#04 (06/11/2025)

Free C compiler (gcc) for Windows PCs

gcc-2.95.2-crtdll.exe (7.1MB)

Extract all the files under the c:\gcc-2.95.2 directory.

Enter cmd in the Windows search box.

cd to c:\gcc-2.95.2 and run mingw32.bat to add the path. If you want to run gcc from any folder, add the folder c:\gcc-2.95.2 to the PATH environmental variable (details in class).

cd c:\gcc-2.95.2
mingw32

gcc MyProgram.c

To compile a C++ program, issue

g++ MyProgram.cpp

To compile a FORTRAN program, issue

g77 MyProgram.f

This will produce an executable file, a.exe instead of a.out.

gcc for the Mac

Misc. notes

Interesting C tutorial site

What if you are trapped in an infinite loop ?

#include <stdio.h>
int main()
{
int i ;

for (i=1; i>0; i++) printf("loop");

return 0;
}

Control-C

`for` Statement

for (initial value; condition ; counter increment) statement;

#include <stdio.h>
int main(){
int i;

for (i=0; i< 10; i++) printf("i=%d\n",i);

return 0;
}

Here is a program to compute

S = 1 + 2 + 3 + 4 + 5 + 6 + …+ 100.

#include <stdio.h>
int main()
{
int i, s = 0;

for (i=0; i<= 100; i++) s = s  + i;

printf("Sum = %d\n", s);

return 0;
}

Try to memorize the following pattern:

sum = 0.0;
for (i=0; i<= 100; i++) sum = sum + function(i);

for

100
∑
i=0

f(i) = f(0) + f(1) + f(2) + …+ f(100).

Note: The iteration variable (i. j, k...) must be declared as int.

`while` Statement

Does the following program output 9 or 10 ?

#include <stdio.h>
int main()
{
int i=0;

while (i<10) i++;

printf("%d\n",i);
return 0;
}

For multiple statements, it is necessary to use a block with curly brackets ({ and }).

#include <stdio.h>
int main()
{
int i=0;

while (i<10) 
 {i++;
  printf("i = %d\n",i);
 }

printf("%d\n",i);
return 0;
}

`do while` Statement

The do while loop is similar to the while loop except that the test occurs at the end of the loop body.

#include <stdio.h>

int main()
{
  int input_value;
do
 { printf("Enter 1 for yes, 0 for no :");
  scanf("%d", &input_value);
 } while (input_value != 1 && input_value != 0);
return 0;
}

`switch` Statement

A switch statement can branch out to different tasks.

#include <stdio.h>
int main()
{
int i;

printf("Enter an integer="); scanf("%d", &i);

switch(i){
case 1: printf("a is 1\n");break;
case 2: printf("a is 2\n");break;
default: printf("a is neither 1 nor 2\n");break;
}

return 0;
}

Common mistakes: Figure out why they are wrong !!

Wrong	Correct
`if (a=0) printf(" Not valid !\n");`	`if (a==0) printf(" illegal !\n");`
`int a=10,b=20; float c; c=a/b;`	`float a=10, b=20, c; c=a/b;`
`gcc math1.c`	`gcc -lm math1.c`
`scanf("%f", a)`	`scanf("%f", &a)`
`for (i=0, i<10, i++)`	`for (i=0; i<10; i++)`
`for (i==0, i<10, i++)`	`for (i=0; i<10; i++)`

Example

By numerically summing up

1−

−

− …,

compute an approximate value of ln 2 ( = 0.693147...).

Programming

#include <stdio.h>
#include <math.h>
int main()
{
int i,n;
float sum=0.0;

printf("Enter # of iterations = ");
scanf("%d", &n);

for (i=1;i<n;i++)
 sum = sum + pow(-1, i+1)/i;

printf("Approximation of ln(2)= %f. ", sum);
printf("Exact value of ln(2)= %f.\n", log(2));

return 0;
}

Note: pow(a,b) is a function found in math.h which returns a^b.

