## Lab 01: Introduction to D

During this lab you will be introduced to the basic D features.

### Syntax

The D programming language uses a C-style syntax that ensures a smooth transition for programmers coming from a C\C++ background. With no further ado, let us take a random C program and see what it takes to compile it with the D compiler:

#include <stdio.h>

int main()
{
int position = 7, c, n = 10;
int array[n] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

for (c = position - 1; c < n - 1; c++)
array[c] = array[c+1];

printf("Resultant array:\n");
for (c = 0; c < n - 1; c++)
printf("%d\n", array[c]);

return 0;
}

The code above simply deletes an element in an array. Now let's take a look on what the minimum modifications are to make the code compile and run with D:

import std.stdio;

int main()
{
int position = 7, c, n = 10;
int[10] array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

for (c = position - 1; c < n - 1; c++)
array[c] = array[c+1];

printf("Resultant array:\n");
for (c = 0; c < n - 1; c++)
printf("%d\n", array[c]);

return 0;
}

As you can see, the only differences are:

• the include directive was replaced by an import statement;
• the array definition and initialization are slightly modified;

Most C programs require minimal changes in order to be compiled with the D compiler. So do not worry, even if you don't have any previous experience in D, you will be able to understand most of the programs written in it because the syntax is extremely similar to the C one.

The full D grammar can be found here.

Now, using the above code snippet, let us delve into the D specific concepts.

### Imports

In D, imports represent the counterpart of the C include directive, however the are 2 fundamental differences:

1. Imports may selectively specify which symbols are to be imported. For example, in the above code snippet, the full standard IO module of the standard library is imported, even though only the printf function is used. This results in a degradation in compile time since there is a larger symbol pool that needs to be examined when trying to resolve a symbol. In order to fix this, we can replace the blunt import with:

import std.stdio : printf;

2. Imports may be used at any scope level.

int main()
{
int position = 7, c, n = 10;
int[10] array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

for (c = position - 1; c < n - 1; c++)
array[c] = array[c+1];

// if the lines calling printf are deleted,
// it is easier to spot the now useless import
import std.stdio : printf;
printf("Resultant array:\n");
for (c = 0; c < n - 1; c++)
printf("%d\n", array[c]);

return 0;
}

### Data Types

The D programming language defines 3 classes of data types:

1. Basic data types: such as int, float, long etc. that are similar to the ones provided by C;
2. Derived data types: pointer, array, associative array, delegate, function;
3. User defined types: class, struct, union, enum;

We will not insist on basic data types and pointers, as those are the same (or slightly modified versions) as the ones in C\C++. We will focus on arrays, associative arrays, classes, structs and unions. Delegates, functions and enums will be treated in a future lab.

Note that in D all types have a default value. This means that there no uninitialized variables.

#### Arrays

The fundamental difference between a C array and a D array is that the former is represented by a simple pointer, while the latter is represented by a pointer and a size. This design decision was taken because of the numerous cases of C buffer overflow attacks that can be simply mitigated by automatically checking the array bounds. Let's take a simple example:

#include <stdio.h>

void main()
{
int a = 5;
int b[10];
b[11] = 9;
printf("%d\n", a);
}

Compiling this code (without the stack protector activated) will lead to a buffer overflow in which the variable a will be modified and the print will output 9. Compiling the same code with D (simply replace the include with an import std.stdio : printf) will issue a compile time error that states that 11 is beyond the size of the array. Aggregating the pointer of the array with its size facilitates a safer implementation of arrays.

D implements 2 types of arrays:

1. Static Arrays: their length must be known at compile time and therefore cannot be modified; all examples up until this point have been using static arrays.
2. Dynamic Arrays: their length may change dynamically at run time.
##### Slicing

Slicing an array means to specify a subarray of it. An array slice does not copy the data, it is only another reference to it:

void main()
{
int[10] a;       // declare array of 10 ints
int[] b;

b = a[1..3];     // a[1..3] is a 2 element array consisting of a[1] and a[2]
int x = b[1];    // equivalent to int x = 0;
a[2] = 3;
int y = b[1];    // equivalent to int y = 3;
}
##### Array Operations

