Lab Session # 6 Functions. ALQUDS University Department of Computer Engineering

Transcription

1 2013/2014 Programming Fundamentals for Engineers Lab Lab Session # 6 Functions ALQUDS University Department of Computer Engineering Objective: After completing this lab, the students should be able to: 1. Understand the concept of functions in general and in C in particular. 2. Explain what a function prototype is, and how that is different from the function definition. 3. Convert the code processing in the main function to a function called from the main function. 4. Understand the concepts or arguments/parameters and returns. 5. Understand and apply recursion whenever applicable. Background: The function is a good mechanism to encapsulate code used repeatedly in a program so that it can be called from other parts of the code. A function does not use a keyword called function but instead the programmer has to define function prototype before the main function (one way to do it) and then define the function again later. A function has the following format type function_name (optional parameter list) { function code; return value; } Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 1

2 The data types in general could be the types of C variable including int, double, char etc. The function does some processing and the calculated/processed value is returned using the return value;. The main function or any other functions c an call this function_name, the value returned can be used like this: calling_value = function_name (parameters); A function does not need to always return a value. A function not returning a value can omit the return statement and the function type is defined as void in this case. A function can also return a pointer to a value. Pointer is covered later. A function can also return a user defined type (called class). These kinds of functions (called methods in some languages like Java or managed C++). Function prototype has the following format: type function_name (list of variable types); Examples are: Example 1. int compute (int); Example 2. void tryout (void); Function prototypes differ from the function definitions in two places: there is no code (no {} with code in between) and the variable names do not follow the types. A function prototype can return nothing, in which case void is the type returned; also it may have no parameters at all like example 2 above. The function prototype is declared before the main function with the function calls inside the main function or the other functions calling this function. The function definitions are put after the main function. Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 2

3 Experiment 6.1 Consider the code snippet that computes the sum of by a for loop. int sum = 0; for (int j = 1; j <= 100; j++) sum += j; Step1: Write a complete program that implements this: This program shows how to compute a sum in a quick way. The program can be easily modified to compute and display the sum of n for a variable n (with n = 100 as above). Such code of (3 lines) computing the sum can be coded as one instruction in the main function compute_sum(n) with the variable n as the parameter. Step2: Write a complete program (a main function+comupte_sum function) to perform the same thing: Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 3

4 - - - Experiment 6.2 Step 1: Write a C usual program to print out f(x) = ax^2 + bx + c for a = 1.0, b = 2.0, and c = 1.0 for x = 3.0, and 4.0 respectively. Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 4

5 Step 2: Write a C function double quadratic (double x) that computes function f(x) with x as the only input parameter and returns f(x) = ax^2 + bx + c with a = 1.0, b = 2.0, and c = 1.0, make sure you are using function implementation. Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 5

6 Step 3: Write a function distance2 that calculates the distance between two points (x1, y1) and (x2, y2) in the plane. Put all numbers and return types in double. Use (3.0, 0.0) and (0.0, 4.0) as one pair of input to verify your program, write the full program to test that Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 6

7 Experiment 6.3 Array Arguments Step 1: Write a that asks the user to enter an array of two dimensions, and then compute the following: a. sum of the array. b. number of positive numbers. Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 7

8 Step 2: Rewrite the same previous, but using functions implementation, i.e. a function to perform each of the following: a. calculate the sum of the array. b. calculate the number of positive numbers. c. sort the array (think about an easy algorithm). And test it. Here we can define the array on upper scope (global for ex), we can define parameters as array_name[], and pass argument as just array_name. Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 8

9 Experiment 6.4 Recursive Functions - A recursive function is one which calls itself. - A useful way to think of recursive functions is to imagine them as a process being performed where one of the instructions is to "repeat the process". - This makes it sound very similar to a loop because it repeats the same code, and in some ways it is similar to looping. - Of course, it must be possible for the "process" to sometimes be completed without the recursive call. - Care must be taken to define functions which will not call themselves indefinitely, otherwise your program will never finish. - Creating a recursive procedure essentially requires defining a "base case" or termination condition, and then defining rules to break down more complex cases into the base case. - Key to a recursive procedure is that with each recursive call, the problem domain must be reduced in such a way that eventually the base case is arrived at. - One simple example is the idea of building a wall that is ten feet high; if I want to build a ten foot high wall, and then I will first build a 9 foot high wall, and then add an extra foot of bricks. Conceptually, this is like saying the "build wall" function takes a height and if that height is greater than one, first calls itself to build a lower wall, and then adds one a foot of bricks. - So: any algorithm that can be defined in terms of multiple copies of itself, can also be implemented as a recursive function. General format: void recurse() { recurse(); /* Function calls itself */ } int main() { recurse(); /* Sets off the recursion */ return 0; } Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 9

10 Step 1: Write a that print the number 0-10, using recursion: Hint: Think first of a traditional loop-style way, then think about formalizing the process as an algorithm that can call itself Step 2: Write a that print ZYXWVUTS..BA using recursive function(s): Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 10

11 Post lab #6 1. Write a function that returns the square of an integer number. 2. Write a function that take a character in a lower case and returns it in an uppercase. 3. Write a that defines a one-dimensional array (10 locations), reads in data, then pass it into the following three functions that: a. Returns the sum of the array. b. Returns the number of positive numbers in the array. c. Returns the largest number. 4. Write the function bool isprime (int n) that takes an integer type parameter n and returns whether n is a prime or not. 5. Write a that prompts for an integer n and computes the primes less than n. 6. Modify and enhance your code to display all prime numbers from 2 to n, 8 in a row (you do not display j is a prime in a line, but display in the first row, in the second row etc. 7. Enhance your program to print out the total number of prime numbers up to n. For example, the total number of primes up to 100 is Can you think of a recursive function based program that sort an array of numbers? Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 11