
```

## Cell & Struct

### Cell Arrays

```
C = {1, 'hello', [1 2 3]};  % mixed types
C{2}                         % access: 'hello'
C{end+1} = true;            % append element
cellfun(@length, C)          % apply func to each
```

### Structs

```
s.name = 'Alice';
s.age = 30;
s.scores = [90 85 92];
fieldnames(s)           % {'name','age','scores'}
rmfield(s, 'age')      % remove field
```

### Struct Arrays

```
people(1).name = 'Alice'; people(1).age = 30;
people(2).name = 'Bob';   people(2).age = 25;
{people.name}           % {'Alice', 'Bob'}
[people.age]            % [30, 25]
```

## Common Patterns

### Vectorized Operations

```
% Avoid loops — use vectorization
v = 1:1000;
result = sum(v.^2);          % fast
idx = v(v > 500 & v < 600);  % logical indexing
```

### Table Operations

```
T = table([25;30], ["A";"B"], 'VariableNames', ...
          {'Age','Grade'});
T.Age             % access column
T(T.Age > 25, :)  % filter rows
```

### Error Handling

```
try
    result = riskyFunction(x);
catch ME
    fprintf('Error: %s\n', ME.message);
end
```

### Timing Code

```
tic
heavyComputation();
toc              % prints elapsed time
```