Reading text files (readtable, textscan) in MATLAB - Step-by-Step Execution

Concept Flow - Reading text files (readtable, textscan)

Open file with fopen

↓

Choose method: readtable or textscan

↓

readtable reads entire table

↓

Store data in table

↓

Close file with fclose

This flow shows how MATLAB opens a file, reads data using readtable or textscan, stores it, and then closes the file.

Execution Sample

MATLAB

fid = fopen('data.txt');
data = textscan(fid, '%s %d %f');
fclose(fid);

This code opens a text file, reads three columns (string, integer, float) using textscan, then closes the file.

Execution Table

Step	Action	Input/Format	Result	Notes
1	fopen	'data.txt'	File ID (fid) assigned	File opened for reading
2	textscan	'%s %d %f'	Cell array with 3 columns	Reads string, int, float columns
3	fclose	fid	File closed	Releases file resource
4	End	-	-	File reading complete

💡 File closed after reading all data with textscan

Variable Tracker

Variable	Start	After fopen	After textscan	After fclose
fid	undefined	file ID (e.g. 3)	file ID (unchanged)	closed (no longer valid)
data	undefined	undefined	cell array with data	cell array with data (unchanged)

Key Moments - 3 Insights

Why do we need to use fclose after fopen?

fclose releases the file resource so other programs or MATLAB can access the file later. See execution_table step 3.

What does '%s %d %f' mean in textscan?

It tells textscan to read a string (%s), then an integer (%d), then a floating-point number (%f) from each line. See execution_table step 2.

Why choose readtable over textscan?

readtable automatically reads the whole file into a table with column names, easier for structured data. textscan is more flexible for custom formats.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'fid' after step 1?

A A file ID number (e.g., 3)

B Undefined

C Cell array with data

D Closed file

💡 Hint

Check execution_table row 1 under Result column

At which step is the file closed according to the execution_table?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for 'fclose' action in execution_table

If the format string in textscan changes to '%f %f %f', what changes in the variable 'data'?

A data will be empty

B data will contain three columns of floating-point numbers

C data will contain strings only

D data will contain integers only

💡 Hint

Refer to key_moments about format specifiers and execution_table step 2

Concept Snapshot

Open file with fopen to get file ID
Use readtable for automatic table import
Or use textscan with format specifiers for custom reading
Close file with fclose to free resource
Data stored as table or cell arrays

Full Transcript

This visual execution shows how MATLAB reads text files using fopen, readtable, and textscan. First, fopen opens the file and returns a file ID. Then, readtable or textscan reads the data. readtable reads the whole file into a table automatically, while textscan reads data according to a format string like '%s %d %f'. After reading, fclose closes the file to release it. Variables like fid and data change during these steps. Key points include why fclose is needed, what format specifiers mean, and when to use readtable versus textscan.

Practice

1. What is the main advantage of using readtable in MATLAB to read text files?

easy

A. It reads files without opening them first.

B. It requires manual specification of each data type.

C. It only reads numeric data, ignoring text.

D. It automatically loads data into a table with proper formatting.

Solution

1. Step 1: Understand readtable functionality

   readtable is designed to load text files directly into tables, automatically detecting and formatting columns.

2. Step 2: Compare options

   It automatically loads data into a table with proper formatting. correctly describes this automatic loading and formatting. Other options describe incorrect behaviors.

3. Final Answer:

   It automatically loads data into a table with proper formatting. -> Option D

4. Quick Check:

   readtable = automatic table loading [OK]

Hint: Remember: readtable = easy table import [OK]

Common Mistakes:

• Thinking readtable needs manual type input
• Assuming readtable only reads numbers
• Believing files don't need opening before reading

2. Which of the following is the correct syntax to open a text file named data.txt for reading in MATLAB?

easy

A. fileID = fopen('data.txt', 'r');

B. fileID = open('data.txt', 'r');

C. fileID = readfile('data.txt');

D. fileID = textscan('data.txt', 'r');

Solution

1. Step 1: Recall file opening function

   In MATLAB, fopen is used to open files with a mode like 'r' for reading.

2. Step 2: Check each option

   fileID = fopen('data.txt', 'r'); uses fopen correctly. fileID = open('data.txt', 'r'); uses a non-existent open function. fileID = readfile('data.txt'); uses readfile which is not a MATLAB function. fileID = textscan('data.txt', 'r'); misuses textscan which reads from an open file, not opens it.

