Chapter 3 : Top Down Design with Functions By Suraya Alias

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 Case study Finding the Area and Circumference of a Circle  1) Problem Definition / Requirement Get the radius of a circle. Compute and display the circle’s area and Circumference.  2) Analysis What are the data requirement and formula needed? Problem constant  PI = Problem Input  radius /*radius of a circle*/ Problem Output  area /*area of a circle*/  Circum /*circumference of a circle*/ Relevant formula  Area of a circle = π X radius²  Circumference of a circle = 2 π X radius 1-4

 3) Design Algorithm  1) Get the circle radius  2) Calculate the area  3) Calculate the circumference  4) Display the area and the circumference Step 2 needs refinement  2.1) Assign PI * radius * radius to area Step 3 needs refinement  3.1) Assign 2 * PI * radius to circum 1-5

1-6 5) Implementation 6) Testing

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 Predefined Function and Code Reuse By reusing predefined functions such as to perform mathematical function is called code reuse C’s standard math library using #include defines a function named sqrt that perform the square root computation  Example; y = sqrt(x), where  sqrt(x) is the function call,  sqrt is the function name, (x) is the argument  Other functions are pow(x,y), log(x), exp(x), cos(x), tan(x), sin(x) 1- 11

) Given y=sqrt(x); 2) x=16.0, so function squareroot computes the 3) The function result, 4.0 is assigned to y

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 We can use the C function pow(power) and sqrt to compute the roots of a quadratic equation in x of the form  /*compute two roots, root_1 and root_2, for disc > 0.0*/ disc= pow(b,2)-4 * a * c; root_1=(-b+sqrt(disc))/2*a; root_2=(-b-sqrt(disc))/2*a; 1- 15

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 Top –Down Design Problem solving method where problems are break into sub problems, then the sub problems are solved to solve the original problem.  Structure Chart A documentation tool that shows the relationships among the sub problems of a problem 1- 17

1- 18 Case study : Drawing Simple Diagram

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 Syntax : fname();  example: draw_circle();  The empty parentheses after the function name indicate that draw_circle requires no argument  Function prototype A function must be declared before it can be referenced. One way is to insert a function prototype before the main function A function prototype tells the C compiler the data type of the function, the function name and the information about the arguments that the function expects 1- 20

 Form : ftype fname(void);  Example : void draw_circle(void); Use void as ftype if the function does not return a value The argument list (void) indicates that the function has no argument The function prototype must appear before the first call to the function 1- 21

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 Function Definitions Syntax : ftype fname (void) { local declaration executable statements } 1- 23

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 Advantages of Using Function Subprograms 1) Procedural Abstraction  A programming technique in which a main function consists of a sequence of a function calls and each function is implemented separately 2) Reuse of Function subprogram  The function can be executed more than once in a program  Example : function draw_intersect is called twice  Displaying User Instructions We can use functions without argument to display message, display lines of program output or instruction to user

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 Input Arguments Arguments used to pass information into a function subprogram  Output Argument Arguments used to return results to the calling function  We can also return a single result from a function by executing a return statement in the function body  Argument make function subprogram more versatile 1- 31

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1- 33 Void Functions with Input Arguments Actual Argument An expression used inside the parentheses of a function call, (135.68) Formal Parameter an identifier that represents a corresponding actual argument in a function definition (rnum)

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1- 38 If PI = , radius=10.0 circum = find_circum(radius) C subtitute the actual argument used in the function call for the formal parameter r

1- 39 Program Style Function Interface comment Pre condition – a condition assumed to be true before a function call Post condition – a condition assumed to be true after a Function executes

1- 40 Actual Argument corresponds toFormal Parameter num_1x num_2n

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 The number of actual arguments used in a call of a function must be the same as the number of formal parameters listed in the function prototype  The order of arguments in the lists determines correspondence. The first actual argument corresponds to the first formal parameter, and so on.  Each actual argument must be of a data type that can be assigned to the responding formal parameter with no unexpected loss of information  Testing functions using driver Driver is a short function written to test another function by defining its arguments, calling it and displaying its result The main function can act as a driver 1- 42

1- 43 The function call scale(2.5,-2) returns the value where (2.5 X 10 ⁻ ²) From the formula (x * scale_factor) scale_factor = pow(10, n)

 ConstructEffect Function Prototype (void function without argument) void star_line (void); Describes star_line as a function with no result and no argument Function Prototype (function with argument and a result) double average (int n, double x); Describes average as function with a type double result, 2 arguments, one type int and one type double Function Call Statement (void function without argument) star_line(); Calls function star_line and causes it to begin execution Function Call (function with argument and a result) money = average(num_kids, funds); Calls function average to compute a result that is stored in money Function Definition (void function without arguments) void star_line(void) { printf(“*\n*\n*\n*\n”); } Defines star_line as a function that prints a vertical line of four asterisks.

1- 45 ConstructEffect Function Definition (function with arguments and a result) /* *Returns the average of its 2 arguments *Pre : x and n are defined, x >= 0, n>0 *Post : result is x / n */ double average (int n, double x); { return (x/n) } Defines average as a function that returns the result of dividing its second argument with its first argument