#pad #padding #string #runtime


Library for padding strings at runtime

7 releases

Uses old Rust 2015

0.1.6 Nov 12, 2019
0.1.5 Mar 10, 2018
0.1.4 Apr 22, 2015
0.1.3 Mar 22, 2015
0.1.1 Jan 29, 2015

#52 in Text processing

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5,098 downloads per month
Used in 35 crates (29 directly)

MIT license

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rust-pad pad on crates.io Build status

This is a library for padding strings at runtime.

It provides four helper functions for the most common use cases, and one main function to cover the other cases.

View the Rustdoc


This crate works with Cargo. Add the following to your Cargo.toml dependencies section:

pad = "0.1"

Padding in the stdlib

You do not need this crate for simple padding! It’s possible to pad strings using the Rust standard library.

For example, to pad a number with zeroes:

// Padding using std::fmt
assert_eq!("0000012345", format!("{:0>10}", 12345));

You can even use a variable for the padding width:

// Padding using std::fmt
assert_eq!("hello       ", format!("{:width$}", "hello", width=12));

The Rust documentation for std::fmt contains more examples. The rest of the examples will use the pad crate.


You can pad a string to have a minimum width with the pad_to_width method:

use pad::PadStr;
println!("{}", "Hi there!".pad_to_width(16));

This will print out “Hi there!” followed by seven spaces, which is the number of spaces necessary to bring it up to a total of sixteen characters wide.

String length is determined with the unicode_width crate, without assuming CJK.


By default, strings are left-aligned: any extra characters are added on the right. To change this, pass in an Alignment value:

use pad::{PadStr, Alignment};
let s = "I'm over here".pad_to_width_with_alignment(20, Alignment::Right);

There are four of these in total:

  • Left, which puts the text on the left and spaces on the right;
  • Right, which puts the text on the right and spaces on the left;
  • Middle, which centres the text evenly, putting it slightly to the left if it can’t be exactly centered;
  • MiddleRight, as above, but to the right.


Another thing that’s set by default is the character that’s used to pad the strings — by default, it’s space, but you can change it:

use pad::PadStr;
let s = "Example".pad_to_width_with_char(10, '_');


Finally, you can override what happens when a value exceeds the width you give. By default, the width parameter indicates a minimum width: any string less will be padded, but any string greater will still be returned in its entirety.

You can instead tell it to pad with a maximum value, which will truncate the input when a string longer than the width is passed in.

use pad::PadStr;
let short = "short".with_exact_width(10);  // "short     "
let long = "this string is long".with_exact_width(10);  // "this strin"

A Full Example

All of the above functions delegate to the pad function, which you can use in special cases. Here, in order to right-pad a number with zeroes, pass in all the arguments:

use pad::{PadStr, Alignment};
let s = "12345".pad(10, '0', Alignment::Right, true);

(The true at the end could just as easily be false. It’s whether to truncate or not.)

A Note on Debugging

One very last point: the width function takes a usize, rather than a signed number type. This means that if you try to pass in a negative size, it’ll wrap around to a positive size, and produce a massive string and possibly crash your program. So if your padding calls are failing for some reason, this is probably why.