Mã hóa thông điệp
#include <stdio.h>
#include <ctype.h>
#include <alloc.h>
char *crypt(char *tdiep, int column)
{
char tam[255], *result;
int i = 0, k = 0, n, j=0;
while(tdiep[i] != 0)
{
if (isalnum(tdiep[i]))
tam[k++] = tdiep[i];
i++;
}
tam[k] = 0;
result = (char *)malloc(k+1);
for (i=0; i<column; i++)
{
n = 0;
while(n+i < k)
{
result[j++] = tolower(tam[n+i]);
n += column;
}
}
resultvoid main()
{
char thongdiep[255], *mahoa;
int col;
printf("\nNhap thong diep can ma hoa : ");
gets(thongdiep);

scanf("%lf", &a);
xn = (a+1)/2;
do {
ketqua = xn;
xn = 0.5 * (xn + a/xn);
} while (fabs(xn-ketqua) > 0.0001);
printf("\nKet qua = %lf", xn);
getch();
}
Cấu trúc và các hàm thao tác trên số phức
#include <math.h>
typedef struct tagcomplex {
float thuc, ao;
} complex;
complex tong(complex a, complex
{
complex c;
c.thuc = a.thuc + b.thuc;
c.ao = a.ao + b.ao;
return c;
}
complex hieu(complex a, complex
{
complex c;
c.thuc = a.thuc - b.thuc;
c.ao = a.ao - b.ao;
return c;
}
complex tich(complex a, complex
{

scanf("%f%f", &a.thuc, &a.ao);
printf("\nNhap he so thuc va phuc cua B : ");
scanf("%f%f", &b.thuc, &b.ao);
printf("\nSo phuc A = ");
print_complex(a);
printf("\nSo phuc B = ");
print_complex( ;
printf("\nTong cua chung = ");
c = tong(a, ;
print_complex©;
printf("\nHieu cua chung = ");
c = hieu(a, ;
print_complex©;
printf("\nTich cua chung = ");
c = tich(a, ;
print_complex©;
printf("\nThuong cua chung = ");
c = thuong(a, ;
print_complex©;
printf("\nArgument cua a = %f", argument(a));
printf("\nModul cua a = %f", modul(a));
getch();
}
Dãy tằng dần
#include <stdio.h>
void main()
{
int a[10], i, maxstart, maxend, maxlen, tmpstart, tmpend, tmplen;
printf("\nNhap vao 10 phan tu nguyen cua day :");
for (i=0; i<10; i++)

printf("\nDay tang co so phan tu nhieu nhat la : \n");
for (i=maxstart; i<=maxend; i++)
printf("%6d", a[i]);
getch();
}
Dãy tăng có tổng dài nhất
#include <stdio.h>
void main()
{
int a[10], i, maxstart, maxend, maxtotal, tmpstart, tmpend, tmptotal;
printf("\nNhap vao 10 phan tu nguyen cua day :");
for (i=0; i<10; i++)
scanf("%d", &a[i]);
printf("Day da cho :\n");
for (i=0; i<10; i++)
printf("%6d", a[i]);
maxstart = maxend = tmpstart = tmpend = 0;
maxtotal = tmptotal = a[0];
for (i=1; i< 10; i++)
{
if (a[i] < a[tmpend])
{
if (maxtotal < tmptotal)
{
maxstart = tmpstart;
maxend = tmpend;
maxtotal = tmptotal;
}
tmpstart = tmpend = i;
tmptotal = a[i];