The document discusses automatic versus static variables in C programming. It provides examples of functions with automatic and static variables. Automatic variables are reinitialized each time the function is called, while static variables retain their value between calls. The document also discusses recursion, providing examples of recursive functions to calculate factorials and generate the Fibonacci series. Recursion involves functions calling themselves until a base case is reached.

1. Computing Fundamentals
Dr. Muhammad Yousaf Hamza
Deputy Chief Engineer, PIEAS

2. Automatic Versus Static Variables
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3. // Auto Variables
#include <stdio.h>
void func1(void);
int main()
{
int count;
for (count = 0; count < 20; count++)
{
printf("At iteration %d: ", count);
func1();
}
getchar(); return 0;
}
void func1(void)
{
int x = 0;
int y = 0;
printf("x = %d, y = %dn", x, y);
x++;
Y++;
} Dr. Yousaf, PIEAS
• At iteration 0: x = 0, y = 0
• At iteration 1: x = 0, y = 0
• At iteration 2: x = 0, y = 0
• At iteration 3: x = 0, y = 0
• At iteration 4: x = 0, y = 0
• At iteration 5: x = 0, y = 0
• At iteration 6: x = 0, y = 0
• At iteration 7: x = 0, y = 0
• At iteration 8: x = 0, y = 0
• At iteration 9: x = 0, y = 0
• and so on

4. //Automatic versus Static Variables, pages 209-211
// Example Page 211 Static Variables
#include <stdio.h>
void func1(void);
int main()
{
int count;
for (count = 0; count < 20; count++)
{
printf("At iteration %d: ", count);
func1();
}
getchar(); return 0;
}
void func1(void)
{
static int x = 0;
int y = 0;
printf("x = %d, y = %dn", x, y);
x++;
y++; }
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• At iteration 0: x = 0, y = 0
• At iteration 1: x = 1, y = 0
• At iteration 2: x = 2, y = 0
• At iteration 3: x = 3, y = 0
• At iteration 4: x = 4, y = 0
• At iteration 5: x = 5, y = 0
• At iteration 6: x = 6, y = 0
• At iteration 7: x = 7, y = 0
• At iteration 8: x = 8, y = 0
• At iteration 9: x = 9, y = 0
• and so on

5. Dr. Yousaf, PIEAS
// static.c
// demonstrates static variables
#include <stdio.h>
float getavg(float); //declaration
int main()
{
float data=1, avg;
while( data != 0 )
{
printf("Enter a number: ");
scanf("%f",&data);
avg = getavg(data);
printf("New average is %fn",avg);
}
return 0;
}
// finds average of old plus
new data
float getavg(float newdata)
{
static float total = 0;
/*static variables are
initialized*/
static int count = 0;
count++; //increment count
total += newdata; /*add
new data to total*/
return total / count;
//return the new average
}

6. Automatic Storage
• Storage class specifiers (auto or static) decide
Storage duration which means how long an object
exists in memory
• Automatic storage
– Object created and destroyed within its block
– auto: default for local variables
auto double x, y;
Variables that are automatically created and
automatically destroyed are called automatic or auto
variables. Non‐static variables that are declared inside a
function are automatically created and destroyed so
non‐static local variables are auto variables
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7. Static Storage
• Static storage
– Variables exist for entire program execution
– Default value of zero
static: local variables defined in functions.
• Keep value after function ends
• Only known in their own function
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8. exit() function
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9. exit()
// About exit() function. Page 236
#include<stdio.h>
#include<stdlib.h> // to use exit() function.
int main()
{
int i, j, k;
for (i = 0; i<5; i++)
printf("%dn", i);
getchar();
exit(0); // it will exit the program
j = 23;
printf("j = %dn", j);
getchar();
return 0;
}
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10. // Following program without exit()
// In the next slide, we will use exit() within a function
#include<stdio.h>
#include<stdlib.h>
int add(int x, int y);
int main()
{
int i = 10, j, k;
printf ("i = %dn", i);
j = add(10,20);
printf ("j = %dn", j);
getchar(); return 0;
}
int add(int x, int y)
{
int z;
z = x+y;
return z;
} Dr. Yousaf, PIEAS
exit()

