According to wikipedia concurrency can be defined like this:

The property of program, algorithm, or problem decomposability into order-independent or partially-ordered components or units.[1] This means that even if the concurrent units of the program, algorithm, or problem are executed out-of-order or in partial order, the result will remain determinate.

or this:

A form of computing in which several computations are executing during overlapping time periods—concurrently—instead of sequentially (one completing before the next starts)

An example of a concurrent program could be a web server that can receive multiple requests “at the same time”. If the server wasn’t concurrent, users would have to wait for another request to be processed before the server could handle their request.

Go is said to make concurrency very easy to program so today I’m going to explore it.

To explain concurrency it’s useful to have parts of a program that can execute asynchronously or in parallel. As an example I’m going to use a pair of functions that pretend to get some information from a database or service. Lets look at a simple example where everything is executed without concurrency:

One of the first things I heard about Go that sounded interesting is that it has only one way to create loops. Go only provides the for statement. This sounds a little weird, but the truth is that they just decided to use the for keyword for while loops instead of having a separate while keyword. Lets see an example:

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package main

import (
  "fmt"
)

func main() {
  people := []string{"Hugo", "Paco", "Luis"}

  // Like a for
  numPeople := len(people)
  for i := 0; i < numPeople; i++ {
    fmt.Println(people[i])
  }

  // Like a while
  numPeople = len(people)
  i := 0
  for i < numPeople {
    fmt.Println(people[i])
    i++
  }
}

A few weeks ago I talked about arrays in Go. This time I’m going to cover slices, which are built on top of arrays.

In my previous post I showed the difference between doing this:

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func doSomething(arr [3]int) {
    arr[0] = 5
}

func main() {
    a := [3]int{1, 2, 3}

    doSomething(a)

    // Prints [1 2 3]
    fmt.Println(a)
}

In a previous post I explained how to create a navigation bar for your Android app. I’m going to build on top of that example a drawer where we can put other options for the user.

Android provides a Drawer Layout that was specifically created for this purpose. Since we are going to be adding the drawer to my previous example lets first look at how the original looks:

I finished building a hobby app a few weeks ago, but after getting all the functionality right I couldn’t help but notice that it looked horrible. I’m going to slowly try to make it less ugly starting with this post.

The first thing that I want to do is get rid of the default title bar because it occupies too much space:

Create the file src/main/res/values/style.xml if it doesn’t exist already and create a new theme with no title:

I have an android project I’ve been working on for a few weeks. I got a new computer recently and I wanted to work on this project. I downloaded the Android SDK and gradle. When I tried to run a build:

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gradle assembleDebug

I got this error:

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Gradle version 2.2 is required. Current version is 2.11. If using the gradle wrapper, try editing the distributionUrl in /home/you/repos/asdf/gradle/wrapper/gradle-wrapper.properties to gradle-2.2-all.zip

Arrays are an interesting case in Go, because you are usually encouraged to not use them and use slices instead. Arrays in Go have a few rules that make them feel counter intuitive but I’m going to start with the parts that look normal. You can declare an array and assign values to it like this:

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var a [3]int
a[0] = 5
a[1] = 11
a[2] = 22

I got asked this question in a code interview and I wanted to make sure my answer was good. Without the pressure of being in an interview I see the problem more clearly and the problem seems pretty easy now.

Lets look at the basics of adding in binary:

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 0         1       1
+0        +0      +1
---       ---     ---
 0         1      10

A few weeks ago I started to learn Go and I wrote an introductory post. I’m going to continue where I left and explain how you can extend structures with methods and later how to use interfaces as arguments.

Methods

Go doesn’t have classes or objects as we know them. It uses structs instead to create object-like structures:

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type Animal struct {
  color string
  size float64
}

This looks very similar to an object but something very important is missing. You can declare properties like this, but not methods. How will our animal do stuff without methods?. Golang actually does have methods, but you have to attach them to the struct after it is created:

I’m having some problems with one of my hobby servers but this time instead of looking at the code to try to figure out what is happening, I decided to try to do it using only tcpdump. I was trying to start my server and I got this error message:

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failed to create GA auth provider: invalid character '\u003c' looking for beginning of value

The character ‘\u003c’ translates to <, so it seemed like the problem was that somewhere in my GA auth library I was getting what looked like an HTML instead of a JSON. The first thing I did was monitor the HTTP traffic using this command:

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tcpdump -c 20 -s 0 -i eth0 -A tcp port http