1. Array setting:

void main()
{
int[3] a = [1, 2, 3];
int[3] c = [ 1, 2, 3, 4];  // error: mismatched sizes
int[] b = [1, 2, 3, 4, 5];
a = 3;                     // set all elements of a to 3
b = 5;                     // set all elements of b to 5
a[] = 2;                   // same as a = 3; using an empty slice is the same as slicing the full array
b = a[0 .. $]; //$ evaluates to the length of the array (in this case 10)
b = a[];                   // semantically equivalent to the one above
b = a[0 .. a.length]       // semantically equivalent to the one above
b[] = a[];                 // semantically equivalent to the one above
b[2 .. 4] = 4              // same as b[2] = 4, b[3] = 4
b[0 .. 4] = a[0 .. 4]      // error, a does not have 4 elements
a[0 .. 3] = b              // error, operands have different length
}

.length is a builtin array property. For an extensive list of array properties click here.

2. Array Concatenation:

void main()
{
int[] a;
int[] b = [1, 2, 3];
int[] c = [4, 5];

a = b ~ c;       // a will be [1, 2, 3, 4, 5]
a = b;           // a refers to b
a = b ~ c[0..0]; // a refers to a copy of b
a ~= c;          // equivalent to a = a ~ c;

Concatenation always creates a copy of its operands, even if one of the operands is a 0-length array. The operator ~= does not always create a copy. For more information click here

3. Vector operations:

void main()
{
int[] a = [1, 2, 3];
int[] b;
int[] c;

b[] = a[] + 4;         // b = [5, 6, 7]
c[] = (a[] + 1) * b[]  // c = [10, 18, 28]

}

Many array operations, also known as vector operations, can be expressed at a high level rather than as a loop. A vector operation is indicated by the slice operator appearing as the left-hand side of an assignment or an op-assignment expression. The right-hand side can be an expression consisting either of an array slice of the same length and type as the left-hand side or a scalar expression of the element type of the left-hand side, in any combination.

Using the concepts of slicing and concatenation, we can modify the original example (that does the removal of an element from the array) so that the for loop is no longer necessary:

int main()
{
int position = 7, c, n = 10;
int[] array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

array = array[0 .. position] ~ array[position + 1 .. \$];

import std.stdio;
writeln("Resultant array:");
writeln(array);

return 0;
}

writeln is a function from the standard D library that does not require a format and is easily usable with a plethora of types.

As you can see, the resulting code is much more expressive and fewer lines of code were utilized.

#### Associative Arrays (AA)

Associative arrays represent the D language hashtable implementation and have the following syntax:

void main()
{
int[string] aa;            // Associative array of ints that are
// indexed by string keys.
// The KeyType is string.
aa["hello"] = 3;           // set value associated with key "hello" to 3
int value = aa["hello"];
}
##### AA operations

1. Removing keys:

void main()
{
int[string] aa;
aa["hello"] = 3;

aa.remove("hello")     // removes the pair ("hello", 3)
}

remove(key) does nothing if the given key does not exist and returns false. If the given key does exist, it removes it from the AA and returns true. All keys can be removed by using the method clear.

2. Testing membership:

void main()
{
int[string] aa;
aa["hello"] = 3;

int* p;
p = "hello" in aa;

if (p)
{
*p = 4;  // update value associated with key; aa["hello"] == 4
}
}

The “in” expression yields a pointer to the value if the key is in the associative array, or null if not.

For more advanced operations on AAs check this link. For an exhaustive list of the AA properties check this link.

