Lesson 18 · 18 of 20

Dynamic arrays · Pascal

Use Pascal dynamic arrays that grow and shrink with SetLength: a shopping cart with routines to add items and total them up.

Pascal · Dynamic arrays in DMarlon Studio: the example's code and its output in the terminal

A dynamic array is declared without a size, array of Double, and resized while the program runs with `SetLength`. It starts empty: nil is a dynamic array with no elements.

Add grows the cart: SetLength(list, Length(list) + 1) adds a slot at the end and list[High(list)] := value fills it. The parameter is var because the procedure changes the caller's array.

Unlike static arrays, dynamic arrays always start at index 0: with three items, the indexes run from 0 to High(cart), which is 2, while Length gives 3.

Sum takes the array as const, without copying it, and walks through it with for x in list. To pass a dynamic array as a parameter, it is best to define its type first, here TPrices.

SetLength(cart, 2) also shrinks the array: the last item disappears. There is no need to free the memory: Free Pascal does it on its own once the array is no longer used.

Code · main.pas

program main;type  TPrices = array of Double;procedure Add(var list: TPrices; value: Double);begin  SetLength(list, Length(list) + 1);  list[High(list)] := value;end;function Sum(const list: TPrices): Double;var  x: Double;begin  Result := 0;  for x in list do    Result := Result + x;end;var  cart: TPrices;  i: Integer;begin  cart := nil;  // nil: an empty dynamic array, with 0 items  Add(cart, 2.50);  Add(cart, 0.99);  Add(cart, 4.75);  for i := 0 to High(cart) do  // index 0 is the first    WriteLn('Item ', i, ': ', cart[i]:5:2);  WriteLn('Total: ', Sum(cart):0:2, ' (', Length(cart), ' items)');  SetLength(cart, 2);  // shrinking drops the last item  WriteLn('Without the last one: ', Sum(cart):0:2);end.

Output · fpc main.pas && ./main

Item 0:  2.50
Item 1:  0.99
Item 2:  4.75
Total: 8.24 (3 items)
Without the last one: 3.49

Key points

array of T is an array with no fixed size.
SetLength(a, n) grows or shrinks it.
Indexes run from 0 to High(a).
The memory is freed automatically.

Exercise

Write a MostExpensive(const list: TPrices): Integer function that returns the index of the most expensive item, and print its position and price.