3 ms·

It's sometimes amazing what low-hanging fruit can be kicking around that you didn't realize until you actually strap up a profiler to something. The other day

It's sometimes amazing what low-hanging fruit can be kicking around that you didn't realize until you actually strap up a profiler to something. The other day I was trying to figure out why one web request in an application was taking an absurd amount of time. I started looking at SQL queries and other external calls, only to realize that it turned out to be a logging statement that used a little innocent looking helper function to stringify a List<T>. Except it was using concatenation, with Aggregate e.g. Aggregate("", (a, b)=> a + b + ", ")... Replace that with a StringBuilder, and I went from seconds to milliseconds on that call. It's one of those things that you go "Eh, I'm just doing this on a few items, it'll be fine", and then month or years later, the wheels come off because something happens to try using it with tens of thousands of items.

usea 8y ago
I've written that logging code, and over the years I've used both the StringBuilder and Aggregate styles. I've discovered that string.Join(", ", list) is a great fit for this, too. It's as quick as using StringBuilder, and also concise and composable (tastes may vary).
chriswarbo 8y ago
Concatenating strings like this is known as "Shlemiel the Painter's Algorithm", since each concatenation starts back at the beginning of the string. From https://www.joelonsoftware.com/2001/12/11/back-to-basics https://www.joelonsoftware.com/2001/12/11/back-to-basics > Shlemiel gets a job as a street painter, painting the dotted lines down the middle of the road. On the first day he takes a can of paint out to the road and finishes 300 yards of the road. “That’s pretty good!” says his boss, “you’re a fast worker!” and pays him a kopeck. > The next day Shlemiel only gets 150 yards done. “Well, that’s not nearly as good as yesterday, but you’re still a fast worker. 150 yards is respectable,” and pays him a kopeck. > The next day Shlemiel paints 30 yards of the road. “Only 30!” shouts his boss. “That’s unacceptable! On the first day you did ten times that much work! What’s going on?” > “I can’t help it,” says Shlemiel. “Every day I get farther and farther away from the paint can!” The more general problem is being "accidentally quadratic", e.g. assuming that N concatenations will be O(N), but it's actually O(N^2) because each must traverse the intermediate string, and (assuming each input string has O(1) length) that intermediate string will have length O(N). There's a Tumblr blog at https://accidentallyquadratic.tumblr.com https://accidentallyquadratic.tumblr.com which collects examples of this, but I can't seem to get past Tumblr's GDPR landing page to see it.

usea 8y ago

I've written that logging code, and over the years I've used both the StringBuilder and Aggregate styles. I've discovered that string.Join(", ", list) is a great fit for this, too. It's as quick as using StringBuilder, and also concise and composable (tastes may vary).

chriswarbo 8y ago

Concatenating strings like this is known as "Shlemiel the Painter's Algorithm", since each concatenation starts back at the beginning of the string. From https://www.joelonsoftware.com/2001/12/11/back-to-basics https://www.joelonsoftware.com/2001/12/11/back-to-basics > Shlemiel gets a job as a street painter, painting the dotted lines down the middle of the road. On the first day he takes a can of paint out to the road and finishes 300 yards of the road. “That’s pretty good!” says his boss, “you’re a fast worker!” and pays him a kopeck. > The next day Shlemiel only gets 150 yards done. “Well, that’s not nearly as good as yesterday, but you’re still a fast worker. 150 yards is respectable,” and pays him a kopeck. > The next day Shlemiel paints 30 yards of the road. “Only 30!” shouts his boss. “That’s unacceptable! On the first day you did ten times that much work! What’s going on?” > “I can’t help it,” says Shlemiel. “Every day I get farther and farther away from the paint can!” The more general problem is being "accidentally quadratic", e.g. assuming that N concatenations will be O(N), but it's actually O(N^2) because each must traverse the intermediate string, and (assuming each input string has O(1) length) that intermediate string will have length O(N). There's a Tumblr blog at https://accidentallyquadratic.tumblr.com https://accidentallyquadratic.tumblr.com which collects examples of this, but I can't seem to get past Tumblr's GDPR landing page to see it.