#### Structs

In D, structs are simple aggregations of data and their associated operations on that data:

struct Rectangle
{
size_t length, width;
int id;

size_t area() { return length*width; }
size_t perimeter() { return 2*(length + width); }
size_t isSquare() { return length == width; }
void setId(int id) {this.id = id; }
}
##### Struct initialization

Structs may be initialized in different forms:

1. Default initialization:

void main()
{
Rectangle s;    // s is default initialized to the default initializers of the members.
// In this case: s.length = 0, s.width = 0
}

2. Static initialization:

void main()
{
Rectangle r = { length:5, width:2; id:1 };        // r.length = 5, r.width = 2, r.id = 1
Rectangle q = { length:7, width: 7};              // r.length = 7, r.width = 7, r.id = 0
Rectangle w = { 3, id:2 };                        // r.length = 0, r.width = 3, r.id = 2
Rectangle z = { width:2, 3};                      // r.length = 0, r.width = 2, r.id = 3
Rectangle x = { 1, length:2}                      // error: cannot initialize a field more than once
}

3. Struct literal initialization:

void main()
{
Rectangle r = Rectangle(4, 2, 1);        // r.length = 4, r.width = 2, r.id = 1
Rectangle q = Rectangle(2, 3);           // r.length = 2, r.width = 3, r.id = 0
}

4. Constructor call

In D, structs may define constructors that are callable via struct literals:

struct Rectangle
{
size_t length, width;
int id;

this(size_t length, size_t width)      // constructor
{
this.length = length;
this.width = width;

/* generate id */
this.id = ...;
}

size_t area() { return length*width; }
size_t perimeter() { return 2*(length + width); }
size_t isSquare() { return length == width; }
void setId(int id) {this.id = id; }
}

void main()
{
Rectangle r = Rectangle(4, 2);    // will call constructor
Rectangle q = Rectangle(4, 2, 1); // will initialize each field (no constructor call)
}

#### Classes

In D, classes are very similar with java classes:

1. Classes can implement any number of interfaces
2. Classes can inherit a single class
3. Class objects are instantiated by reference only
4. super has the same meaning as in Java
5. overriding rules are very similar
class Square
{
size_t length;
this(size_t length) { this.length = length; }
size_t area() { return length*length; }
size_t perimeter() { return 4*length; }
}

class Rectangle : Square
{
size_t width;
this(size_t length, size_t width)
{
super(length);
this.width = width;
}

override size_t area() { return length*width; }
override size_t perimeter() { return 2*(length + width); }
}

The fundamental difference between structs and classes is that the former are value types, while the latter are reference types. This means that whenever a struct is passed as an argument to an lvalue function parameter, the function will operate on a copy of the object. When a class is passed as an argument to an lvalue function parameter, the function will receive a reference to the object.

### Functions

Functions are declared the same as in C. In addition, D offers some convenience features like:

1. Uniform function call syntax (UFCS): allows that any call to a free function fun(a) can be written as member function call a.fun()

import std.algorithm : group;
import std.range : chain, dropOne, front, retro;
[1, 2].chain([3, 4]).retro; // 4, 3, 2, 1
[1, 1, 2, 2, 2].group.dropOne.front; // (2, 3)

front(dropOne(group([1, 1, 2, 2, 2])));

void fun(int a) {}
void fun(int a, int b) {}

Overloading is done based on the type of parameters, not on the return type.

3. Default parameters:

void fun(int a, int b=8) {}

void main()
{
fun(7);    // calls fun(7, 8)
fun(2, 3); // calls fun(2, 3)
}

A function may have any number of default parameters. If a default parameter is given, all following parameters must also have default values.

4. Auto return type:

auto fun()
{
return 7;
}

void main()
{
int b = fun();
auto c = fun();   // works for variables also
}

Auto functions have their return type inferred based on the type of the return statements. Auto can be also used to infer the type of a variable declaration.

### Control flow instructions

if, while, do/while, for, switch statements have the same syntax and semantics as the C ones.

### Nested scopes

D offers a flexible approach when it comes to defining scopes. Functions, struct declarations, class declarations and other types of declarations may appear at any scope level (not just the global scope):

void main()
{
struct A
{
this(int a, int b)
{
this.a = a;
this.b = b;
this.c = B(a+1, "hello");
}

struct B
{
int a;
string b;
}
int a, b;
B c;
}
int fun(A a)
{
return a.c.a + a.a;
}

if (fun(A(2, 3)) == 2)
{
import std.stdio : writeln;
writeln("hooray");
}
}

### Unittests

Unit tests are a builtin framework of test cases applied to a module to determine if it is working properly. A D program can be run with unit tests enabled or disabled.

