Merge pull request #195 from Seanny123/memory-allocation

Memory allocation exercise

build.zig

@@ -483,6 +483,10 @@ const exercises = [_]Exercise{
         .main_file = "095_for_loops.zig",
         .output = "1 2 4 7 8 11 13 14 16 17 19",
     },
+    .{
+        .main_file = "096_memory_allocation.zig",
+        .output = "Running Average: 0.30 0.25 0.20 0.18 0.22",
+    },
     .{
         .main_file = "999_the_end.zig",
         .output = "\nThis is the end for now!\nWe hope you had fun and were able to learn a lot, so visit us again when the next exercises are available.",

exercises/096_memory_allocation.zig

@@ -0,0 +1,75 @@
+// In most of the examples so far, the inputs are known at compile
+// time, thus the amount of memory used by the program is fixed.
+// However, if responding to input whose size is not known at compile
+// time, such as:
+//  - user input via command-line arguments
+//  - inputs from another program
+//
+// You'll need to request memory for your program to be allocated by
+// your operating system at runtime.
+//
+// Zig provides several different allocators. In the Zig
+// documentation, it recommends the Arena allocator for simple
+// programs which allocate once and then exit:
+//
+//     const std = @import("std");
+//
+//     // memory allocation can fail, so the return type is !void
+//     pub fn main() !void {
+//
+//         var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+//         defer arena.deinit();
+//
+//         const allocator = arena.allocator();
+//
+//         const ptr = try allocator.create(i32);
+//         std.debug.print("ptr={*}\n", .{ptr});
+//
+//         const slice_ptr = try allocator.alloc(f64, 5);
+//         std.debug.print("ptr={*}\n", .{ptr});
+//     }
+
+// Instead of an simple integer or a constant sized slice, this
+// program requires a slice to be allocated that is the same size as
+// an input array.
+
+// Given a series of numbers, take the running average. In other
+// words, each item N should contain the average of the last N
+// elements.
+
+const std = @import("std");
+
+fn runningAverage(arr: []const f64, avg: []f64) void {
+    var sum: f64 = 0;
+
+    for (0.., arr) |index, val| {
+        sum += val;
+        avg[index] = sum / @intToFloat(f64, index + 1);
+    }
+}
+
+pub fn main() !void {
+    // pretend this was defined by reading in user input
+    var arr: []const f64 = &[_]f64{ 0.3, 0.2, 0.1, 0.1, 0.4 };
+
+    // initialize the allocator
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+
+    // free the memory on exit
+    defer arena.deinit();
+
+    // initialize the allocator
+    const allocator = arena.allocator();
+
+    // allocate memory for this array
+    var avg: []f64 = ???;
+
+    runningAverage(arr, avg);
+    std.debug.print("Running Average: ", .{});
+    for (avg) |val| {
+        std.debug.print("{d:.2} ", .{val});
+    }
+}
+
+// For more details on memory allocation and the different types of
+// memory allocators, see https://www.youtube.com/watch?v=vHWiDx_l4V0

patches/patches/096_memory_allocation.patch

@@ -0,0 +1,4 @@
+65c65
+<     var avg: []f64 = ???;
+---
+>     var avg: []f64 = try allocator.alloc(f64, arr.len);