Tcl format Command

last modified April 3, 2025

The Tcl format command creates formatted strings by substituting values into a format specifier. It's similar to printf in other languages. The command provides precise control over string formatting.

Basic Definition

The format command uses format specifiers to control how values are converted to strings. Each specifier starts with % followed by optional modifiers and a conversion character.

Syntax: format formatString ?arg arg ...?. The formatString contains literal text and format specifiers. Each specifier processes one argument.

Basic String Formatting

This example demonstrates simple string formatting with the format command.

basic_format.tcl

set name "John"
set age 30
set result [format "Name: %s, Age: %d" $name $age]
puts $result

Here %s formats a string and %d formats a decimal integer. The variables name and age are substituted into the format string. The result is stored in result and printed.

Number Formatting

The format command can control number display with width and precision.

number_format.tcl

set pi 3.1415926535
set formatted [format "%.2f" $pi]
puts "Pi rounded to 2 decimal places: $formatted"

The %.2f specifier formats the floating-point number to 2 decimal places. This is useful for displaying monetary values or measurements where precision matters.

Width and Alignment

Format specifiers can control field width and alignment of values.

width_format.tcl

set items [list "Apple" "Banana" "Cherry"]
foreach item $items {
    puts [format "%-10s : %4.2f" $item [expr {rand()}]] 
}

This example shows left-aligned strings in 10-character fields and right-aligned floating-point numbers. The - modifier left-aligns, while numbers default to right alignment.

Hexadecimal and Octal Formatting

The format command can convert numbers to different bases like hexadecimal.

hex_format.tcl

set num 255
puts [format "Decimal: %d, Hex: 0x%x, Octal: %o" $num $num $num]

This demonstrates number formatting in different bases. %x formats as hexadecimal, %o as octal, and %d as decimal. The 0x prefix is added manually for clarity.

Positional Specifiers

Format specifiers can reference arguments by position instead of order.

positional_format.tcl

set first "John"
set last "Doe"
set phone "555-1234"
puts [format "%2\$s, %1\$s - Phone: %3\$s" $first $last $phone]

The %n$ syntax specifies which argument to use (1-based index). This allows reordering or reusing arguments without changing their positions in the call.

Formatting Lists

The format command can process list elements with some additional Tcl commands.

list_format.tcl

set numbers {1 2 3 4 5}
set formatted [join [lmap n $numbers {format "%03d" $n}] ", "]
puts "Formatted numbers: $formatted"

This formats each list element to 3 digits with leading zeros, then joins them with commas. The lmap command applies format to each element, creating a new list of formatted strings.

Best Practices

Clarity: Use format for complex string construction.
Localization: Consider format for number localization.
Performance: Format is efficient for multiple values.
Readability: Break complex formats into smaller parts.
Validation: Validate inputs before formatting.

This tutorial covered the Tcl format command with practical examples showing its usage in different string formatting scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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