{"id":2377,"date":"2016-01-01T17:17:46","date_gmt":"2016-01-01T23:17:46","guid":{"rendered":"http:\/\/dev.iachieved.it\/iachievedit\/?p=2377"},"modified":"2016-01-01T17:17:46","modified_gmt":"2016-01-01T23:17:46","slug":"swift-substring-on-a-c-string","status":"publish","type":"post","link":"https:\/\/dev.iachieved.it\/iachievedit\/swift-substring-on-a-c-string\/","title":{"rendered":"Swift Substring on a C String"},"content":{"rendered":"

String handling in Swift has been a moving target. While following the evolution of performing certain string operations in Swift, I’ve been reminded of the post<\/a> Joel Spolsky wrote over a decade ago admonishing all of us to get with the program and support Unicode<\/a> in our applications from day one. While Swift 1.0 didn’t ignore the request, it certainly did make it easier to use “natural” expressions to extract substrings with ranges. Swift 2.0 tossed that out the window, and a top-rated answer<\/a> on Stackoverflow becomes useless. <\/p>\n

Don’t try this at home, it won’t work, but it sure would be nice if it did:<\/p>\n

\r\nvar helloWorld = \"Hello, world!\"\r\nlet hello      = helloWorld[0...4]\r\nprint(hello)\r\n<\/pre>\n
I mean, it works for Python with `hello = helloWorld[0:5]`, why can’t it work in Swift?<\/p>\n
Like we said, Swift 2 is taking Joel’s 2003 plea to heart, and working to make us all (painfully) more aware that these aren’t your father’s strings.  There is a great summary as to the rationale at mikeash.com<\/a> so there’s no reason for me to rehash characters vs. graphemes here.  And while I “get it”, there’s no doubt its an aggravation trying to remember what magic I need to invoke to get a substring out of what I know is going to be a `const char*` C-style ASCII-based string.  So, I won’t bother trying to remember, I’ll just write it here for you and for me.<\/p>\n
A Simple Substring<\/h3>\n
I’ve been looking at ways to blend more Swift programming in my day-to-day Linux tasks.  I really think it has an opportunity to displace some<\/a> other<\/a> languages<\/a> for quick scripts and “devops”-oriented tasks.  In this case I wanted a quick way to print out the `git` revisions of the projects that make up an open source Swift build.  Here’s what I came up with:<\/p>\n
\r\nimport Glibc\r\n\r\nfunc getGitRevision(dirname:String) -> String {\r\n  let BUFSIZE = 128\r\n  let cwd     = String.fromCString(getcwd(nil, 0))!\r\n  var rc      = chdir(dirname)\r\n  \r\n  guard rc == 0 else {\r\n    return \"ERROR\"\r\n  }\r\n  \r\n  var rev  = \"\"\r\n  let pipe = popen(\"\/usr\/bin\/git rev-parse HEAD\", \"r\")\r\n  var buf  = [CChar](count:BUFSIZE, repeatedValue:CChar(0))\r\n  while fgets(&buf, Int32(BUFSIZE), pipe) != nil {\r\n    rev = String.fromCString(buf)!\r\n  }\r\n  rev = rev[rev.startIndex...rev.startIndex.advancedBy(9)]\r\n\r\n  chdir(cwd)\r\n\r\n  return rev\r\n}\r\n\r\nlet dirs = [\"swift\", \"llvm\", \"clang\", \"lldb\", \"cmark\", \"llbuild\", \"swiftpm\", \"swift-corelibs-xctest\", \"swift-corelibs-foundation\", \"swift-integration-tests\"]\r\n\r\nfor dir in dirs {\r\n  let rev = getGitRevision(dir)\r\n  print(\"\\(dir):\\(rev)\")\r\n}\r\n<\/pre>\n
Getting the first 10 characters of the `git` revision hash with `rev = rev[rev.startIndex…rev.startIndex.advancedBy(9)]` is a little more verbose than it should be, but if you are dealing with what you know to be a C-style string, it gets the job done.<\/p>\n
Extend It<\/h3>\n
Fortunately for us, Swift supports extensions<\/a>, that awesome feature that allows you to tack on some methods to a class you didn’t write and don’t have source access to.  Many folks have posted gists<\/a> of this particular extension which provides a little syntactic sugar for accessing substrings:<\/p>\n
\r\nextension String {\r\n  subscript (r: Range) -> String {\r\n    get {\r\n      let startIndex = self.startIndex.advancedBy(r.startIndex)\r\n      let endIndex   = self.startIndex.advancedBy(r.endIndex)\r\n            \r\n      return self[Range(start: startIndex, end: endIndex)]\r\n    }\r\n  }\r\n}\r\n<\/pre>\n
With our extension we can indeed write:<\/p>\n
\r\nlet helloWorld = \"Hello, world!\"\r\nvar hello      = helloWorld[0...4]\r\n\r\nprint(hello)\r\n<\/pre>\n
Life is good.<\/p>\n","protected":false},"excerpt":{"rendered":"
String handling in Swift has been a moving target. While following the evolution of performing certain string operations in Swift, I’ve been reminded of the post Joel Spolsky wrote over a decade ago admonishing all of us to get with the program and support Unicode in our applications from day one. While Swift 1.0 didn’t […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"class_list":["post-2377","post","type-post","status-publish","format-standard","hentry","category-swift"],"_links":{"self":[{"href":"https:\/\/dev.iachieved.it\/iachievedit\/wp-json\/wp\/v2\/posts\/2377"}],"collection":[{"href":"https:\/\/dev.iachieved.it\/iachievedit\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/dev.iachieved.it\/iachievedit\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/dev.iachieved.it\/iachievedit\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/dev.iachieved.it\/iachievedit\/wp-json\/wp\/v2\/comments?post=2377"}],"version-history":[{"count":10,"href":"https:\/\/dev.iachieved.it\/iachievedit\/wp-json\/wp\/v2\/posts\/2377\/revisions"}],"predecessor-version":[{"id":2387,"href":"https:\/\/dev.iachieved.it\/iachievedit\/wp-json\/wp\/v2\/posts\/2377\/revisions\/2387"}],"wp:attachment":[{"href":"https:\/\/dev.iachieved.it\/iachievedit\/wp-json\/wp\/v2\/media?parent=2377"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/dev.iachieved.it\/iachievedit\/wp-json\/wp\/v2\/categories?post=2377"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/dev.iachieved.it\/iachievedit\/wp-json\/wp\/v2\/tags?post=2377"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}