String formatting (sprintf) in MATLAB - Step-by-Step Execution

Concept Flow - String formatting (sprintf)

Start

↓

Call sprintf with format and values

↓

Parse format string

↓

Replace placeholders with values

↓

Return formatted string

↓

End

The sprintf function takes a format string and values, replaces placeholders with those values, and returns the formatted string.

Execution Sample

MATLAB

str = sprintf('Hello %s, your score is %d.', 'Alice', 95);

Formats a string inserting a name and a score into placeholders.

Execution Table

Step	Action	Format String	Values	Resulting String
1	Call sprintf	'Hello %s, your score is %d.'	['Alice', 95]	'' (empty, before processing)
2	Parse format string	'Hello %s, your score is %d.'	['Alice', 95]	'' (placeholders identified)
3	Replace %s with 'Alice'	'Hello %s, your score is %d.'	['Alice', 95]	'Hello Alice, your score is %d.'
4	Replace %d with 95	'Hello %s, your score is %d.'	['Alice', 95]	'Hello Alice, your score is 95.'
5	Return formatted string	'Hello %s, your score is %d.'	['Alice', 95]	'Hello Alice, your score is 95.'

💡 All placeholders replaced, formatted string returned.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
str	''	'Hello Alice, your score is %d.'	'Hello Alice, your score is 95.'	'Hello Alice, your score is 95.'

Key Moments - 2 Insights

Why does the %s placeholder get replaced before the %d placeholder?

sprintf processes placeholders in order from left to right in the format string, so %s is replaced first as shown in execution_table step 3.

What happens if the number of values does not match the number of placeholders?

sprintf will throw an error or produce unexpected output because each placeholder expects a corresponding value, as implied by the exit_note.

Visual Quiz - 3 Questions

Look at the execution table, what is the string after step 3?

A 'Hello Alice, your score is 95.'

B 'Hello %s, your score is 95.'

C 'Hello Alice, your score is %d.'

D '' (empty string)

💡 Hint

Check the 'Resulting String' column at step 3 in the execution_table.

At which step does sprintf return the final formatted string?

A Step 4

B Step 5

C Step 2

D Step 1

💡 Hint

Look for the step labeled 'Return formatted string' in the execution_table.

If the value 95 was replaced with 100, how would the final string change?

A 'Hello Alice, your score is 100.'

B 'Hello Alice, your score is 95.'

C 'Hello %s, your score is 100.'

D 'Hello %s, your score is %d.'

💡 Hint

Refer to the variable_tracker and imagine changing the value replacing %d.

Concept Snapshot

sprintf(format, values) formats a string by replacing placeholders like %s (string) and %d (integer) with given values.
Placeholders are processed left to right.
Returns the formatted string without printing.
Useful for building strings with variables.
Example: sprintf('Hi %s, %d', 'Bob', 10) -> 'Hi Bob, 10'.

Full Transcript

This visual execution shows how MATLAB's sprintf function works step-by-step. First, sprintf is called with a format string containing placeholders and values to insert. It parses the format string, identifies placeholders like %s and %d, then replaces them one by one with the provided values in order. The variable 'str' changes from empty to the final formatted string 'Hello Alice, your score is 95.' The process ends when all placeholders are replaced and the string is returned. Common confusions include the order of replacement and matching values to placeholders. The quizzes test understanding of intermediate string states and final output.

Practice

1. What does the sprintf function in MATLAB do?

easy

A. Creates a formatted string by inserting values into a template

B. Reads input from the user

C. Plots data on a graph

D. Saves variables to a file

Solution

1. Step 1: Understand the purpose of sprintf

   sprintf is used to create text with values inserted in a specific format.

2. Step 2: Compare options to function behavior

   Only Creates a formatted string by inserting values into a template describes creating formatted strings, which matches sprintf's role.

3. Final Answer:

   Creates a formatted string by inserting values into a template -> Option A

4. Quick Check:

   sprintf formats strings = A [OK]

Hint: Remember: sprintf formats text, not input or files [OK]

Common Mistakes:

• Confusing sprintf with input functions
• Thinking sprintf plots graphs
• Assuming sprintf saves files

2. Which of the following is the correct syntax to format an integer variable num using sprintf in MATLAB?

easy

A. str = sprintf('%s', num);

B. str = sprintf('%c', num);

C. str = sprintf('%d', num);

D. str = sprintf('%f', num);

Solution

1. Step 1: Identify format specifier for integers

   The format specifier %d is used for integers in sprintf.

2. Step 2: Match specifier with variable type

   Since num is an integer, %d is correct; others are for strings, floats, or characters.

3. Final Answer:

   str = sprintf('%d', num); -> Option C

4. Quick Check:

   Integer format = %d = D [OK]

Hint: Use %d for integers, %s for strings, %f for floats [OK]

Common Mistakes:

• Using %s for numbers
• Using %f for integers
• Using %c for non-character data

3. What is the output of the following MATLAB code?

str = sprintf('Value: %0.2f', 3.14159); disp(str);

medium

A. Value: 3.1

B. Value: 3.14

C. Value: 3.14159

D. Value: 03.14

Solution

1. Step 1: Understand the format specifier %0.2f

   This means a floating-point number with 2 digits after the decimal point, padded with zeros if needed.

2. Step 2: Apply formatting to 3.14159

   Rounded to 2 decimals, 3.14159 becomes 3.14, no extra zero padding before the number.

3. Final Answer:

   Value: 3.14 -> Option B

4. Quick Check:

   %.2f rounds float to 2 decimals = Value: 3.14 [OK]

Hint: %.2f rounds float to 2 decimals in sprintf [OK]

Common Mistakes:

• Expecting full decimal without rounding
• Confusing zero padding before decimal
• Ignoring rounding rules

4. Identify the error in this MATLAB code:

str = sprintf('Name: %s, Age: %d', 25, 'John');

medium

A. sprintf cannot format multiple variables

B. Missing semicolon at the end

C. Incorrect format specifier for string

D. The order of variables does not match format specifiers

Solution

1. Step 1: Check format specifiers and variable order

   The format expects a string first (%s) then an integer (%d), but variables are 25 (int) then 'John' (string).

2. Step 2: Identify mismatch cause

   Variables are swapped, causing wrong data types for specifiers.

3. Final Answer:

   The order of variables does not match format specifiers -> Option D

4. Quick Check:

   Variable order must match specifiers = A [OK]

Hint: Match variable order exactly to format specifiers [OK]

Common Mistakes:

• Swapping variable order
• Ignoring type mismatch
• Assuming sprintf auto-corrects order

5. You want to create a formatted string showing a product name and price with 2 decimals, like "Product: Laptop, Price: $999.99". Which sprintf call correctly formats name = 'Laptop' and price = 999.987?

hard

A. sprintf('Product: %s, Price: $%0.2f', name, price)

B. sprintf('Product: %s, Price: $%d', price, name)

C. sprintf('Product: %s, Price: $%f', name, price)

D. sprintf('Product: %s, Price: $%.2d', name, price)

Solution

1. Step 1: Identify correct format specifiers

   %s for string name, %0.2f for float price with 2 decimals.

2. Step 2: Check variable order and formatting

   sprintf('Product: %s, Price: $%0.2f', name, price) uses correct order and formats price to 2 decimals, rounding 999.987 to 999.99.

3. Final Answer:

   sprintf('Product: %s, Price: $%0.2f', name, price) -> Option A

4. Quick Check:

   String + float with 2 decimals = B [OK]

Hint: Use %s for strings and %0.2f for 2-decimal floats [OK]

Common Mistakes:

• Swapping variable order
• Using %d for floats
• Using incorrect precision specifier