MATLAB Data Import/Export

MATLAB Data Import/Export

Guidance for MATLAB data I/O — correct patterns for delimiters, locales, format-specific quirks, and common error messages.

When to Use

• Reading or writing CSV, Excel, Parquet, or JSON files in MATLAB with readtable/writetable/detectImportOptions
• Troubleshooting data I/O errors (misleading messages, "file not found" variants)
• Reading data from URLs or authenticated REST API endpoints
• Importing non-English locale data (European decimals, semicolons)
• Validating imported data for silent corruption (NaN, 65535, type widening)
• Reading compressed files or JSON with non-identifier keys
• Reading or writing text files (use readlines/writelines, not fopen/fgetl/fprintf)
• Reading or writing XML files (use MAXP provider, not legacy JAXP)

When NOT to Use

• Large files that may not fit in memory, or choosing between tall arrays, datastores, and parallel workflows (use matlab-choose-big-data-solution skill)
• Database access via ODBC/JDBC — reading, writing, or querying relational databases (use matlab-use-database skill)
• SQL-based queries on large CSV/Parquet/JSON files for reduction before analysis (use matlab-use-duckdb skill)
• Vehicle data from MDF/MF4/BLF/ASC log files or CAN/LIN bus decoding (use matlab-import-export-vehicle-data skill)
• Vehicle network communication setup with CAN/CAN FD/J1939 (use matlab-use-vehicle-network skill)
• Tracking data import for sensor fusion workflows (use matlab-import-tracking-data skill)
• Medical image data — DICOM, NIfTI, or Analyze formats (use matlab-read-medical-data skill)
• Market or financial data feeds (use matlab-access-datafeed skill)
• Simulink data logging or signal I/O (use Simulink-specific workflows)
• Image or audio file I/O (imread, audioread — different domain)
• Streaming or real-time data acquisition (use Data Acquisition Toolbox)
• File system operations, path manipulation, or folder traversal

General principles

• Validate after import. Append 2-3 assertion-style checks that verify the imported data matches expectations.
• Always set TextType="string" for text and Excel imports — the string type is more efficient and easier to work with than char or cell arrays of character vectors. This only needs to be set for delimited text and spreadsheet formats; XML, JSON, and other formats already return strings by default.
• Specify FileType when reading from URLs that lack a recognizable extension — MIME type detection handles some cases, but API endpoints and non-standard URLs still need explicit format (see Topic 4). When using detectImportOptions with readtable, pass FileType on detectImportOptions — readtable does not accept FileType when an import options object is provided.
• Handle missing value placeholders at read time — use TreatAsMissing on readtable instead of calling standardizeMissing after import.
• Read directly from compressed files — readtable, readmatrix, readtimetable, and other read functions accept ZIP, GZ, and TAR file paths directly without manual extraction (R2025a+).
• European CSVs use ; as delimiter because , is the decimal separator — set both Delimiter and DecimalSeparator when contextual cues suggest European-format data.

Topics