Understanding User Defined Functions and Variables in Code

1. 4. User Defined Functions(part 2)

2. Revision: Maths - Substitution • Expression for y, in terms of m, x, and c:y = mx + cthis is the formula of a straight line graph. • Given values for m, x, and cm = 2, x = 3, c = 5 • Possible to calculate y: y = mx + c from above y = (m * x) + c computing terms y = (2 * x) + c substitute m with 2 y = (2 * 3) + c substitute x with 3 y = (2 * 3) + 5 substitute c with 5

3. Substitution Examples • Calculate b given that: b = z + yf, z = 10, y=2, and f = 0.5 • Calculate L given that: L = v + y + x + 2v, v = 4, y = 3, x = 2

4. Revision: Variables • Variables – storedata in computer’s memory • Variables have • Identifier (name) – you choose this, used to refer to (reference) variable • Type – you choose this (to suit purpose) • Value – you set/change this x 23 Integer Identifier Type Value Memory

5. Revision: Variables • Variables are: • Declared, using the following syntax (grammar/form):var<identifier>:<type>;for example:var weight: double; • Assigned values, using the following syntax:<identifier>:=<value expression>;for example:weight := 109.45;Note: the data flows backwards (from right to left)

6. Variable Declaration: Examples • Write a line of code that: • Declares a variable called x of type double • Declares a variable called y of type integer • Declares a variable called surname of type string • Declares a variable called age of type integer

7. Variable Assignment: Examples • Write a line of code that: • Assigns the value of 23 to the variable y • Assigns the value of 14.6 to the variable x • Assigns the value of ‘John’ to the variable surname • Assigns the value of 21 to the variable age

8. Variable Assignment: Examples • Write a line of code that: • Increases the value of x by 2.89 • Increases the value of y by 43 • Decreases the value of age by 1 • Decreases the value of x by y

9. Functions • Functions often used to hide detail of calculations • For example, the following: m = k / 1.6 • gives • expression for m in terms of k • expression for miles in terms of kilometres • So, if we’ve walked 8 kilometres (k = 8) m = k / 1.6 m = 8 / 1.6 m = 5

10. Functions • To convert from km to m in code, we could type:m := k / 1.6; • Assuming: • m and k have been declared as variables earlier in the program. • This would become monotonous, if the program did it many times • Especially if the calculation were more complex • To solve this we can define and use a function

11. Functions k m := k / 1.6 m • To replace the following with a function:m := k / 1.6; • Ask: • what goes into the calculation, and • what comes out • we: • Put a value for kinto the calculation, and • Get a value for mout

12. Functions: defining k: double m := k / 1.6 m: double • The diagram (with types added): • The function code:function m(k: double): double;begin Result := k / 1.6;end;

13. Functions: calling • The function code:function miles(km: double): double;begin Result := km / 1.6;end; • Note: • m has changed to miles and k to km • to be meaningful, and unique (different from other identifiers) • Can be called:m := miles(k);

14. Functions: calling • Function Definition:function miles(km: double): double;begin Result := km / 1.6;end; • Function Call:var m: double;var k: double; k := 8; m := miles(k);

15. Example: Functions Demo (form)

16. Example: Functions Demo (code) implementation ... const KMperMile = 1.6; ... function Miles(km: real): real; begin Result := km / KMperMile; end; ... procedure TfrmFuncts.cmdMilesClick(Sender: TObject); var tmpKM: real; var resMiles: real; begin tmpKM := StrToFloat(txtKM.Text); resMiles := Miles(tmpKM); lblMiles.Caption := FloatToStr(resMiles); end; end.