分类： C/C++
2009-12-19 23:53:44
1. When computing the square root of 2 by one variant of Newton's method, you were surprised to find that the result printed was 1.5. What's the mistake?
#include
#define abs( x ) ((x) >= 0 ? (x) : -(x))
double sqrt( double y )
{
double x = 1.0; /* initial estimate */
int error;
if( y <= 0 ) return 0; /* ensure positive value */
while( abs( error = y - x * x ) > .0005 )
x = x + error / (2*x); /* adjust estimate */
return x;
}
int main()
{
printf( "sqrt(%d) = %8.3f\n", 2, sqrt(2) );
return 0;
}
2. You want to calculate the 16 bits crc for first 3 members (serial_number, data, and hw_id) of nvram. Instead, you get unexpected result. You know for sure that crc16()is correct. What is the mistake?
#include
#include
typedef unsigned short uint16;
typedef unsigned long uint32;
typedef unsigned char uint8;
uint16 crc16(char *p, int size); /* calculate 16 bits crc */
typedef struct
{
uint32 serial_number; /* 4 bytes serial number */
uint8 data[16]; /* 16 bytes data */
uint32 hw_id; /* 4 bytes hardware id*/
uint16 crc16; /* 2 bytes crc16 */
} nvram;
nvram nv;
int main()
{
uint8 i;
memset(&nv, 0x0,sizeof(nv));
/* assign serial number */
nv.serial_number = 0x11112222;
/* assign data */
for(i = 0; i < sizeof(nv.data); i++)
nv.data[i] = i;
/* assign hardware id */
nv.hw_id= 0x00110005;
/* calculate crc16 for serial_number, data, and hw_id */
nv.crc16 = crc16((char *)&nv, sizeof(nv)-sizeof(nv.crc16));
return 0;
}
3. The function, count_vowels(), which is intended to count the vowels in the input string. When running the program, it is taking a long time to do so. Can you spot the difficulty?
int count_vowels( char *s )
{
int sum = 0;
for(;;)
{
switch( *s++ )
{
case'a':
case'e':
case'i':
case'o':
case'u':
case'A':
case'E':
case'I':
case'O':
case'U':
sum++;
continue;
default:
continue;
case'\0':
break;
}
}
return sum;
}
int main(void)
{
count_vowels("abcdefghijklmnop");
return 0;
}
4. A programmer wrote the following function in an embedded system. Everything works fine until compiler's optimization option is turned on. What are the possible reasons ?
// io_reg is a memory-mapped IO register
char send_ack()
{
extern char *io_reg;
// Wait until the device is ready
while( *io_reg == 0 );
// write 'A', 'C', 'K' to the register
*io_reg = 'A';
*io_reg = 'C';
*io_reg = 'K';
// Read the status from the register
// and return the status to caller
return (*io_reg);
}
5. The programmer expected the test for equality to be true, instead, most compilers test this as not equal. Why?
#include
#include
int main()
{
Const double three = 3.0;
double x, y, z;
x = 2 / three;
y = 5 / three;
z = 7 / three;
if( x + y == z )
printf("2/3 + 5/3 == 7/3 \n");
else
printf("2/3 + 5/3 != 7/3 \n");
return 0;
}
6. Why does this program say that the power, and LED is OFF, when we just turned it ON?
#include
typedef struct
{
int power:1;
int fan:1;
int led:1;
} system;
static system s;
bool is_power_on()
{
if( s.power > 0 )
return true;
else
return false;
}
bool is_fan_on()
{
if( s.fan > 0 )
return true;
else
return false;
}
bool is_led_on()
{
if( s.led > 0 )
return true;
else
return false;
}
int main()
{
s.power = 1;
s.fan= 0;
s.led = 1;
if( is_power_on() )
printf( "power is ON\n" );
else
printf( "power is OFF\n" );
if( is_fan_on() )
printf( "fan is ON\n" );
else
printf( "fan is OFF\n" );
if( is_led_on() )
printf( "led is ON\n" );
else
printf( "led is OFF\n" );
return 0;
}
