基礎版：利用strcmp函式進行字串的比較

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
/* 簡單密碼比較 */
/* written by Chen Gengru */
/* updated on 2018-11-30 */
int main()
{
	char password1[20], password2[20];
	
	int i;
	
	printf("Please input password\n");
	scanf("%s", &password1);
	
	printf
 
("check your password\n");
	scanf("%s", &password2);
	
	if (strcmp(password1, password2) == 0)
	{
		printf("correct!\n");
	}
	else
	{
		printf("wrong!\n");
	}
	
	return 0;
 } 

進階版：加入重新輸入密碼

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
/* 簡單密碼比較 */
/* written by Chen Gengru */
 

/* updated on 2018-11-30 */
int main()
{
	char password1[20], password2[20];
	
	int iWrongcheck;
	
	retype: printf("Please input password\n");
	        scanf("%s", &password1);
	
	printf("check your password\n");
	scanf("%s", &password2);
	
		if (strcmp(password1, password2) == 0)
	    {
	       printf
 
("correct!\n");
	    }
	    else
    	{
	    	printf("wrong! Press 1 to retype password. Press 0 to end.\n");
	    	scanf("%d", &iWrongcheck);
	    	if (iWrongcheck)
	    	{
	    		goto retype;
			}
			else
			{
				return 0;
			}
     	}
	
	return 0;
 } 

高階版：在密碼輸入和確認的基礎上，判斷密碼，若重複輸錯三次，結束

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
/* 密碼比較（確認加判斷） */
/* written by Chen Gengru */
/* updated on 2018-12-6 */ 
int main()
{
	char password1[20], password2[20], password3[20];
	
	int i, iWrongcheck;
	
	retype: printf("Please define password\n");
	        scanf("%s", &password1);
	
	printf("check your password\n");
	scanf("%s", &password2);
	
		if (strcmp(password1, password2) == 0)
	    {
	       printf("correct!\n");
	    }
	    else
    	{
	    	printf("wrong! Press 1 to retype password. Press 0 to end.\n");
	    	scanf("%d", &iWrongcheck);
	    	if (iWrongcheck)
	    	{
	    		goto retype;
			}
			else
			{
				return 0;
			}
     	}
     	
     printf("Please input your password.\n");
	 scanf("%s", &password3);
	
	for (i == 0; i < 3; i++)
	{
		if (strcmp(password1, password3) == 0)
		{
			printf("Correct! Welcome!\n");
			return 0;
		}
		else
		{
			printf("Wrong password! Please input again!\n");
			scanf("%s", &password3);
		}
	}
	
	printf("You have input the password wrongly for three times! END\n");
	
	return 0;
 }