c - 在 C 中执行 while 循环

Question

我正在使用数组实现多项式。这是问题陈述:

Write a menu-driven program to represent Polynomials as a data structure using arrays. and write functions to add, subtract and multiply two polynomials; multiply a polynomial with a constant, find whether a polynomial is a "zero- polynomial, return the degree of the polynomial. Assume that a new polynomial is created after each operation. How would you input and output polynomials?

我已经创建了输入和输出函数。但是我的 do while 循环运行了两次。请帮助我找出原因。

do-while 循环

do{
        print_menu();

        scanf("%c",&ch);
        printf("\nch = %c\n",ch);
    switch(ch){
        case '1':
            create_poly(poly,termpool,&next_poly);
            break;
        case '2':
            print_poly(poly,termpool,&next_poly);
            break;
        case 'q':
            break;
        default:
            printf("Invalid choice.");
    }
}while(ch != 'q');

return 0;

print_menu() 函数

void print_menu()
{
    printf("\n1. Create a new polynomial.");
    printf("\n2. Print polynomial.");
    printf("\nq. Exit");
    printf("\nEnter Choice:");
}

create_poly() 函数

void create_poly(int poly[][2], int termpool[][2], int *next_poly)
{
    int beg = poly[*next_poly][0];
    int end, size, i, j;

    printf("Enter size of the polynomial:");
    scanf("%d",&size);
    poly[*next_poly][1] = beg + size - 1;
    end = poly[*next_poly][1];

    printf("Enter terms of the polynomial(coeff then exponent):\n");
    for(i=beg; i<=end; i++){
        for(j=0; j<2; j++){
            scanf("%d ",&termpool[i][j]);
        }
    }

    poly[++(*next_poly)][0] = end + 1;
}

print_poly() 函数

void print_poly(int poly[][2],int termpool[][2],int *next_poly)
{
    int pos,beg,end;
    int i;

    printf("Enter position of the polynomial:");
    scanf("%d",&pos);
    if(pos-1 > *next_poly){
        printf("Invalid position.");
        return;
    }

    beg = poly[pos-1][0];
    end = poly[pos-1][1];
    for(i=beg; i<=end; i++){
        printf(" %dx^%d +",termpool[i][0],termpool[i][1]);
    }
    printf("\b = 0");

}

这是一个示例输出:

1. Create a new polynomial.2. Print polynomial.q. ExitEnter Choice:1ch = 1Enter size of the polynomial:2Enter terms of the polynomial(coeff then exponent):2 46 71. Create a new polynomial.2. Print polynomial.q. ExitEnter Choice:ch = Invalid choice.1. Create a new polynomial.2. Print polynomial.q. ExitEnter Choice:qch = q

尝试刷新 stdin...问题仍然存在。每一步打印ch的值，我觉得是空格。空白从哪里来？

---

abnormal behavior of scanf的答案也回答了这个问题。

Answer 1

如果你测试下一段代码，你会发现同样的问题

int main() {
    char c;

    do {
        scanf_s("%c", &c);
        if (c != 'q')
             printf("test scanf() function\n");

    } while (c);
}

scanf() 函数在按下 enter 键时起作用，但这会在缓冲区输入中插入另一个字符，即新行 '\n' 的字符，因为循环阻塞，它会再次被 scanf() 获取。尝试通过以下代码更改以前的代码:`

do {
    scanf_s("%c", &c); // or c = getchar();
    switch (c){
    case '\n':
        break;
    default:
        printf("test scanf() function\n");
    }

} while (c);`

并且会正常工作。在您的代码中，仅在 switch block 中添加一个新案例:

 switch(ch) {
    case '1':
        create_poly(poly,termpool,&next_poly);
        break;
    case '2':
        print_poly(poly,termpool,&next_poly);
        break;

    case '\n':
        break;

    case 'q':
        break;
    default:
        printf("Invalid choice.");
 }

对不起，英语不是我的母语