C Functions

If there is no value to be returned, use void instead of int or others.

#include <stdio.h>

void write()
{
 printf("Hello world !\n");
}

int main()
{
write();
return 0;
}

Otherwise, the type of the function you use must be declared.

#include <stdio.h>

float cube(float x)
{
return x*x*x;
}

int main()
{
float x;

printf("Enter x = "); scanf("%f",&x);

printf("The cube of x is %f.\n", cube(x));

return 0;
}

Example: Alternative to pow(x, y)

Background

Here is an example of a C program to compute x^y without using pow(a, b).

#include <stdio.h>
#include <math.h>

float power(float x, float y)
 {
 return exp(y*log(x));
 }

int main()
{
 float x,y;
 printf("Enter x and y separated by space =");
 scanf("%f %f",&x,&y);

 if (x<0)
  {
  printf("x must be positive !!\n");
   return 0;
   }

 printf("%f to power of exponent %f is %f.\n", x, y, power(x,y));

 return 0;
}

Variables used in a function are local, i.e. they do not retain the values outside the function.

#include <stdio.h>
float f(int n)
 {
  int i; float sum=0;
  for (i=1;i < n;i++) sum=sum+1.0/i;
  return sum;
 }

int main()
{
 float sum=0;
 int i;
  for (i=1;i < 20;i++) sum=sum+1.0/i/i;
  printf("%f %f\n", sum, f(20));
  return 0;
 }

#include <stdio.h>

int i=20;

void f()
{
    printf("%d\n",i);
}

int main()
{

  int i=30;
  printf("%d\n", i);
  f();
  return 0;
}

C functions are recursive (i.e. a function can call its own. Examples follow.)

Fibonacci numbers

from Da Vinci Code.

The Fibonacci numbers are defined as

a_n = a_n−1+ a_n−2, a₁ = 1, a₂ = 1.

This is related to the problem of how fast rabbits could breed in ideal circumstances studied by Fibonacci (Click this link).

Suppose a newly-born pair of rabbits, one male, one female, are put in a field. Rabbits are able to mate at the age of one month so that at the end of its second month a female can produce another pair of rabbits. Suppose that our rabbits never die and that the female always produces one new pair (one male, one female) every month from the second month on. The puzzle that Fibonacci posed was...

How many pairs will there be in two years (a₂₄ = ?) ?

Code:

#include <stdio.h>

int fibo(int n)
{
if (n==1) return 1;
if (n==2) return 1;
return fibo(n-1) + fibo(n-2);
}

int main()
{
int i;

printf("Enter n = "); scanf("%d", &i);

printf("%d\n", fibo(i));

return 0; 

}

The explicit form of the Fibonacci sequence is given by

a_n=

⎛
⎝

(1+√5)

⎞
⎠

−

⎛
⎝

(1−√5)

⎞
⎠

√5

Alternative to 1 + 2 + 3 + 4 + …+ n.

The following program computes

1+2+3+ …+ n.

#include <stdio.h>

int mysum(int n)
{
if (n==0) return 0;
return n + mysum(n-1);
}

int main()
{
int n;
printf("Enter n = ");
scanf("%d", &n);
printf("1+2+....+%d =%d \n", n, mysum(n));
return 0;
}

Another example of recursive algorithm:

If a sequence of numbers is generated by the following rules, what is a₂₀ ?

a_n = 4a_n−1 − 6 a_n−2, a₁ = 1, a₂ = 3.

(1)

#include <stdio.h>

int mysequence(int n)
{
if (n==1) return 1;
if (n==2) return 3;
return 4*mysequence(n-1)-6*mysequence(n-2);
}

int main()
{
int n;
printf("Enter n = ");
scanf("%d", &n);
printf("%d\n",mysequence(n));
return 0;
}

File translated from T_EX by T_TH, version 4.03.
On 11 Jun 2025, 10:37.

Free C compiler (gcc) for Windows PCs

Misc. notes

for Statement

while Statement

do while Statement