What does String formatter "<<<&" in Informix 4GL code mean?

Demystifying the Informix 4GL String Formatter: <<<

Explore the powerful yet often misunderstood <<< string formatting operator in Informix 4GL, understanding its syntax, usage, and practical applications for precise output control.

Informix 4GL, a powerful fourth-generation language for database application development, includes a unique set of operators for string manipulation and formatting. Among these, the <<< (triple less-than) operator stands out as a specialized tool for creating formatted output strings. This article delves into the functionality of <<<, explaining how it works, its syntax, and providing practical examples to help you master its use in your 4GL applications.

Understanding the <<< Operator

The <<< operator in Informix 4GL is a string formatting operator. It takes a target string variable on its left-hand side and a format string on its right-hand side, followed by a list of expressions whose values are to be formatted. Its primary purpose is to assign a formatted string to a variable, similar to how printf works in C or sprintf in other languages, but with a specific 4GL syntax and set of formatting directives.

Unlike simple string concatenation, <<< allows for precise control over data presentation, including alignment, padding, numeric precision, and date/time formatting. This makes it invaluable for generating reports, user interface displays, and data exports where consistent and well-structured output is crucial.

flowchart TD
    A[Start]
    B["Target String Variable (e.g., `my_string`)"]
    C["Formatting Operator (`<<<`)"]
    D["Format String (e.g., `"%-10s %05d"`)"]
    E["Expressions (e.g., `name, id_num`)"]
    F["Formatted Output Assigned to Target Variable"]

    A --> B
    B --> C
    C --> D
    D --> E
    E --> F
    F --> G[End]

Conceptual flow of the <<< string formatting operation in Informix 4GL.

Syntax and Basic Usage

The general syntax for the <<< operator is as follows:

TARGET_STRING_VARIABLE <<< FORMAT_STRING, EXPRESSION1, EXPRESSION2, ...

Where:

• TARGET_STRING_VARIABLE: A STRING or VARCHAR variable that will receive the formatted output.
• FORMAT_STRING: A string literal or variable containing formatting directives.
• EXPRESSION1, EXPRESSION2, ...: A comma-separated list of expressions (variables, literals, function calls) whose values will be formatted according to the FORMAT_STRING.

DEFINE l_name VARCHAR(20),
       l_age INTEGER,
       l_formatted_output VARCHAR(50)

LET l_name = "John Doe"
LET l_age = 30

-- Basic usage: format name and age into a string
LET l_formatted_output <<< "Name: %-20s Age: %03d", l_name, l_age

DISPLAY l_formatted_output
-- Expected output: "Name: John Doe             Age: 030"

Basic example of using the <<< operator to format a string.

Formatting Directives

The power of <<< comes from its formatting directives, which are similar to those found in C's printf. Each directive starts with a % character, followed by optional flags, width, precision, and a type specifier. Here are some common directives:

• %s: String
• %d or %i: Signed decimal integer
• %f: Floating-point number (decimal notation)
• %g: Floating-point number (shorter of %f or %e)
• %c: Single character
• %x or %X: Hexadecimal integer
• %o: Octal integer
• %p: Pointer (not commonly used in 4GL for direct output)
• %%: A literal percent sign

Optional flags and specifiers include:

• -: Left-justify within the field width.
• +: Always print a sign for signed numbers.
• 0: Pad with leading zeros for numeric types.
• FIELD_WIDTH: Minimum field width.
• .PRECISION: For floating-point numbers, number of digits after the decimal point. For strings, maximum number of characters to print.

DEFINE l_product_code VARCHAR(10),
       l_price DECIMAL(8,2),
       l_quantity INTEGER,
       l_report_line VARCHAR(100)

LET l_product_code = "P12345"
LET l_price = 123.456
LET l_quantity = 7

-- Formatting with various directives
LET l_report_line <<< "Code: %-10s | Price: $%8.2f | Qty: %04d", 
                      l_product_code, l_price, l_quantity

DISPLAY l_report_line
-- Expected output: "Code: P12345     | Price: $  123.46 | Qty: 0007"

Advanced formatting with field width, precision, and zero-padding.

Date and Time Formatting

Informix 4GL also provides specific directives for formatting DATE and DATETIME values. These directives are often prefixed with %Y, %M, %D, etc., or use specific format strings within the <<< operator that leverage the MDY or YMD functions for conversion, or rely on the DBDATE and DBTIME environment variables for default formatting.

However, for more explicit date/time formatting within <<<, it's common to first convert the DATE or DATETIME to a string using TO_CHAR or FMT_DATE (if available in your specific 4GL version or library) and then use %s.

DEFINE l_current_date DATE,
       l_formatted_date_str VARCHAR(20),
       l_output_line VARCHAR(50)

LET l_current_date = TODAY

-- Convert date to a specific string format first, then use %s
LET l_formatted_date_str = MDY(MONTH(l_current_date), DAY(l_current_date), YEAR(l_current_date))

LET l_output_line <<< "Today's Date: %s", l_formatted_date_str

DISPLAY l_output_line
-- Expected output (e.g.): "Today's Date: 01/15/2024"

Formatting a DATE value by converting it to a string first.

Common Pitfalls and Best Practices

While powerful, the <<< operator can lead to issues if not used carefully. Here are some common pitfalls and best practices:

1. Mismatch between format specifiers and expressions: Ensure the number and type of format specifiers in the FORMAT_STRING match the number and data types of the EXPRESSION list. A mismatch can lead to runtime errors or unexpected output.
2. Buffer overflow: The TARGET_STRING_VARIABLE must be large enough to hold the resulting formatted string. If the formatted string exceeds the declared length of the target variable, it will be truncated without warning, potentially leading to data loss or incorrect display.
3. Readability: For very complex format strings, consider breaking them down or using intermediate variables to improve readability.
4. Performance: For simple concatenations, the || operator might be marginally faster, but for complex formatting, <<< is the clear choice for maintainability and control.

DEFINE l_value1 INTEGER = 10
DEFINE l_value2 INTEGER = 20
DEFINE l_result VARCHAR(10)

-- Pitfall: Target variable too small
-- LET l_result <<< "Value1: %d, Value2: %d", l_value1, l_value2
-- This would truncate the output if l_result is too short.

-- Best Practice: Ensure target variable is large enough
DEFINE l_safe_result VARCHAR(40)
LET l_safe_result <<< "Value1: %d, Value2: %d", l_value1, l_value2
DISPLAY l_safe_result
-- Expected output: "Value1: 10, Value2: 20"

Demonstrating the importance of adequate target string variable size.