1. Functions / Procedures
In C, a function is equivalent to a procedure or a function in Pascal
A function provides a convenient way to encapsulate some computation, which can then be used without worrying about its implementation
Main idea: “To ignore how a job is done; knowing what is done is sufficient”
Functions we have used so far: printf, scanf, getchar, putchar
Statement structure
return-type function-name (parameters declarations, if any) {
declarations
statements
return expression; }

2. Simple functions #include <stdio.h> void prn_message( );{
printf(“Message for you: “);
printf(“Have a nice day!\n”); }
int main( )
prn_message( );
return 0;
#include <stdio.h>
void prn_message(int n);
void prn_message(int n) {
int i;
printf(“Message for you: “);
for (i = 0; i < n; i++)
printf(“%d. Cheers :D \n”, i);
printf(“Have a nice day!\n”); }
int main( )
int n;
printf(“How many CHEERS? “);
scanf(“%d”, &n);
prn_message(n);
return 0;

3. Function power(x, y) #include <stdio.h>
int power(int x, int y); /* declares the parameters types
and names */
int main() {
int i;
for (i = 0; i < 10; i++)
printf(“%d %d %d\n”, i, power(2, i), power(-3,i));
return 0; }
/* Power: raise base to n-th power; n >=0 */
int power(int base, int n)
int i, p;
p = 1;
for (i = 1; i <= n; i++)
p = p * base;
return p;

4. Something related to functions
Main program is a function, too
Function definitions can appear in any order
What are parameters, formal & actual arguments
Formal arguments are local variables
The scope of the variables
A function need not return a value, a return statement with no expression cause control, but no useful value to be returned to the caller
The function-type -- the data type when the function returns
Return-types conversion

5. Functions C2F(int), and Function F2C(int)
#include <stdio.h>
/* Instead of float C2F(int x); */
float C2F(int);
float F2C(int);
int main() {
float tmp;
int i;
for (i = 0; i < 100; i++){
tmp = C2F(i);
printf(“%d C = %f F\n”, i, tmp); }
for (i = 0; i < 213; i++){
tmp = F2C(i);
printf(“%d F = %f C\n”, i, tmp);
return 0;
float C2F(int C) {
float tmp;
tmp = (C * 9.0 / 5.0) ;
return tmp; }
float F2C(int F)
tmp = (F ) * 5.0 / 9.0;

6. The scope of the variables
#include <stdio.h>
void proc1(int a, int b);
void proc2(int a, int b);
int i = 0, j = 0;
int main(){
int i;
printf("i=%d j=%d\n", i, j);
i = 5; j = 10;
proc1(i, j);
proc2(i, j);
return 0; }
void proc1(int a, int b){
printf("i=%d j=%d a=%d b=%d\n", i, j, a, b);
a = 15; b = 20; i = 25; j = 30;
void proc2(int a, int b){
int j = 35;
a = 40; b = 45; i = 50; j = 55;
Output:
i= j=0
i=5 j=10
i=0 j=10 a=5 b=10
i=25 j=30 a=15 b=20
i=5 j=30
i=25 j=35 a=5 b=30
i=50 j=55 a=40 b=45

7. Arguments -- Call by value
In C, all function arguments are passed by value
The called function is given the values of its arguments in temporary variables rather than the originals
#include <stdio.h> Program Output:
void try_to_change(int x); i = 5
int main() x = 5
{ x = 10
int i = 5; i = 5
printf(“i = %d\n”, i);
try_to_change(i);
return 0; }
void try_to_change(int x) {
printf(“x = %d\n”, x);
x = 10;

8. Advantage for call by value
int power(int base, int n) int power(int base, int n)
{ {
int i, p; int p;
p = 1; for (p = 1; n > 0; n--)
for (i = 1; i <= n; i++) p = p * base;
p = p * base; return p;
return p; } }
Structure Programming (char arrays & function)
Problem: To write a program that reads a set of text lines and prints the longest line out.
Using Top-Down approach:
1. declarations and initializations
2. reading and text processing
3. printing out the longest line

9. Refinements 2. Processing the text 2.2 (refinement) 2.2.1 (refinement)
2.1 While there is another line do
2.2 The corresponding actions when you find a longer line
2.2 (refinement)
2.2.1 if (it’s longer than the previous longest line) do
2.2.1 (refinement)
save its length
save the line for later printing
After we all these refinements, we can build our program by substituting the smallest refinements by C code
Functions or procedures could be used to encapsulate some computations

10. Final Coding #include <stdio.h> int getline(char s[], int lim)
#define MAXLINE 1000 /* max. input line size */ {
int getline(char line[], int maxline); int c, i;
void copy(char to[], char from[]); for (i = 0; i < lim-1 &&
/* print longest input line */ (c = getchar()) != EOF &&
int main() c != ‘\n’; i++)
{ s[i] = c;
int len; if (c == ‘\n’){
int max; s[i] = c;
char line[MAXLINE]; i++;
char longest[MAXLINE]; }
max = 0; s[i] = ‘\0’;
while ((len = getline(line, MAXLINE)) > 0) return i;
if (len > max){ }
max = len;
copy(longest, line); }
if (max > 0) printf(“%s”, longest);
return 0;
void copy(char to[], char from[]) {
int i = 0;
while ((to[i] = from[i]) != ‘\0’) i++;

11. Function atoi() Converting a string to its number equivalent. (atoi)
Steps:
1. Skip white space, if any
2. Get sign, if any
3. Get integer part and convert it
#include <ctype.h> n = 0;
int atoi(char s[]) while (isdigit(s[i])){
{ n = n * 10 + (s[i] - ‘0’);
int i, n, sign; i++;
i = 0; }
while (isspace(s[i])) i++; return sign * n;
if (s[i] == ‘-’){ }
sign = -1;
i++;
}else{
sign = 1;
if (s[i] == ‘+’) i++; }

12. Function reverse() Reverse: reverse string S in place
#include <string.h>
void reverse(char s[]) {
int c, i, j;
i = 0;
j = strlen(s) - 1;
while ( i < j){
c = s[i];
s[i] = s[j];
s[j] = c;
i++;
j--; }