选题[tech]: 20200813 How to use printf to format output

sources/tech/20200813 How to use printf to format output.md

sources/tech/20200813 How to use printf to format output.md

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+[#]: 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