3. Final Answer:

   fileID = fopen('data.txt', 'r'); -> Option A

4. Quick Check:

   Open file = fopen(filename, 'r') [OK]

Hint: Use fopen to open files before reading [OK]

Common Mistakes:

• Using open instead of fopen
• Trying to read file directly without fopen
• Confusing textscan with fopen

3. Given the following MATLAB code snippet, what will be the output stored in data?

fileID = fopen('numbers.txt', 'r');
data = textscan(fileID, '%d %f');
fclose(fileID);

Assuming numbers.txt contains:

10 3.14
20 2.71
30 1.62

medium

A. A 1x2 cell array with first cell [10;20;30] and second cell [3.14;2.71;1.62]

B. A 3x2 numeric matrix with all numbers

C. A single numeric array combining all values

D. An error because textscan cannot read mixed data

Solution

1. Step 1: Understand textscan output format

   textscan returns a cell array where each cell contains a column of data matching the format specifiers.

2. Step 2: Analyze the format and file content

   Format '%d %f' reads an integer and a float per line. The file has 3 lines, so the first cell will be [10;20;30] and the second cell [3.14;2.71;1.62].

3. Final Answer:

   A 1x2 cell array with first cell [10;20;30] and second cell [3.14;2.71;1.62] -> Option A

4. Quick Check:

   textscan returns cell array columns [OK]

Hint: textscan returns cells, each for one format column [OK]

Common Mistakes:

• Expecting a numeric matrix instead of cell array
• Thinking textscan merges all data into one array
• Assuming textscan cannot read mixed types

4. Consider this MATLAB code snippet:

fileID = fopen('data.txt', 'r');
data = textscan(fileID, '%s %d');

What is the main problem with this code?

medium

A. textscan cannot read strings and integers together.

B. The format specifier '%s %d' is invalid.

C. The file is not closed after reading.

D. fopen should be called with 'w' mode to read.

Solution

1. Step 1: Check file handling steps

   The code opens the file with fopen and reads with textscan, but does not close the file.

2. Step 2: Validate format and usage

   The format '%s %d' is valid for reading a string and an integer. textscan supports mixed types. fopen with 'r' is correct for reading.

3. Final Answer:

   The file is not closed after reading. -> Option C

4. Quick Check:

   Always fclose after fopen [OK]

Hint: Always fclose after reading files [OK]

Common Mistakes:

• Forgetting to close the file
• Thinking '%s %d' is invalid format
• Confusing fopen modes

5. You have a text file records.txt with mixed data: a name (string), age (integer), and height (float) on each line, separated by commas, like:

John,25,5.9
Anna,30,5.5
Mike,22,6.1

Which MATLAB approach correctly reads this file into a table with proper data types?

hard

A. Use readtable('records.txt', 'Delimiter', ',', 'Format', '%s%d%f')

B. Use readtable('records.txt') without specifying delimiter

C. Use textscan(fopen('records.txt', 'r'), '%s %d %f', 'Delimiter', ',') and fclose after

D. Use textscan(fopen('records.txt'), '%s %d %f') without closing file

Solution

1. Step 1: Identify correct reading method for mixed CSV data into table

   readtable automatically detects common delimiters like commas and loads data into a table with inferred types (string for names, double for numbers).

2. Step 2: Compare options

   Use readtable('records.txt') without specifying delimiter: readtable('records.txt') auto-detects comma delimiter. Use readtable('records.txt', 'Delimiter', ',', 'Format', '%s%d%f') uses invalid 'Format' parameter (readtable lacks it). Use textscan(fopen('records.txt', 'r'), '%s %d %f', 'Delimiter', ',') and fclose after: textscan syntax invalid ('Delimiter' not for fid), returns cell array not table. Use textscan(fopen('records.txt'), '%s %d %f') without closing file: textscan fails to parse commas (default whitespace), no fclose.

3. Final Answer:

   Use readtable('records.txt') without specifying delimiter -> Option B

4. Quick Check:

   readtable = auto delimiter + table types [OK]

Hint: readtable auto-detects CSV delimiters into tables [OK]

Common Mistakes:

• Using textscan expecting table output
• Adding invalid 'Format' to readtable
• Omitting delimiter handling in textscan