第四章编程题

每一题中，上边为自己写的代码；下边的为Instructor’s Guide的答案。

2.求1~100的质数。

/*
    求1~100中的质数。
*/

#include <stdio.h>
int main(void)
{
    printf("1\n2\n");

    int i, j;
    for (i = 3; i <= 100; i++)
    {
        for (j = 2; j <= i; j++)
        {
            if (i % j == 0)
                break;

            if (j + 1 == i)
                printf("%d\n", i);
        }
    }
}

/*
**Compute and print all the prime numbersfrom 1 to 100.
*/

#include <stdio.h>

int
main ()
{

    int number;
    int divisor;

    /*
    **One and two are easy.
    */
    printf("1\n2\n");

    /*
    **No other even numbers are prime;look at the remaining odd ones.
    */
    for (number = 3; number <= 100; number =number + 2)
    {
        /*
        **See if any divisor from 3 up to the number evenly divides the
        **number.
        */
        for (divisor = 3; divisor < number; divisor = divisor + 2)
        {
            if (number % divisor == 0)
                break;
        }
            if (divisor >= number )
                printf("%d\n",number);

    }

}

/*
**tricks:1.去掉偶数提高了效率；
**       2.第二个for语句跟自己的不同(if在{}后面)。
*/

 

3.判断是等边三角形，等腰三角形还是普通三角形。

此题中，自己的代码效率不高，应先比较三条边的大小。

还要注意多个相同类型的scanf的语法。

#include <stdio.h>
int main (void)
{
    int a, b, c;

    scanf("%d %d %d", &a, &b, &c);

    if (a < 0 || b < 0 || c < 0 || a+b<=c || a+c<=b || b+c<=a)
        {
            printf("not an triangle.\n");
            return 0;
        }

    if ( a== b && b == c)
        printf("equilateral triangle\n");
    else if (a == b || b == c || a == c)
        printf("isoceles triangle\n");
    else
        printf("normal.\n");

    return 0;
}

 

/*
**Classify the type of a triangle given the lengths of its sides.
*/
#include <stdio.h>
#include <stdlib.h>

int main ()
{
    float a, b, c, temp;
    printf("Enter the lengths of the three sides of the triangle:");
    scanf("%f %f %f", &a, &b, &c);

    if (a < b)
    {
        temp = a;
        a = b;
        b = temp;
    }
    if (a < c)
    {
        temp = a;
        a = c;
        c = temp;
    }
    if (b < c)
    {
        temp = b;
        b = c;
        c = temp;
    }

    if (c <= 0 || b + c < a)
        printf("not an triangle.\n");
    else if (a == b & b == c)
        printf("equilateral.\n");
    else if (a == b || b == c)
        printf("isosceles.\n");
    else
        printf("scalene.\n");
    return 0;

}

 

4.这题自己的代码实在没啥出彩的地方。但guide里头的很美。所以就贴一下。尤其值得注意的是如何去补充那个NUL字符。

题：编写函数copy_n(char dst[], char src[], int n);

用于复制数组src到dst，要求如下：必须正好复制n个字符到dst中，不能多也不能少。如果src的字符串长度小于n，就补充足够的NUL字符。

void copy_n (char dst[], char src[], int n)
{
    int dst_index, src_index;
    
    src_index = 0;
    
    for (dst_index = 0; dst_index < n; dst_index += 1)
    {
        dst[dst_index] = src[src_index];
        if (src[src_index] != 0)
            src_index += 1;
    }
}

 

int substr (char dst[], char src[], int start, int len)
{
    int i;

    if (src[start] == '\0' || start < 0 || len < 0)
    {
        dst[0] = '\0';
        return 0;
    }
    else
    {
        for (i = 0;i < len; i++)
        {
            if (!src[i+start])
            {
                dst[i] = src[i+start];
                break;
            }
            dst[i] = src[i+start];
        }
    dst[i+1] = '\0';
    return i;N
    }
}

 

int substr_1 (char dst[], char src[], int start, int len)
{
    int srcindex;
    int dstindex;

    dstindex = 0;

    if (start >= 0 && len > 0)
    {
        for (srcindex = 0;
            src[srcindex] != '\0' && srcindex < start; srcindex += 1)
            ;

        while (src[srcindex] != '\0' && len > 0)
        {
            dst[dstindex] = src[srcindex];
            dstindex++;
            srcindex++;
            len--;
        }
    }

    dst[dstindex] = '\0';
    return dstindex;
}

 

 

 

void deblank (char string[])
{
    int i, j, a = 0;
    char dst[20];

    for (i = 0; string[i] != '\0'; i++)
    {
        for(j = i; string[j] == '\0'; j++)
        ;

        if (j - i < 2)
            i = j;

        dst[a] = string[i];
        a++;
    }
}

 

这题答案的代码编译有错？？？

#define NUL ¡¯\0¡¯
void
deblank(char *string)
{
    char *dest;
    char *src;
    int  ch;
/*
**Set source and destination pointers to beginning of the string,then
**move to 2nd character in string.
*/
    src = string;
    dest = string++;
/*
**Examine each character from the source string.
*/
    while((ch = *src++) != NUL)
    {
        if(is_white(ch))
        {
            if(src==string||!is_white(dest[¨C1]))
                *dest++=' ';
        }
        else
        {
            *dest++ = ch;
        }
    }
        *dest = NUL;
        //*dest =  NUL;
}
int is_white(int ch)
{
    return  ch==' '||ch== '\t'||ch=='\v'||ch=='\f'||ch=='\n'||ch=='\r';
}