C puzzles详解【31-33题】

第三十一题

The following is a simple C program to read and print an integer. But it is not working properly. What is(are) the mistake(s)?
#include <stdio.h>
int main()
{
int n;
printf("Enter a number:\n");
scanf("%d\n",n); printf("You entered %d \n",n);
return ;
}

题目讲解:

运行后发生段错。

问题出在这一行

scanf("%d\n",n);

首先,scanf的第二个参数应该是个地址。若是从标准输入读入数据到变量n中,这一行应该改成

scanf("%d\n",&n);

再次编译运行,发现要输入一个数值后回车,程序并不会返回,再次输入一个数值后才会返回,n的值是第一次输入的值。

参考http://book.51cto.com/art/200901/106938.htm

‘\n’在scanf格式中不表示等待换行符,而是读取并放弃连续的空白字符,因此“%d\n”中的’\n’会让scanf读到非空白字符为止。要使用户输入一个数据回车后程序立马返回,去掉scanf中的’\n’即可。

scanf("%d",&n);

第三十二题

The following is a simple C program which tries to multiply an integer by  using the bitwise operations. But it doesn't do so. Explain the reason for the wrong behaviour of the program.
#include <stdio.h>
#define PrintInt(expr) printf("%s : %d\n",#expr,(expr))
int FiveTimes(int a)
{
int t;
t = a<< + a;
return t;
} int main()
{
int a = , b = ,c = ;
PrintInt(FiveTimes(a));
PrintInt(FiveTimes(b));
PrintInt(FiveTimes(c));
return ;
}

题目讲解:

函数FiveTimes中,

t = a<< + a;

‘+’的优先级高于’<<’,应改成

t = (a<<) + a;

第三十三题

Is the following a valid C program?
#include <stdio.h>
#define PrintInt(expr) printf("%s : %d\n",#expr,(expr))
int max(int x, int y)
{
(x > y) ? return x : return y;
} int main()
{
int a = , b = ;
PrintInt(a);
PrintInt(b);
PrintInt(max(a,b));
}

题目讲解:

编译有错误:

test.c: In function ‘max’
test.c:: error: expected expression before ‘return’

(x > y) ? return x : return y;

改成

return (x > y) ? x : y;
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