Intro to Computer Programming

3.2 Formatting Output and String Interpolation

4 min readLast Updated on August 9, 2024

Formatting output and string interpolation are essential skills for presenting data clearly in programming. These techniques allow you to control how information is displayed, from simple string formatting to complex date and currency representations.

By mastering these tools, you'll be able to create more readable and user-friendly output in your programs. Whether you're using format specifiers, f-strings, or specialized formatting methods, these skills will enhance your ability to communicate information effectively through your code.

Format Specifiers and Styles

Understanding Format Specifiers and Printf-style Formatting

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  • Format specifiers act as placeholders in strings to represent different data types
  • Printf-style formatting originated in C programming language, now widely used in many languages
  • Common format specifiers include:
    • %d for integers
    • %f for floating-point numbers
    • %s for strings
    • %c for characters
  • Printf-style formatting uses % followed by a letter to indicate data type
  • Syntax typically involves a string with format specifiers and a list of values to insert
  • Python example: print("My name is %s and I am %d years old" % ("Alice", 25))
  • C# example: Console.WriteLine("The price is ${0:F2}", 19.99);

Escape Sequences and Precision Formatting

  • Escape sequences represent special characters in strings
  • Common escape sequences include:
    • \n for newline
    • \t for tab
    • \\ for backslash
    • \" for double quote
  • Precision formatting controls the number of decimal places displayed for floating-point numbers
  • Syntax varies by language but often uses a dot followed by a number
  • Python example: %.2f displays two decimal places
  • JavaScript example: toFixed(2) method rounds to two decimal places

Alignment, Padding, and Advanced Formatting Techniques

  • Alignment determines the position of text within a specified width
  • Left alignment (default), right alignment, and center alignment options available
  • Padding adds extra characters (spaces or specified characters) to achieve desired width
  • Width specification often precedes the format specifier
  • Python example: %10s right-aligns a string in a 10-character wide field
  • C# example: {0,-10} left-aligns a value in a 10-character wide field
  • Advanced techniques include:
    • Zero-padding for numbers (%05d pads with leading zeros)
    • Thousands separators ({:,} in Python f-strings)
    • Scientific notation (%e or %E)

String Interpolation

Understanding String Interpolation and F-strings

  • String interpolation embeds expressions directly into string literals
  • F-strings (formatted string literals) introduced in Python 3.6
  • F-strings start with f or F before the opening quotation mark
  • Expressions enclosed in curly braces {} are evaluated at runtime
  • Python f-string example: f"The sum of {a} and {b} is {a + b}"
  • F-strings support formatting options within the curly braces
  • Can include method calls, arithmetic operations, and function calls
  • F-strings offer better performance compared to older formatting methods

Template Literals and Advanced Interpolation Techniques

  • Template literals in JavaScript use backticks (`) instead of quotes
  • Expressions embedded using ${} syntax
  • JavaScript example: `Hello, ${name}! You are ${age} years old.`
  • Multi-line strings supported without explicit newline characters
  • Tagged template literals allow custom parsing of template literals
  • Raw string literals (prefixed with r in Python) treat backslashes as literal characters
  • Nested interpolation possible in some languages
  • Python example of nested f-strings: f"The result is {f'{x:.{precision}f}'}" where precision is a variable

Specialized Formatting

Currency Formatting Techniques

  • Currency formatting ensures consistent display of monetary values
  • Locale-specific formatting considers regional differences in currency symbols and separators
  • Python's locale module provides currency formatting functions
  • locale.currency() function formats numbers as currency strings
  • JavaScript's Intl.NumberFormat object handles currency formatting
  • Options include specifying currency code, number of decimal places, and use of symbols
  • Python example using locale:
    import locale
    locale.setlocale(locale.LC_ALL, 'en_US.UTF-8')
    formatted_currency = locale.currency(1234.56, grouping=True)
    
  • JavaScript example using Intl.NumberFormat:
    const formatter = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' });
    const formattedCurrency = formatter.format(1234.56);
    

Date and Time Formatting Strategies

  • Date/time formatting converts datetime objects into human-readable strings
  • Python's datetime module provides extensive date and time manipulation capabilities
  • strftime() method formats date and time objects using format codes
  • Common format codes include:
    • %Y for four-digit year
    • %m for two-digit month
    • %d for two-digit day
    • %H for 24-hour clock hour
    • %M for minutes
    • %S for seconds
  • JavaScript's Date object includes methods for formatting components
  • toLocaleString() method provides locale-specific formatting
  • Python example using strftime():
    from datetime import datetime
    now = datetime.now()
    formatted_date = now.strftime("%Y-%m-%d %H:%M:%S")
    
  • JavaScript example using toLocaleString():
    const now = new Date();
    const formattedDate = now.toLocaleString('en-US', { dateStyle: 'full', timeStyle: 'long' });
    
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.