APS 105 Functions and Pointers 1 Modularity Modularity

APS 105 Functions (and Pointers) 1

Modularity • Modularity – Break a program into manageable parts (modules) – Modules interoperate with each other • Benefits of modularity: modularity/modules in C: functions 2

Functions 3

C pre-defined functions • main • Math functions – Sqrt(), fabs(), etc • I/O functions – printf() , scanf() 4

Two Types of Functions • Commands – perform a process, but don’t return anything • Queries – evaluate something, return a result • Some languages: – commands: called procedures, subroutines – queries: called functions • C language: – commands: called functions – queries: called functions 5

Defining a Function • Define a function that prints a blank line. • Note: this is called a "function definition" 6

Incorporating a Function in a Program. 7

Function Prototype • Sometimes must summarize a function – tell the compiler what it needs to know – eg. , if function has parameters or return value • Function Prototype: – similar to a copy of the first line of the function definition – Example of a prototype: . 8

Prototype Example: Declare main First.

Parameters 10

Functions with Parameters • Define a function that prints n blank lines. • Note: n is called a "parameter" 11

Calling a Function with Parameters. 12

When A Function is Called… 1) argument(s) is/are evaluated 2) control passes to the function 3) parameter(s) is/are assigned value(s) • • • the values of the argument(s) are copied i. e. , param gets a copy of the value of the arg this method is called "call-by-value" • • the only method used by C other languages provide other methods 4) the function is executed 5) control passes back to the calling function 13

Example of A Query Function • a function that returns the factorial of an int. 14

Function Scope of Identifiers/Variables • An identifier/variable exists within its scope – if declared within a function • the scope is within the function • Implications: for a var declared within a func – can't be accessed outside that function – can re-use the identifier in multiple functions 15

Example of Scope. 16

Example of Multiple Parameters. 17

Parameter Type Correspondence • Param and argument types needn't match – as long as they are "assignment compatible" • Example: . 18

Returning 19

Multiple Returns • A function can have multiple return statements • Example: a function that returns max value. 20

Avoiding Multiple Returns • Example: a function that returns max value. 21

No Return • Example: print n blank lines. 22

Style: Returning Bool • Functions that return bool should be named: is<something> • Produces more readable code • Example: . 23

is. Perfect. Square. 24

is. Perfect. Square (cont'd). 25

is. Perfect. Square (cont'd). 26

Pointers 27

Swap • Example: a function to swap two values. 28

Pointers • Need a way to refer to locations of things – not just a copy of a variable’s value • a pointer: – points to a variable – eg. , like a house address points to a house • Example: 29

Memory • RAM – random access memory – composed of memory chips Memory – organized like a giant table of locations Address Value • Addresses – every location in table has an address – just like every house on a street – use the address to read/write a location • In C – addresses are stored in pointer variables – a pointer is a memory address 0 0 1 23 2 0 3 0 … … 232 -2 102 232 -1 7 30

Understanding Pointers • Declaring a pointer variable int *p; • Declaring and assigning int x = p =. *p; x; 8; &x; Memory Addr Value 0 0 1 23 2 0 3 0 … … 232 -2 102 232 -1 7 31

Using Pointers • Declaring, assigning, and using a pointer int *p; int x; x = 8; p = &x; printf("%dn", *p); // print what's at p. 32

Pointers Example double x, y; double *p, *q; x = 3. 6; y = 6. 7; p = &x; q = &y; *p = 1. 0; *q = *p + 1. 0; Memory Addr Value 0 0 1 0 2 0 3 0 4 0 5 0 … … 232 -2 0 232 -1 0 33

Pointers Example Again double x, y; double *p, *q; x = 3. 6; y = 6. 7; p = &x; q = &y; *p = 1. 0; *q = *p + 1. 0; p x q y 34

Fixed Swap • Example: a function to swap two values . 35

Functions that Return Pointers • Create a function called large. Loc – Takes addresses of two doubles – Returns the address of the larger value • Example usage: double x = 2. 6; double y = 3. 4; double *p; p = large. Loc(&x, &y); . 36

large. Loc. 37

Scope 38

Scope of Internal Identifiers • Scope of an identifier: – the range within which it is recognized – an identifier is not recognized outside its scope • i. e. , it cannot be used, or compiler will complain • Ex: for (int i = 1; . . . ) – scope of i is within the for loop only • Scope of a function parameter: – only within the body of the function • Scope of a variable declared in a function – starts at the point of declaration – ends at the end of the block {} it was declared in – called an internal identifier 39

Scope Example int foo(int x) { int y=5; for (int i=0; i<10; i++) { int z = 3; y *= z + x; } return y; } 40

Overlapping Scope int i = 1; printf(“i = %dn”, i); { int i = 2; printf(“i = %dn”, i); } printf(“i = %dn”, i); • What is the output? 41

Scope of External Identifiers • External identifier – one that is declared outside of any function – ex: a function identifier/name is external • Example: int x; // x is an external identifier void main() {. . . } • If external ident. declared before all func. s: – then it is called global 42

Top-Down Modular Design • Functions and scope provide modularity – can build them independently and combine • Modularity eases large programs – break the problem into smaller & smaller parts – until the parts are small and manageable – make these into functions – combine them into the whole 43

Ex: Goldbach’s Conjecture • Can an even number larger than 2 be expressed as the sum of two primes? . 44