Data Import and Export

## Learning Objectives - Import data from files - Export data to files - Work with different file formats - Handle missing data ## Text Files ### Reading Text Files ```matlab % Read entire file as text fid = fopen('data.txt', 'r'); text = fread(fid, '*char')'; fclose(fid); ``` ### Reading with fscanf ```matlab % Formatted reading fid = fopen('data.txt', 'r'); data = fscanf(fid, '%f %f %f', [3, inf]); fclose(fid); data = data'; % Transpose to get rows as records ``` ### Reading Line by Line ```matlab fid = fopen('data.txt', 'r'); while ~feof(fid) line = fgetl(fid); disp(line) end fclose(fid); ``` ## CSV Files ### Reading CSV ```matlab % Basic CSV reading data = readmatrix('data.csv'); % With header row data = readmatrix('data.csv', 'NumHeaderLines', 1); % Specify range data = readmatrix('data.csv', 'Range', 'A2:D10'); ``` ### Reading with readtable ```matlab % readtable returns a table T = readtable('data.csv'); % Access by variable name T.Name T.Age T.Height % Access by index T{1, :} ``` ### Writing CSV ```matlab % Write matrix M = [1, 2, 3; 4, 5, 6]; writematrix(M, 'output.csv'); % Write table T = table(['John'; 'Jane'], [25; 30], [170; 165], ... 'VariableNames', {'Name', 'Age', 'Height'}); writetable(T, 'people.csv'); ``` ## Excel Files ### Reading Excel ```matlab % Read entire sheet data = readmatrix('data.xlsx'); % Read specific sheet data = readmatrix('data.xlsx', 'Sheet', 'Sheet2'); % Read as table T = readtable('data.xlsx'); ``` ### Writing Excel ```matlab % Write matrix M = magic(5); writematrix(M, 'magic.xlsx'); % Write table writetable(T, 'output.xlsx'); % Append to existing file writetable(T, 'existing.xlsx', 'WriteMode', 'append'); ``` ## Delimited Files ### readmatrix Options ```matlab % Tab-delimited data = readmatrix('data.txt', 'Delimiter', '\t'); % Semicolon-delimited data = readmatrix('data.txt', 'Delimiter', ';'); % With multiple delimiters data = readmatrix('data.txt', 'Delimiter', {',', ';', '\t'}); ``` ### writematrix Options ```matlab % Tab-delimited writematrix(M, 'output.txt', 'Delimiter', '\t'); % Specify precision writematrix(M, 'output.txt', 'Precision', 4); ``` ## Native Format (.mat) ### Saving Variables ```matlab % Save all workspace variables save('data.mat') % Save specific variables save('data.mat', 'x', 'y', 'z') % Save with compression save('data.mat', 'x', 'y', '-v7') % Save with no compression save('data.mat', 'x', 'y', '-v6') ``` ### Loading Variables ```matlab % Load all variables load('data.mat') % Load specific variables load('data.mat', 'x', 'y') % Load to structure S = load('data.mat'); S.x S.y ``` ## Tables ### Creating Tables ```matlab % From arrays Age = [25, 30, 35]'; Height = [170, 165, 180]'; Name = {'John'; 'Jane'; 'Bob'}'; T = table(Name, Age, Height); ``` ### Table Operations ```matlab % Display first few rows head(T) % Display last few rows tail(T) % Get summary summary(T) % Add row T(end+1, :) = {'Mike', 28, 175}; % Remove row T(2, :) = []; ``` ### Table Indexing ```matlab % By row/column index T{1, 2} % Age of first person T{2, 'Name'} % Name of second person T.Age % Age column as array T(:, {'Name', 'Age'}) % Specific columns ``` ## Missing Data ### Detecting Missing Values ```matlab % NaN in numeric data data = [1, NaN, 3, 4, NaN]; isnan(data) % [0, 1, 0, 0, 1] any(isnan(data)) % true ``` ### Handling Missing Values ```matlab data = [1, NaN, 3, 4, NaN]; % Remove NaN clean = data(~isnan(data)); % [1, 3, 4] % Fill NaN with value filled = fillmissing(data, 'constant', 0); % Fill with interpolation filled = fillmissing(data, 'linear'); % Fill with previous value filled = fillmissing(data, 'previous'); ``` ### Table with Missing Data ```matlab T = readtable('data.csv'); % Find missing values ismissing(T) % Remove rows with missing T = rmmissing(T); % Fill missing T = fillmissing(T, 'constant', 0); ``` ## Low-Level File I/O ### File Opening Modes | Mode | Description | |------|-------------| | `r` | Read (default) | | `w` | Write (create/truncate) | | `a` | Append (create/write at end) | | `r+` | Read and write | | `w+` | Read and write (create/truncate) | ### Writing to File ```matlab fid = fopen('output.txt', 'w'); fprintf(fid, 'Hello, %s!\n', 'World'); fprintf(fid, 'Value: %d\n', 42); fclose(fid); ``` ### Reading from File ```matlab fid = fopen('data.txt', 'r'); name = fscanf(fid, '%s', 1); value = fscanf(fid, '%d', 1); fclose(fid); ``` ## Binary Files ### Reading Binary ```matlab fid = fopen('data.bin', 'r'); data = fread(fid, 100, 'float32'); fclose(fid); ``` ### Writing Binary ```matlab fid = fopen('data.bin', 'w'); fwrite(fid, myData, 'float32'); fclose(fid); ``` ## XML Files ```matlab % Parse XML tree = xmlread('data.xml'); % Navigate and extract data root = tree.getDocumentElement; child = root.getFirstChildElement; ``` ## Image Files ```matlab % Read image img = imread('image.png'); % Display image imshow(img) % Write image imwrite(img, 'output.png'); % Write JPEG with quality imwrite(img, 'output.jpg', 'Quality', 90); ``` ## Importing Examples ### Import Fixed-Width Text ```matlab % Define format format = '%2s %5f %4f %3f'; % Read with textscan fid = fopen('data.txt', 'r'); C = textscan(fid, format); fclose(fid); ``` ### Import with Header ```matlab opts = detectImportOptions('data.csv'); opts.VariableNamesLine = 1; opts.DataLines = 2:10; T = readtable('data.csv', opts); ``` ## Summary - `readmatrix()` for numeric data in delimited files - `readtable()` for tabular data with headers - `save()`/`load()` for MATLAB .mat files - Use `'Delimiter'` option for different separators - Handle missing data with `isnan()`, `rmmissing()`, `fillmissing()` - Use `fopen()`/`fprintf()`/`fscanf()` for custom file formats - `imread()`/`imwrite()` for image files