11. // Use of exit() within a function
#include<stdio.h>
#include<stdlib.h> // to use exit function
int add(int x, int y);
int main()
{
int i = 10, j, k;
printf ("i = %dn", i);
getchar();
j = add(10,20);
printf ("j = %dn", j);
getchar(); return 0;
}
int add(int x, int y)
{
int z;
z = x+y;
exit(0); // It will exit the entire program (not the function only).
return z;
}
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exit()

12. Recursion
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13. Recursion
Example: factorials
– 5! = 5 * 4 * 3 * 2 * 1
– Notice that
• 5! = 5 * 4!
• 4! = 4 * 3! ...
– Can compute factorials recursively
– Solve base case (1! = 0! = 1) then plug in
• 2! = 2 * 1! = 2 * 1 = 2;
• 3! = 3 * 2! = 3 * 2 = 6;
Write a recursive function int fact(int num) that should return
the factorial of given number num
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14. Dr. Yousaf, PIEAS
//Calculate Factorial using Recursion
# include<stdio.h>
int factorial(int num);
int main ()
{
int f,x;
printf("Enter integer value in the range 1
to 10: ");
scanf("%d", &x);
if (x > 10 || x < 1)
printf ("Illegal input!n");
else
{
f = factorial(x);
printf ("%d factorial equals %dn", x,f);
}
getchar(); return 0; }
int factorial (int a)
{
if (a==1) // base case
return 1;
else
{
a *= factorial(a-1);
return a;
}
}

15. Recursion
• Recursive functions
• A function that calls itself inside its body is called a
recursive function
• Can only solve a base case
– Divide a problem into
• What it can do
• What it cannot do
– What it cannot do resembles original problem
– The function launches a new copy of itself (recursion
step) to solve what it cannot do
– Eventually base case gets solved
• Gets plugged in, works its way up and solves whole problem
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16. Example Using Recursion
/*This function prints all the number between zero and a given number that is
greater than zero. */
#include <stdio.h>
void printSeries(int num);
int main()
{
printSeries(6);
getchar(); return 0;
}
void printSeries(int num)
{
if(num<=0)
return;
printf("%d", num);
printSeries(num-1);
}
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17. Example Using Recursion:
The Fibonacci Series
• Fibonacci series: 0, 1, 1, 2, 3, 5, 8, 13, ...
– Each number is the sum of the previous two
– fib( n ) = fib( n - 1 ) + fib( n – 2 )
– Write a recursive function that can print first N terms of
Fibonacci series ,
– Can be solved recursively:
Code for the fibaonacci function
int fibonacci(int n )
{
if (n == 0 || n == 1) // base case
return n;
else
return fibonacci(n - 1)+ fibonacci(n – 2);
}
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18. Example Using Recursion:
The Fibonacci Series
• Set of recursive calls to function fibonacci
f( 3 )
f( 1 )f( 2 )
f( 1 ) f( 0 ) return 1
return 1 return 0
return +
+return
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19. Dr. Yousaf, PIEAS
// To generate Fibonacci series
using Recursion
#include<stdio.h>
int fibonacci(int n);
int main()
{
int i, terms, result;
printf("Please enter the number of
terms from 1 to 20: ");
scanf("%d",&terms);
for ( i = 0; i<= terms; i++)
{
result = fibonacci(i);
printf("%d, ", result);
}
getchar(); return 0; }
// Function Definition
int fibonacci(int n)
{
if ( n == 0 || n == 1 )
return n;
else
return fibonacci( n - 1 ) +
fibonacci( n - 2 );
}

20. Recursion vs. Iteration
• Repetition
– Iteration: explicit loop
– Recursion: repeated function calls
• Termination
– Iteration: loop condition fails
– Recursion: base case recognized
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