added C math exercise

exercises/093_hello_c.zig

@@ -57,7 +57,7 @@ pub fn main() void {
     const c_res = write(2, "Hello C from Zig!", 17);
 
     // let's see what the result from C is:
-    std.debug.print(" - C result ist {d} chars\n", .{c_res});
+    std.debug.print(" - C result ist {d} chars written.\n", .{c_res});
 }
 //
 // Something must be considered when compiling with C functions.

exercises/094_c_math.zig

@@ -0,0 +1,33 @@
+//
+// Often C functions are used where no equivalent Zig function exists
+// yet. Since the integration of a C function is very simple as already
+// seen in the last exercise, it naturally offers itself to use the
+// very large variety of C functions for the own programs.
+// In addition immediately an example:
+//
+// Let's say we have a given angle of 765.2 degrees. If we want to
+// normalize that, it means that we have to subtract X * 360 degrees
+// to get the correct angle. How could we do that? A good method is
+// to use the modulo function. But if we write "765.2 % 360", it won't
+// work, because the standard modulo function works only with integer
+// values. In the C library "math" there is a function called "fmod".
+// The "f" stands for floating and means that we can solve modulo for
+// real numbers. With this function it should be possible to normalize
+// our angel. Let's go.
+
+const std = @import("std");
+
+const c = @cImport({
+    // What do wee need here?
+    ???
+});
+
+pub fn main() !void {
+    const angel = 765.2;
+    const circle = 360;
+
+    // Here we call the C function 'fmod' to get our normalized angel.
+    const result = c.fmod(angel, circle);
+
+    std.debug.print("The normalized angle of {d: >3.1} degrees is {d: >3.1} degrees.\n", .{ angel, result });
+}