CSCI 171 Presentation 6 Functions and Variable Scope

Functions Functions: –are named uniquely –perform specific tasks –are independent should not interfere with other parts of program –may need data to accomplish their task parameters (arguments) –may send data back to their initiator returning a value

Function Example #include double calcForce (double, double);//function prototype int main( void ) { double mass = 3; double acceleration = 4; double force = 0; force = calcForce (mass, acceleration);//calling the function printf("%lf", force); } double calcForce(double m, double a)//function header { return (m * a);//function body }

Function Example - cont’d Function definition (header and body): double calcForce(double m, double a) { return (m * a); } Function name is: calcForce Function parameters are: 2 doubles (m and a) Function returns: 1 double

Sample Program 7.1 #include float findArea(float, float); void main() { float base = 0, height = 0, area = 0; printf("Enter triangle base: "); scanf("%f", &base); printf("Enter triangle height: "); scanf("%f", &height); area = findArea(base, height); printf("The area is: %.2f", area); } float findArea(float b, float h) { float area = 0; area = 0.5 * b * h; return area; }

How a function works Is not executed unless called “Calling function” sends in any arguments –information to perform the process Control returns to –statement following function call (if no return) –function call (if a value is returned)

Parts of a function Prototype –function name –argument type(s) –return type Definition –header –body (actual code)

Local Variables Variables declared within a function are local –only defined within the function –not accessible by other functions Arguments act locally as well

Note the local variables in 7.1 #include float findArea(float, float); void main() { float base = 0, height = 0, area = 0; printf("Enter triangle base: "); scanf("%f", &base); printf("Enter triangle height: "); scanf("%f", &height); area = findArea(base, height); printf("The area is: %.2f", area); } float findArea(float b, float h) { float area = 0; area = 0.5 * b * h; return area; }

Return values Functions may have more than 1 return value, but only 1 is ever returned –Generally this is bad structure, not suggested Ex: if (a > b) return a; else return b;

Return types Return type is declared in function header & prototype Actual value being returned is in return statement Examples: double calcForce(double m, double a) { return (m * a); } float findArea(float b, float h) { float area = 0; area = 0.5 * b * h; return area; }

No return type void specifies that there is no return value no return statement in function Ex: void writeErrorMessage() { printf(“\nInvalid input”); printf(“\nPlease check specifications”); errorCount += 1; }

Calling a function Calling a function with no return  writeToFile( ); Calling a function with a return value  x = square(3);  printf(“The square of 3 is: %d”, square(3)); Calling a function as a parameter  c = sumTheValues(square(3), square(4)); Multiple functions in an expression  b = square(3) + square(4);

Global Variables Placed outside of all functions Scope of variable is entire program –can be accessed by any function within the file Generally considered bad programming style

Sample Program 7.6 #include double force = 0.0;//Global variable – accessible to all functions in file double calcForce (double, double); int main( void ) { double mass = 3.0; double acceleration = 4.0; force = calcForce (mass, acceleration); printf("%lf", force); } void calcForce(double m, double a)//function header { force = m * a; }

What is scope? Parts of program which can access a variable –accessibility –visibility How long variable takes up system resources (memory) –Dynamic memory allocation

Scope Program 1 (compiles) | Program 2 (does not compile) int x = 999;|void squareX() | void squareX();|void main() { |int x = 999; void main() {|x = x * x; x = x * x;|} }| |void squareX() { void squareX () { |x = x * x; x = x * x;|} }|

Importance of scope Modularization –each function should be independent –variables isolated from interference Scope allows for control over degree of isolation

Local Variables Defined within a code block Visibility limited to that code block Usually a function’s local variable’s value is re-initialized with each call to the function Can request variable not be destroyed –static keyword

Local variables - example void main() { outputNumbers(); } ____________________________ void outputNumbers() {|Program output: static int x = 0;|0 0 int y = 0;|1 0 printf(“\n%d %d”, x++, y++);|2 0 }|

Parameters and Scope A variable in the parameter list acts locally void function1(int x) { int y = 0; printf(“%d %d”, x, y); } In the preceding: –x is a parameter (and so is only accessible within the function) –y is a local variable (and so is only accessible within the function)

Register variables Suggests that a local variable be stored in processor register (not regular memory) void function1() { register int x; …. } Any processing with x will be done quicker Can only be used with simple numeric variables –can’t use with arrays, structures, etc.

Code Blocks Variables can be declared as local within program blocks (statements enclosed in {}) void main() { int count = 0; printf(“%d”, count); { int count = 10; printf(“%d”, count); } printf(“%d”, count); }

Guidelines Initialize all variables Pass data as function parameters unless most functions use the data Use symbolic constants for constant data that is used in most of the functions –PI in a math library Put definitions at beginning of scope –functions, files, etc.