Question

I am trying to run a simple shell command that runs and returns text rather quickly inside of a loop of indeterminate size at compile time that is generated by an NSArray. In scripting languages like perl, I would be able to do something like this:

for(i=0;i<=$myinputarraysize;i++){
    $output[i]=`/my/task $inputarray[i]`;
}

This would build a new array for me from the expected output of my task. In Obj-C this seems to be much more difficult and a bit confusing to me. Right now my loop looks like this:

for(int i=0; i<[inputarray count]; i++){
    NSTask *task;
    task = [[NSTask alloc] init];
    [task setLaunchPath:nsdchat];

    NSArray *args;
    args = [NSArray arrayWithObjects:@"/my/task", [inputarray objectAtIndex:i], nil];
    [task setArguments:args];

    NSPipe *pipe;
    pipe = [NSPipe pipe];
    [task setStandardOutput:pipe];

    NSFileHandle *file;
    file = [pipe fileHandleForReading];

    [task launch];

    NSData *data;
    data = [file readDataToEndOfFile];

    NSString *desc;
    desc = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];    
    desc = [string stringByReplacingOccurrencesOfString:@"\n" withString:@""];

    [descriptions insertObject:desc atIndex:i];

    [task release];
    [args release];
    [pipe release];
    [file release];
    [data release]; 
}

My goal is to fill descriptions (an NSMutableArray) with the output from my task (which I know is always a string and always ends in a newline that I want to strip out). It seems I’m missing something about memory releasing as when I run this and NSLog the output, I get the same result for the entire count of the loop.

Is there any easier or more way to loop through simple tasks like this? Am I overcomplicating it for myself?

Answer 1

First of all, there’s no need to do most of that work inside the loop. task, pipe and file all look like they could be handled outside the loop. You should also consider using Objective-C 2.0 dot syntax and fast enumeration to cut things down a little.

More significantly:

NSArray *args;
args = [NSArray arrayWithObjects:@"/my/task", [inputarray objectAtIndex:i], nil];
[task setArguments:args];

This says that the first argument passed to the executable at path ndschat is /my/task. Which doesn’t seem to match your PERL usage. Probably you want just:

NSArray *args;
args = [NSArray arrayWithObject:[inputarray objectAtIndex:i]];
[task setArguments:args];

Or, with the style comments taken into account:

for(NSString *argument in inputarray)
{
    ...
    task.arguments = [NSArray arrayWithObject:argument];
    ...
}

EDIT: you’re also releasing a lot of objects you don’t own, which as well as adding heft to your code is a memory management error possibly leading to a crash. So, to cut the whole thing down and correct that fault:

for(NSString *argument in inputarray)
{
    NSTask *task = [[NSTask alloc] init];  // you now own this
    task.launchPath = nsdchat;

    NSPipe *pipe = [NSPipe pipe];          // you don't own this
    task.standardOutput = pipe;

    NSFileHandle *file = [pipe fileHandleForReading];  // you also don't own this

    task.arguments = [NSArray arrayWithObject:argument];
    [task launch];
    [task waitUntilExit]; // you should wait until the task is done

    NSData *data = [file readDataToEndOfFile];  // this is another thing
                                                // you don't own. Note also that
                                                // readDataToEndOfFile advances
                                                // the current read pointer, so
                                                // it should be fine to do this
                                                // successively 

    NSString *desc;
    desc = [[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding] 
                                                                   autorelease];    
                                                // you don't own this because
                                                // of the autorelease; you
                                                // don't want to own it since
                                                // the next line will throw
                                                // it away


    desc = [string stringByReplacingOccurrencesOfString:@"\n" withString:@""];
                                                // you don't own this

    [descriptions addObject:desc];
    [task release];
}