digitalmars.D.learn - Declaring run time variables
- splatterdash (29/29) Aug 04 2014 Hello everyone,
- anonymous (14/27) Aug 04 2014 }
- splatterdash (7/37) Aug 04 2014 Indeed I do. I'm not sure which type I should use for the common
- anonymous (26/32) Aug 04 2014 Let MyFileReader implement an interface that has the operations
- splatterdash (2/36) Aug 05 2014 That does it, thanks :)!
Hello everyone,
I'm trying to write a command-line application that can detect
whether the input file is gzipped or not. Sounds simple enough,
but I can't seem to do it properly in D (this is my first foray
to the language).
After checking whether the file is gzipped or not, I try to
create a new instance of a class that is basically an InputRange
returning each line of the input file:
```
File f = File("input_file")
// detect gzip ...
if (isGzip)
auto fileIter = new MyFileReader!GzipIterator(f)
else
auto fileIter = new MyFileReader!NormalIterator(f)
foreach(string line; fileIter) {
// do things
}
```
However I always get the compiler error `undefined identifier
lineIter`.
Now, my questions are:
1. What is causing the error? Is this caused by the compiler not
being able to figure out what fileIter is at compile time?
2. I realize the current way of creating file handles may not be
the best way to handle gzipped or non-gzipped file. Is there a
better way to detect and create such file handles based on run
time values?
Thanks in advance :).
Aug 04 2014
On Monday, 4 August 2014 at 22:00:18 UTC, splatterdash wrote:``` File f = File("input_file") // detect gzip ... if (isGzip){auto fileIter = new MyFileReader!GzipIterator(f)}else{auto fileIter = new MyFileReader!NormalIterator(f)}foreach(string line; fileIter) { // do things } ```(Added braces for clarity.)However I always get the compiler error `undefined identifier lineIter`.You have two independent variables called fileIter. Both are only available in their respective scopes. Declare the variable before the `if` to get one variable with a wider scope: SomeCommonBaseTypeOfThoseMyFileReaderVariants fileIter; if (isGzip) fileIter = new MyFileReader!GzipIterator(f); else fileIter = new MyFileReader!NormalIterator(f);
Aug 04 2014
On Monday, 4 August 2014 at 22:09:49 UTC, anonymous wrote:On Monday, 4 August 2014 at 22:00:18 UTC, splatterdash wrote:Indeed I do. I'm not sure which type I should use for the common base type, though. MyFileReader is a templated class, so using it plainly did not work. I also tried `InputRange!string` to no avail despite `MyFileReader` implementing the three InputRange requirement (popFront(), front, and empty). Any ideas on what I should as the class?``` File f = File("input_file") // detect gzip ... if (isGzip){auto fileIter = new MyFileReader!GzipIterator(f)}else{auto fileIter = new MyFileReader!NormalIterator(f)}foreach(string line; fileIter) { // do things } ```(Added braces for clarity.)However I always get the compiler error `undefined identifier lineIter`.You have two independent variables called fileIter. Both are only available in their respective scopes. Declare the variable before the `if` to get one variable with a wider scope: SomeCommonBaseTypeOfThoseMyFileReaderVariants fileIter; if (isGzip) fileIter = new MyFileReader!GzipIterator(f); else fileIter = new MyFileReader!NormalIterator(f);
Aug 04 2014
On Monday, 4 August 2014 at 22:18:24 UTC, splatterdash wrote:Indeed I do. I'm not sure which type I should use for the common base type, though. MyFileReader is a templated class, so using it plainly did not work. I also tried `InputRange!string` to no avail despite `MyFileReader` implementing the three InputRange requirement (popFront(), front, and empty). Any ideas on what I should as the class?Let MyFileReader implement an interface that has the operations you need. That interface can be std.range.InputRange!string, or you can define your own. Note that a type is an input range when it has the input range primitives (front, popFront, empty), but it's only a std.range.InputRange!T when it implements the interface in the OOP sense: class C : InputRange!E {...}. Phobos generally doesn't use InputRange, but templatizes everything. You can go that way, too, and move the foreach loop to a templated function: void main() { File f = File("input_file") // detect gzip ... if (isGzip) doThings(new MyFileReader!GzipIterator(f)); else doThings(new MyFileReader!NormalIterator(f)); } void doThings(I)(I fileIter) { foreach(string line; fileIter) { // do things } }
Aug 04 2014
"splatterdash" <toa.konuva yahoo.com> On Monday, 4 August 2014 at 22:45:15 UTC, anonymous wrote:On Monday, 4 August 2014 at 22:18:24 UTC, splatterdash wrote:That does it, thanks :)!Indeed I do. I'm not sure which type I should use for the common base type, though. MyFileReader is a templated class, so using it plainly did not work. I also tried `InputRange!string` to no avail despite `MyFileReader` implementing the three InputRange requirement (popFront(), front, and empty). Any ideas on what I should as the class?Let MyFileReader implement an interface that has the operations you need. That interface can be std.range.InputRange!string, or you can define your own. Note that a type is an input range when it has the input range primitives (front, popFront, empty), but it's only a std.range.InputRange!T when it implements the interface in the OOP sense: class C : InputRange!E {...}. Phobos generally doesn't use InputRange, but templatizes everything. You can go that way, too, and move the foreach loop to a templated function: void main() { File f = File("input_file") // detect gzip ... if (isGzip) doThings(new MyFileReader!GzipIterator(f)); else doThings(new MyFileReader!NormalIterator(f)); } void doThings(I)(I fileIter) { foreach(string line; fileIter) { // do things } }
Aug 05 2014