---
tags: Matlab Workshop
---

# Lesson 4: Functions

## Functions

**Functions** is a block of code which only runs when it is called. We can pass data (called arguments/parameters) into a function and the function can return some data as results.


In Matlab, we can construct a function using the ```function``` keyword.

:::info

Basic syntax of a **function**

function [out1, out2,... outN] = function_name(in1, in2, ...inN)
    equation
    expression
end

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## Example

function y = double_value(x) 
% The function "double_value" takes "x" as input, and output "y"
    y = 2 * x;
end

When creating a user-defined function in MATLAB, **save the file in the current directory** or **add its directory to MATLAB's search path** so MATLAB can find and use the function.

>> double_value(3)

% output: 6

In MATLAB, **input parameters and output results** can be **any data type**, allowing functions to handle different types of data flexibly.

function out = flexibleFunction(input)
    if isnumeric(input)
        out = input + 10;        % If input is numeric, add 10
    elseif ischar(input)
        out = strcat("Hello, ", input);  % If input is a string, concatenate
    elseif iscell(input)
        out = length(input);     % If input is a cell array, return its length
    else
        out = "Unsupported type"; % Handle unsupported data types
    end
end

>> result1 = flexibleFunction(5)       % Numeric input
result1 =
    15

>> result2 = flexibleFunction('John')  % String input
result2 =
    "Hello, John"

>> result3 = flexibleFunction({1, 2, 3})  % Cell array input
result3 =
    3

## Basics of Function Declaration

1. **Functions with no inputs.**

function names = get_weekday_names()
% or function names = get_weekday_names
    names = ["Monday", "Tuesday", "Wednesday", "Thurdays", "Friday", "Saturday", "Sunday"];
end

>> names = get_weekday_names();

2. **Functions with no outputs.**

function print_info(name, age, gender)
% or function [] = print_info(name, age, gender)
    fprintf("%s is a %d-year-old %s\n", name, age, gender);
end

>> print_info("Amy", 25, "girl"); 
% Amy is a 25-year-old girl

>> print_info("Tom", 30); 
% Error: Not enough input arguments.

3. **Multiple inputs and outputs.** We need to use square brackets for the multiple outputs.

function [out1, out2] = get_min_max(in)
    out1 = min(in);
    out2 = max(in);
end

>> arr = [-1, 12, 3, 0];

>> [a, b] = get_min_max(arr);
>> disp(a);
>> disp(b);

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### Example

From our previous exercises, we can write a function that checks the number of mismatches between two sequences.

function [mismatches] = checkNucleotideSequences(seq1, seq2)
% This function compares two nucleotide sequences and counts mismatches.
% Input: seq1, seq2 - two strings representing nucleotide sequences
% Output: mismatches - number of mismatches

    % Initialize counter for mismatches
    mismatches = 0;
    
    % Ensure both sequences are the same length
    if length(seq1) ~= length(seq2)
        error('Sequences must be of equal length');
    end
    
    % Loop through each nucleotide in the sequences
    for i = 1:length(seq1)
        if seq1(i) ~= seq2(i)
            mismatches = mismatches + 1; % Increment mismatches if nucleotides are different
        end
    end
end

Run the function and see the results.

>> [mismatchCount] = checkNucleotideSequences('AGTCGA', 'AGTCCA')

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:::info

### Let's do an Exercise

Write a function that creates a matrix with dimensions (length of seq1+1)×(length of seq2+1), initialized with zeros. Set the element at position (i+1,j+1) to 1 if the character at position i in seq1 matches the character at position j in seq2.

**seq1**: AGTCGA
**seq2**: AGTGCA

function 
    % b = zeros(3, 7)　
    First create a zeros matrix
    % for loop
    Set the element at (i+1,j+1) to 1 if position i in seq 1 == position j in seq2
end

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:::spoiler

function match_matrix = create_match_matrix(seq1, seq2)
    % Create a matrix with dimensions (length(seq1)+1) x (length(seq2)+1)
    % Each element starts as 0, and if seq1(i) == seq2(j), the element at
    % (i+1, j+1) is set to 1.

    % Get lengths of sequences
    len1 = length(seq1);
    len2 = length(seq2);

    % Initialize the matrix with zeros
    match_matrix = zeros(len1 + 1, len2 + 1);

    % Fill the matrix based on character matches
    for i = 1:len1
        for j = 1:len2
            if seq1(i) == seq2(j)
                match_matrix(i + 1, j + 1) = 1;  % Match found, set to 1
            end
        end
    end
end

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