From 5bc556decd64861d70fb17ffb5df9cfbeaadf3ba Mon Sep 17 00:00:00 2001 From: DarkSun Date: Fri, 14 Aug 2020 05:02:15 +0800 Subject: [PATCH] =?UTF-8?q?=E9=80=89=E9=A2=98[tech]:=2020200813=20How=20to?= =?UTF-8?q?=20use=20printf=20to=20format=20output?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit sources/tech/20200813 How to use printf to format output.md --- ...0813 How to use printf to format output.md | 222 ++++++++++++++++++ 1 file changed, 222 insertions(+) create mode 100644 sources/tech/20200813 How to use printf to format output.md diff --git a/sources/tech/20200813 How to use printf to format output.md b/sources/tech/20200813 How to use printf to format output.md new file mode 100644 index 0000000000..1682cef9c9 --- /dev/null +++ b/sources/tech/20200813 How to use printf to format output.md @@ -0,0 +1,222 @@ +[#]: collector: (lujun9972) +[#]: translator: ( ) +[#]: reviewer: ( ) +[#]: publisher: ( ) +[#]: url: ( ) +[#]: subject: (How to use printf to format output) +[#]: via: (https://opensource.com/article/20/8/printf) +[#]: author: (Seth Kenlon https://opensource.com/users/seth) + +How to use printf to format output +====== +Get to know printf, a mysterious, flexible, and feature-rich alternative +to echo, print, and cout. +![Person drinking a hot drink at the computer][1] + +When I started learning Unix, I was introduced to the `echo` command pretty early in the process. Likewise, my initial [Python][2] lesson involved the `print` function. Picking up C++ and [Java][2] introduced me to `cout` and `systemout`. It seemed every language proudly had a convenient one-line method of producing output and advertised it like it was going out of style. + +But once I turned the first page of intermediate lessons, I met `printf`, a cryptic, mysterious, and surprisingly flexible function. In going against the puzzling tradition of hiding `printf` from beginners, this article aims to introduce to the world the humble `printf` function and explain how it can be used in nearly any language. + +### A brief history of printf + +The term `printf` stands for "print formatted" and may have first appeared in the [Algol 68][3] programming language. Since its inclusion in C, `printf` has been reimplemented in C++, Java, Bash, PHP, and quite probably in whatever your favorite (post-C) language happens to be. + +It's clearly popular, and yet many people seem to regard its syntax to be complex, especially compared to alternatives such as `echo` or `print` or `cout`. For example, here's a simple echo statement in Bash: + + +``` +$ echo hello +hello +$ +``` + +Here's the same result using `printf` in Bash: + + +``` +$ printf "%s\n" hello +hello +$ +``` + +But you get a lot of features for that added complexity, and that's exactly why `printf` is well worth learning. + +### printf output + +The main concept behind `printf` is its ability to format its output based on style information _separate_ from the content. For instance, there is a collection of special sequences that `printf` recognizes as special characters. Your favorite language may have greater or fewer sequences, but common ones include: + + * `\n`: New line + * `\r`: Carriage return + * `\t`: Horizontal tab + * `\NNN`: A specific byte with an octal value containing one to three digits + + + +For example: + + +``` +$ printf "\t\123\105\124\110\n" +     SETH +$ +``` + +In this Bash example, `printf` renders a tab character followed by the ASCII characters assigned to a string of four octal values. This is terminated with the control sequence to produce a new line (`\n`). + +Attempting the same thing with `echo` produces something a little more literal: + + +``` +$ printf "\t\123\105\124\110\n" +\t\123\105\124\110\n +$ +``` + +Using Python's `print` function for the same task reveals there's more to Python's `print` command than you might expect: + + +``` +>>> print("\t\123\n") +        S + +>>> +``` + +Obviously, Python's `print` incorporates traditional `printf` features as well as the features of a simple `echo` or `cout`. + +These examples contain nothing more than literal characters, though, and while they're useful in some situations, they're probably the least significant thing about `printf`. The true power of `printf` lies in format specification. + +### Format