Functions

Why use functions? They can break your problem down into smaller sub-tasks (modularity).  easier to solve complex problems They make a program much easier to read and maintain  abstraction – we don’t have to know how a function is implemented to use it (printf, scanf, etc.). Generalize a repeated set of instructions  we don’t have to keep writing the same thing over and over.

Functions A group of declarations and statements that is assigned a name  effectively, a named statement block.  usually return a value A sub-program  when we write our program we always define a function named main  inside main we can call other functions  which can themselves use other functions, and so on…

Example - Square #include double square(double a) { return a*a; } void main() { double num; printf("enter a number\n"); scanf("%lf",&num); printf("square of %g is %g\n",num,square(num)); } This is a function defined outside main Here is where we call the function square

Characteristics of Functions returnType name(argType1 arg_name1, argType2,arg_name2, …) { function body; return value; }

Return Statement Return causes the execution of the function to terminate and usually returns a value to the calling function. The type of the value returned must be the same as the return-type defined for the function (or a ‘lower’ type). If no value is to be returned, the return-type of the function should be set to ‘void’.

Factorial calculation example int factorial(int n) { int i, fact = 1; for (i=2; i<=n; i++) fact *= i; return fact; } void main() { int num; printf("enter a number\n"); scanf("%d",&num); printf(“The factorial of %d is %d\n",num,factorial(num)); } #include

A Detailed Example Write a program that receives a nominator and a denominator from the user, and displays the reduced form of the number. For example: if the input is 6 and 9, the program should display 2/3.

Example – solution (step I) #include int main(void) { int n, d; printf("Please enter nominator and denominator: "); scanf("%d%d", &n, &d); Calculate n’s and d’s Greatest Common Divisor printf("The reduced form of %d/%d is %d/%d", n, d, n/gcd, d/gcd); return 0; }

Example – solution (step II) #include int main(void) { int n, d, g; printf("Please enter nominator and denominator: "); scanf("%d%d", &n, &d); g = gcd(n, d); printf("The reduced form of %d/%d is %d/%d", n, d, n/g, d/g); return 0; }

Example – solution (step III) /* Returns the greatest common divisor of its two parameters. Assumes both are positive. The function uses the fact that if r = mod(y, x) then the gcd of y and x equals the gcd of x and r. */ int gcd(int x, int y) { int tmp; while(x > 0) { tmp = x; x = y % x; y = tmp; } return y; }

The Great Void Sometimes there’s no reason for a function to return a value In these cases, the function return type should be ‘void’ If the ‘return’ keyword is used within such a function it exits the function immediately. No value needs be specified. If the function receives no parameters, the parameter list should be replaced by ‘void’.

Pass-by-value Function arguments are passed to the function by copying their values rather than giving the function direct access to the actual variables A change to the value of an argument in a function body will not change the value of variables in the calling function

Scope of variables The scope of a variable is the boundaries within which it can be used in a program. Normally variables are local in scope - this means they can only be used in the function where they are declared. A variable declared within a function is unrelated to variables declared elsewhere, even if they have the same name Global variables can be declared, which can be used in any function. Global variables are a BAD PRACTICE.

Wrong way to do it int add_one(int b) { a=b+1; } void main() { int a=34,b=1; add_one(b); printf("a = %d, b = %d\n", a, b); return 0; }

Function swap void swap(int x, int y){ int temp; temp = x; x = y; y = temp; } void main() { int a=7,b=8; printf("a = %d, b = %d\n", a, b); swap(a,b); printf("a = %d, b = %d\n", a, b); }

Bad global variable use example This program only prints ONE row of stars Variables here are global #include void printRow(int); int i, size; void main() { printf(“Please enter a size:\n”); scanf(“%d”, &size); for (i = 1; i <= size; i++) printRow(i); } void printRow(int length) { for(i = length; i <= size; i++) printf("*"); printf("\n"); }

Function Declaration Most software projects in C are composed of more than one file. We want to be able to define the function in one file, and to use it in all files. For this reason, the function must be declared in every file in which it’s called, before it’s called for the first time. the declaration contains:  the function name.  the data types of the arguments (their names are optional).  the data type of the return value.

Function Declaration stdio.h actually contains a large set of function declarations The #include directive tells the compiler to insert these declarations into the file, so that these functions could be called

Function Declaration - Prototype #include int factorial(int a); /* Function Declaration! – Prototype */ void main(){ int num; printf("enter a number\n"); scanf("%d",&num); printf("%d!=%d\n",num,factorial(num)); } int factorial(int a){ int i,b=1; for(i=1; i<=a; i++) b=b*i; return b; }

The math library A collection of mathematical functions Need to include the header file math.h (#include ) Use functions of the library, e.g. double s,p; s=sqrt(p); Declared in math.h : double sqrt ( double x );

The math library sin(x), cos(x), tan(x)  x is given in radians asin(x), acos(x), atan(x) log(x) sqrt(x) pow(x,y) – raise x to the yth power. ceil(x), floor(x) …and more

Passing arrays to functions Functions can accept arrays as arguments. Usually the array’s size also needs to be passed (why?). For example - int CalcSum(int arr[], int size); It is not necessary when the size of the array fix in a #define declaration. Within the function, arr is accessed in the usual way.

Passing arrays to functions Arrays can be passed to functions and have their values changed from within the function!  Unlike regular variables This is possible because an array variable is actually an address and the function receive this address.

Two Dimensional Arrays When passing a two-dimensional array to a function, it is necessary to indicate the size of the second dimension in the function header  But this does not mean the array’s size needn’t be passed as well int func(int arr[][4], int a);  Same was true when initializing the array double B[][2] = {{1,2}, {2,3}, {3,4}};