C语言学习5-1：字符加法，字符串操作函数编写，查找数组中第二大的数

1，字符加法

#include <stdio.h>
#include <string.h>

void reverse(char s[]) //反向遍历
{
    int i;
    int len = strlen(s);

    for (i = 0; i < len / 2; i++)
    {
        s[i] ^= s[len - i - 1];
        s[len - i - 1] ^= s[i];
        s[i] ^= s[len - i - 1];
    }
}

void add_str(char sum[], char s1[], char s2[])//字符串数字相加
{
    int len1 = strlen(s1);
    int len2 = strlen(s2);
    int len_max = len1 > len2 ? len1 : len2;//选取最大的作为循环长度

    reverse(s1);//切换高位和地位的位置
    reverse(s2);//切换高位和地位的位置

    int i;
    int digit, carry;//digit是存储当前位数相加的值
    for (i = 0, digit = 0; i < len_max; i++, digit = 0)
    {
        if (s1[i] != '')    
            digit += s1[i] - '0';//变成数字
        if (s2[i] != '')    
            digit += s2[i] - '0';//变成数字

        digit += sum[i];     //把上次的进位加上

        sum[i] = digit % 10;//当前数字
        carry = digit / 10;//进位
        if (carry != 0)
            sum[i + 1] = carry;//把进位加到下一个位中

        sum[i] += '0';//变成字符
    }

    if (sum[i] != '')
        sum[i] += '0';//最后看是否有进位，有的话就加上，比如1000+9000的10000结果就是

    reverse(sum);
}

int main(void)
{
    char s1[31] = { 0 };
    char s2[31] = { 0 }; 
    char sum[32] = { 0 };

    printf("input num1: ");
    gets(s1);
    printf("input num2: ");
    gets(s2);

    add_str(sum, s1, s2);

    printf("the sum is: %s
", sum);

    return 0;
}

结果：

will@will-Inspiron-N4010:~/c/4th$ ./a.out
input num1: 1234
input num2: 9899
the sum is: 11133

2，字符串操作函数编写

#include <stdio.h>

int my_strlen(char s[])
{
    int i = 0;

    while (s[i] != '')
        i++;

    return i;
}

void my_strcpy(char dst[], char src[])
{
    int i = 0, j = 0;

    while ((dst[i++] = src[j++]) != '')
        ;
}

void my_strcat(char dst[], char src[])
{
    int i = 0, j = 0;

    while (dst[i] != '')
        i++;

    while ((dst[i++] = src[j++]) != '')
        ;
}

int my_strcmp(char s1[], char s2[])
{
    int i = 0, j = 0;

    while (s1[i] == s2[j] && s1[i] != '')
        i++, j++;

    return s1[i] - s2[j];
}

int my_strchr(char s[], int ch)
{
    int i;

    for (i = 0; s[i] != ''; i++)
    {
        if (ch == s[i])    
            return i;
    }

    return -1;
}

int my_strstr(char s[], char substr[])
{
    int i = 0, j;
    int start_cmp;

    while (s[i] != '')
    {
        start_cmp = i, j = 0;
        while (s[i] == substr[j] && substr[j] != '')    
            i++, j++;

        if (substr[j] == '')
            return start_cmp;

        i = start_cmp;
        i++;
    }

    return -1;
}

int main(void)
{
    char s1[128];
    char s2[128];

    printf("test strlen, input s1:");
    gets(s1);
    printf("len of s1 is %d
", my_strlen(s1));

    printf("test strcpy, input s1: ");
    gets(s1);
    my_strcpy(s2, s1);
    printf("s2: %s
", s2);

    printf("test strcat, input s1: ");
    gets(s1);
    my_strcat(s2, s1);
    printf("s2: %s
", s2);

    printf("test strcmp, input s1: ");
    gets(s1);
    printf("test strcmp, input s2: ");
    gets(s2);
    if (strcmp(s1, s2) > 0)
        printf("%s > %s
", s1, s2);
    else if (strcmp(s1, s2) < 0)
        printf("%s < %s
", s1, s2);
    else
        printf("%s == %s
", s1, s2);

    int ch, index;
    printf("test strchr, input s1: ");
    gets(s1);
    printf("input ch: ");
    ch = getchar();
    if ((index = my_strchr(s1, ch)) != -1)
        printf("%s has %c, index = %d
", s1, ch, index);
    else
        printf("not find.
");

    while (getchar() != '
')
        ;

    printf("test strstr, input s1: ");
    gets(s1);
    printf("input s2: ");
    gets(s2);
    if ((index = my_strstr(s1, s2)) != -1)
        printf("%s has %s, index = %d
", s1, s2, index);
    else
        printf("not find.
");

    return 0;
}

3，查找第二个最大的

#include <stdio.h>

void rand_a(int a[], int len)
{
    int i;
    for (i = 0; i < len; i++)
        a[i] = rand() % 100;
}

void print_a(int a[], int len)
{
    int i;
    for (i = 0; i < len; i++)
        printf("%d ", a[i]);
    putchar('
');
}

int second_max(int a[], int len)
{
    int i, max = a[0], min = a[0];

    for (i = 0; i < len; i++)
    {
        if (a[i] > max)    
            max = a[i];
        if (a[i] < min)
            min = a[i];
    }

    int sec_max = min;
    for (i = 0; i < len; i++)
    {
        if (sec_max < a[i] && a[i] < max)    
            sec_max = a[i];
    }

    return sec_max;
}

int main(void)
{
    int a[10];

    rand_a(a, 10);
    print_a(a, 10);

    printf("second max is %d
", second_max(a, 10));

    return 0;
}

这个太简单我就不说了