Unit tests are a special function defined like:

unittest
{
...test code...
}

Individual tests are specified in the unit test using assert expressions. There can be any number of unit test functions in a module, including within struct and class declarations. They are executed in lexical order. Unit tests, when enabled, are run after all static initialization is complete and before the main() function is called.

struct Sum
{
int add(int x, int y) { return x + y; }

unittest
{
Sum sum;
}
}

### Contract Programming

Contracts are a breakthrough technique to reduce the programming effort for large projects. Contracts are the concept of preconditions, postconditions, errors, and invariants.

#### Preconditions/Postconditions

The pre contracts specify the preconditions before a statement is executed. The most typical use of this would be in validating the parameters to a function. The post contracts validate the result of the statement. The most typical use of this would be in validating the return value of a function and of any side effects it has. In D, pre contracts begin with in, and post contracts begin with out. They come at the end of the function signature and before the opening brace of the function body.

int fun(ref int a, int b)
in (a > 0)
in (b >= 0, "b cannot be negative!")
out (r; r > 0, "return must be positive")
out (; a != 0)
{
// function body
}

int fun(ref int a, int b)
in
{
assert(a > 0);
assert(b >= 0, "b cannot be negative!");
}
out (r)
{
assert(r > 0, "return must be positive");
assert(a != 0);
}
do
{
// function body
}

The two functions are almost identical semantically. The expressions in the first are lowered to contract blocks that look almost exactly like the second, except that a separate block is created for each expression in the first, thus avoiding shadowing variable names.

#### Invariants

Invariants are used to specify characteristics of a class or struct that must always be true (except while executing a member function). For example, a class representing a date might have an invariant that the day must be 1..31 and the hour must be 0..23:

class Date
{
int day;
int hour;

this(int d, int h)
{
day = d;
hour = h;
}

invariant
{
assert(1 <= day && day <= 31);
assert(0 <= hour && hour < 24, "hour out of bounds");
}
}

For public or exported functions, the order of execution is:

1. preconditions
2. invariant
3. function body
4. invariant
5. postconditions

## Exercises

The exercises for lab-01 are located in this repo.

### 1. C to D

Go to this link. You will find a series of searching algorithms, each implemented in C in its own file.

1. Choose one algorithm and port it to D with minimum modifications.
2. Update the code using D specific features to improve the code (fewer lines of code, increase in expressiveness etc.).

### 2. Array Median Element

Compute the median element of an unsorted integer array. For this, you will have to:

1. Sort the array: implement any sorting algorithm you wish.
2. Eliminate the duplicates: once the array is sorted, eliminating the duplicates is trivial.
3. Select the (n+1)/2th element

### 3. The standard library

D has a standard library called phobos. Implement exercise 2 using functions from the std.algorithm package. Use UFCS for an increase in expressiveness.

### 4. Majority Element

Find the majority element in a string array using builtin associative arrays.

[Bonus] Implement a version with O(n) time complexity and O(1) space complexity

### 5. Struct vs. Class

Navigate to the 5-struct-class directory. You will find 2 files implementing the same program. One uses a struct as a data container, while the other uses a class.

1. Compile both files and measure the run time of each program. How do you explain the differences?
2. In both situations, print the value of the field a0 after the loop ends. How do you explain the differences?

### 6. Voldemort types

Navigate to the 6-voldemort directory. Inspect the source file voldermort.d. The declaration of struct Result is declared as private (nobody has access to it, except the members of the current file). Move the declaration of the struct Result inside the fun function. Compile the code. Does it compile? Why? Fix the issue.

### 7. Sanitization & Unittesting

Navigate to the 7 - BinarySearch directory. Inspect the source file binarySearch.d. As the name implies, a binarySearch algorithm is implemented on integers. Compile and run the file.

1. Write a unittest function that tests some corner cases. Are there any bugs in the algorithm implementation? If yes, fix them.
2. The binarySearch function may be called with invalid data (for example: l = -1, r = -2). Write an in contract that halts the code execution in the case of invalid input (invalid values for l and r, the array is not sorted etc.)
3. Rewrite the binarySearch algorithm to make use of slices.