程序代码：

#include<stdio.h>
#include<string.h>
int main()
{
    char s1[80], s2[40];
    int i = 0, j = 0;
    printf("Input s1:\n");
    scanf_s("%s", s1, 80);
    printf("\n");
    printf("Input s2:\n");
    scanf_s("%s", s2, 40);
    while (s1[i] != '\0')               //(1)这里只要s1[i] != '\0'，i++;例如输入s1=123,s2=456,此时s1[2]=3,跳到(2)
        i++;
    while (s2[j] != '\0')               //(2)和(1)类似
        s1[i++] = s2[j++];               //(3)此时i++是从3即s1[3]开始+的，因为(1)中i的计数，一直加到5
    s1[i] = '\0';                       //(4)？这怎么实现？此时i是几，为什么？
    printf("\nThe new string is:\n");
    printf("%s", s1);
    printf("\n");
    return 0;
}

[此贴子已经被作者于2016-12-28 15:47编辑过]

程序代码：

#include <stdio.h>
#include <conio.h>
#include <math.h>
main()
{
    char c,str1[100],str2[100],str3[300];
    int i,j;
    printf("Please input the first string\nConfirm by Enter\n");
    gets(str1);
    printf("Please input the second string\nConfirm by Enter\n");
    gets(str2);
    for(i=0;(c=str1[i])!='\0';i++)
    {
        str3[i]=str1[i];
    };
    j=i;
    for(i=0;(c=str2[i])!='\0';i++,j++)
    {
        str3[j]=str2[i];
    };

    str3[j] = '\0';    //添加'\0'

    printf("The string which has been connected is:\n%s",str3);

    getch();
}