练习4-4 在栈操作中添加几个命令，分别用于在不弹出元素的情况下打印栈顶元素；复制栈顶元素；交换栈顶两个元素的值。另外增加一个命令用于清空栈。
最后添加一个简单测试。
#include <stdio.h>#define MAXVAL 100int sp = 0;
double val[MAXVAL];void push(double f)
{if (sp < MAXVAL)val[sp++] = f;elseprintf("error: stack full, can't push %g\n", f);return;
}double pop(void)
{if (sp > 0)return val[--sp];else{printf("error:stack empty\n");return 0.0;}
}void print_no_pop(void)
{if (sp > 0)printf("%g\n", val[sp-1]);elseprintf("error:stack empty\n");return;
}void stack_top_cpy(void)
{if (sp > 0 || sp < MAXVAL)val[sp++] = val[sp-1];else if (sp <= 0)printf("error:stack empty\n");elseprintf("error:stack full\n");return ;
}void swap_stack_top(void)
{double temp;if (sp >= 2){temp = val[sp-1];val[sp-1] = val[sp-2];val[sp-2] = temp;}elseprintf("not enough number\n");return;
}void empty_stack(void)
{sp = 0;return;
}int main()
{double i;for (i = 1.0; i < 20.0; i++)push(i);printf("affirm:%g\n", pop());swap_stack_top();printf("swap:%g\n", pop());printf("swap:%g\n", pop());stack_top_cpy();printf("copy:%g\n", pop());printf("copy:%g\n", pop());printf("print_no_pop:\n");print_no_pop();empty_stack();printf("empty:%g\n", pop());return 0;
}