2. Why functions segments of code that repeat several times
better readability
abstraction of code
smaller program size
IAG0581

3. Structural programming
int main(void){ int a, b, c; do{ printf("Please enter A: "); scanf("%d", &a); if(a <= 0) printf("A must be positive!\n"); }while(a <= 0); printf("Please enter B: "): scanf("%d", &b); if(b <= 0) printf("B must be positive!\n"); }while(b <= 0); //... return 0; }
IAG0581

4. Functional programming
int readPositiveInt(char *name){ int x; do{ printf("Please enter %s: ", name); scanf("%d", &x); if(x <= 0) printf("%s must be positive!\n", name); }while(x <= 0); return x; } int main(void){ int a, b, c; a = readPositiveInt("A"); b = readPositiveInt("B"); //... return 0;
IAG0581

5. Declaring a function
returntype functionName(parameters){ functionBody }
parameters is just a list of variables separated by comma: int a, int b
use "void" if no parameters
IAG0581

6. Datatypes, simple types
char a single character or small number:
int integer number
float floating point number
void signifies no type
When used as parameters, these are unidirectional
IAG0581

7. Datatypes, pointers char * pointer to a character or character array
int * pointer to integer or integer array
float * pointer to float or floating point array
void * pointer to memory without type specified
When used as parameters, these are bidirectional
IAG0581

8. Datatypes, arrays char[] a character array int[] an array of integers
float[] a array of floating point numbers
When used as parameters, these are bidirectional
IAG0581

9. Calling a function even if no parameters, ( ) are necessary
even if function returns something it can be called without assignment
For example:
int doSmth(void){
//...
return 1; }
int main(void){
doSmth();
return 0;
IAG0581

10. C compiler single pass compiler
accepts only variables and functions that have been declared before they are used
sometimes necessary to declare that a function exists before providing code for that function
IAG0581

11. Function prototypes
used to tell compiler that a function by that name exists and what parameter types it takes
parameter names are unimportant
For example:
int readPositiveInt(char *);
IAG0581

12. Function prototypes
int readPositiveInt(char *); int main(void){ //... a = readPositiveInt("A"); } int readPositiveInt(char *name){
IAG0581

13. Variable scope variable exists in block where it is declared
outside block it doesn't exist
variable can be redefined in an inner block
each function is separate block
IAG0581

14. Variable scope
int fn1(void){ int x = 2; //... } int main(void){ int a, b, x; x = 5; a = fn1(); if(a < b){ float x = 3.0; x++;
IAG0581

15. Function with no data exchange
void welcome(void){ printf("This program ..."); printf("Written by..."); } int main(void){ welcome(); ...
IAG0581

16. Getting a result from a function
result is returned with return keyword.
what's returned should be of same type as declared returntype, if not it will be converted
IAG0581

17. Getting a result from a function
int test(void){ float f = 0.8; return f; // will return 0, cos converted to int } int test2(void){ ... return; //warning !! void test3(void){ int x = 10; return x; //warning !!
IAG0581

18. Getting multiple results
only 1 item of returntype can be returned
returntype could be complex type, like record or pointer etc.
use pointers as parameters so function can access memory outside of its scope
Example you should know:
scanf("%d", &a);
& operator returns address of where variable is located in memory
IAG0581

19. Getting multiple results
int div(int a, int b, int *result, int *remainder){ if(b == 0) return 0; *result = a / b; *remainder = a % b; return 1; } int main(void){ //... int s, t; div(15, 6, &s, &t);
IAG0581

20. Getting multiple results with arrays
arrays are always pointers to first element
function can access all elements if it knows how many there are
IAG0581

21. Getting multiple results with arrays
void doSmth(int n, float M[n]){ int i; for(i = 0; i < n; i++) M[i] = //... } int main(void){ int Arr[10]; doSmth(10, Arr); ...
IAG0581

22. Conditions
a function call can be put anywhere in code, also in place of conditions
if you have complex condition then make it into a function, especially if needed more than once
For example:
while( a < b && a < c || a + b < c + 1 ){
//... }
IAG0581

23. Conditions
int isTrue(int a, int b, int c){ return a < b && a < c || a + b < c + 1; } int main(void){ //... if(!isTrue(3, b, x)) while(isTrue(3, b, x)){
IAG0581

24. Function as parameter
permits you call a function that has been passed as a parameter to your code
alternatively you can write a function that behaves as a parameter for some function you want to call
typical example qsort
void qsort (void* base, size_t num, size_t size,
int (*compar)(const void*,const void*));
IAG0581

25. Function qsort()
qsort() sorts an array if you supply it a function that can compare 2 elements in it, essentially a condition function
int isLess(const void *aa, const void *bb){
int *a = (int*)aa;
int *b = (int*)bb;
return (*a) - (*b); }
int main(void){
int M[10];
//...
qsort(M, 10, sizeof(int), isLess);
IAG0581

26. Function as parameter
int readInt(char *name, int (*suits)(int), char *cond){ int x; do{ printf("Please enter %s: ", name); scanf("%d", &x); if(!suits(x)) printf("%s must be %s!\n", name, cond); }while(!suits(x)); return x; } int isPos(int a){ return a > 0; int main(void){ int a, b, c; a = readInt("A", isPos, "positive"); b = readInt("B", isPos, "positive"); //...
IAG0581