output with printf + +Format specifiers are characters preceded by a percent sign (`%`). +Common ones include: + + * `%s`: String + * `%d`: Digit + * `%f`: Floating-point number + * `%o`: A number in octal + + + +These are placeholders in a `printf` statement, which you can replace with a value you provide somewhere else in your `printf` statement. Where these values are provided depends on the language you're using and its syntax, but here's a simple example in Java: + + +``` +string var="hello\n"; +system.out.printf("%s", var); +``` + +This, wrapped in appropriate boilerplate code and executed, renders: + + +``` +$ ./example +hello +$ +``` + +It gets even more interesting, though, when the content of a variable changes. Suppose you want to update your output based on an ever-increasing number: + + +``` +#include <stdio.h> + +int main() { +  int var=0; +  while ( var < 100) { +    var++; +  printf("Processing is %d% finished.\n", var); +  } +  return 0; +} +``` + +Compiled and run: + + +``` +Processing is 1% finished. +[...] +Processing is 100% finished. +``` + +Notice that the double `%` in the code resolves to a single printed `%` symbol. + +### Limiting decimal places with printf + +Numbers can get complex, and `printf` offers many formatting options. You can limit how many decimal places are printed using the `%f` for floating-point numbers. By placing a dot (`.`) along with a limiter number between the percent sign and the `f`, you tell `printf` how many decimals to render. Here's a simple example written in Bash for brevity: + + +``` +$ printf "%.2f\n" 3.141519 +3.14 +$ +``` + +Similar syntax applies to other languages. Here's an example in C: + + +``` +#include <math.h> +#include <stdio.h> + +int main() { +  [fprintf][4](stdout, "%.2f\n", 4 * [atan][5](1.0)); +  return 0; +} +``` + +For three decimal places, use `.3f`, and so on. + +### Adding commas to a number with printf + +Since big numbers can be difficult to parse, it's common to break them up with a comma. You can have `printf` add commas as needed by placing an apostrophe (`'`) between the percent sign and the `d`: + + +``` +$ printf "%'d\n" 1024 +1,024 +$ printf "%'d\n" 1024601 +1,024,601 +$ +``` + +### Add leading zeros with printf + +Another common use for `printf` is to impose a specific format upon numbers in file names. For instance, if you have 10 sequential files on a computer, the computer may sort `10.jpg` before `1.jpg`, which is probably not your intent. When writing to a file programmatically, you can use `printf` to form the file name with leading zero characters. Here's an example in Bash for brevity: + + +``` +$ printf "%03d.jpg\n" {1..10} +001.jpg +002.jpg +[...] +010.jpg +``` + +Notice that a maximum of 3 places are used in each number. + +### Using printf + +As you can tell from these `printf` examples, including control characters, especially `\n`, can be tedious, and the syntax is relatively complex. This is the reason shortcuts like `echo` and `cout` were developed. However, if you use `printf` every now and again, you'll get used to the syntax, and it will become second nature. I don't see any reason `printf` should be your _first_ choice for printing statements during everyday activities, but it's a great tool to be comfortable enough with that it won't slow you down when you need it. + +Take some time to learn `printf` in your language of choice, and use it when you need it. It's a powerful tool you won't regret having at your fingertips. + +-------------------------------------------------------------------------------- + +via: https://opensource.com/article/20/8/printf + +作者：[Seth Kenlon][a] +选题：[lujun9972][b] +译者：[译者ID](https://github.com/译者ID) +校对：[校对者ID](https://github.com/校对者ID) + +本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出 + +[a]: https://opensource.com/users/seth +[b]: https://github.com/lujun9972 +[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/coffee_tea_laptop_computer_work_desk.png?itok=D5yMx_Dr (Person drinking a hot drink at the computer) +[2]: https://opensource.com/resources/python +[3]: https://opensource.com/article/20/6/algol68 +[4]: http://www.opengroup.org/onlinepubs/009695399/functions/fprintf.html +[5]: http://www.opengroup.org/onlinepubs/009695399/functions/atan.html