Содержание

Глава 04

Операторы

  1. Demonstrate the % operator.
  2. Demonstrate the difference between prefix postfix forms of ++.
  3. Demonstrate the relational and logical operators.
  4. Create an implication operator in C#.
  5. Demonstrate the short-circuit operators.
  6. Side-effects can be important.
  7. Use bitwise AND to make a number even.
  8. Use bitwise AND to determine if a number is odd.
  9. Display the bits within a byte.
  10. Use bitwise OR to make a number odd.
  11. Use XOR to encode and decode a message.
  12. Demonstrate the bitwise NOT.
  13. Demonstrate the shift << and >> operators.
  14. Use the shift operators to multiply and divide by 2.
  15. Prevent a division by zero using the ?.
  16. Prevent a division by zero using the ?.
listing 1
// Demonstrate the % operator.

using System;

class ModDemo {
public static void Main() {
int iresult, irem;
double dresult, drem;

iresult = 10 / 3;
irem = 10 % 3;

dresult = 10.0 / 3.0;
drem = 10.0 % 3.0;

Console.WriteLine("Result and remainder of 10 / 3: " +
iresult + " " + irem);
Console.WriteLine("Result and remainder of 10.0 / 3.0: " +
dresult + " " + drem);

}
}

Результат:

Result and remainder of 10 / 3: 3 1
Result and remainder of 10.0 / 3.0: 3,33333333333333 1

Анализ:

Вверх

listing 2
/*
Demonstrate the difference between prefix
postfix forms of ++.
*/
using System;

class PrePostDemo {
public static void Main() {
int x, y;
int i;

x = 1;
Console.WriteLine("Series generated using y = x + x++;");
for(i = 0; i < 10; i++) {

y = x + x++; // postfix ++

Console.WriteLine(y + " ");
}
Console.WriteLine();

x = 1;
Console.WriteLine("Series generated using y = x + ++x;");
for(i = 0; i < 10; i++) {
y = x + ++x; // prefix ++

Console.WriteLine(y + " ");
}
Console.WriteLine();

}
}

Результат:

Series generated using y = x + x++;
2
4
6
8
10
12
14
16
18
20

Series generated using y = x + ++x;
3
5
7
9
11
13
15
17
19
21

Анализ:

Вверх

listing 3
// Demonstrate the relational and logical operators.

using System;

class RelLogOps {
public static void Main() {
int i, j;
bool b1, b2;

i = 10;
j = 11;
if(i < j) Console.WriteLine("i < j");
if(i <= j) Console.WriteLine("i <= j");
if(i != j) Console.WriteLine("i != j");
if(i == j) Console.WriteLine("this won't execute");
if(i >= j) Console.WriteLine("this won't execute");
if(i > j) Console.WriteLine("this won't execute");

b1 = true;
b2 = false;
if(b1 & b2) Console.WriteLine("this won't execute");
if(!(b1 & b2)) Console.WriteLine("!(b1 & b2) is true");
if(b1 | b2) Console.WriteLine("b1 | b2 is true");
if(b1 ^ b2) Console.WriteLine("b1 ^ b2 is true");

}
}

Результат:

i < j
i <= j
i != j
!(b1 & b2) is true
b1 | b2 is true
b1 ^ b2 is true

Анализ:

Вверх

listing 4
// Create an implication operator in C#.
using System;

class Implication {
public static void Main() {
bool p=false, q=false;
int i, j;

for(i = 0; i < 2; i++) {

for(j = 0; j < 2; j++) {
if(i==0) p = true;
if(i==1) p = false;
if(j==0) q = true;
if(j==1) q = false;

Console.WriteLine("p is " + p + ", q is " + q);
if(!p | q) Console.WriteLine(p + " implies " + q +
" is " + true);
Console.WriteLine();

}
}
}
}

Результат:

p is True, q is True
True implies True is True

p is True, q is False

p is False, q is True
False implies True is True

p is False, q is False
False implies False is True

Анализ:

Вверх

listing 5
// Demonstrate the short-circuit operators.

using System;

class SCops {
public static void Main() {
int n, d;

n = 10;
d = 2;
if(d != 0 && (n % d) == 0)
Console.WriteLine(d + " is a factor of " + n);

d = 0; // now, set d to zero

// Since d is zero, the second operand is not evaluated.
if(d != 0 && (n % d) == 0)
Console.WriteLine(d + " is a factor of " + n);

/* Now, try the same thing without short-circuit operator.
This will cause a divide-by-zero error. */
if(d != 0 & (n % d) == 0)
Console.WriteLine(d + " is a factor of " + n);

}
}

Результат:

2 is a factor of 10
0 is a factor of 10

Анализ:

Пришлось заремировать инструкцию

if(d != 0 & (n % d) == 0)

так как компилятор выдал исключение о делении на нуль.

Вверх

listing 6
// Side-effects can be important.

using System;

class SideEffects {
public static void Main() {
int i;

i = 0; /* Here, i is still incremented even though
the if statement fails. */
if(false & (++i < 100))
Console.WriteLine("this won't be displayed");
Console.WriteLine("if statement executed: " + i); // displays 1

/* In this case, i is not incremented because
the short-circuit operator skips the increment. */
if(false && (++i < 100))
Console.WriteLine("this won't be displayed");
Console.WriteLine("if statement executed: " + i); // still 1 !!

}
}

Результат:

if statement executed: 1
if statement executed: 1

Анализ:

Вверх

listing 7
// Use bitwise AND to make a number even.
using System;

class MakeEven {
public static void Main() {
ushort num;
ushort i;

for(i = 1; i <= 10; i++) {

num = i;

Console.WriteLine("num: " + num);

num = (ushort) (num & 0xFFFE); // num & 1111 1110

Console.WriteLine("num after turning off bit zero: "
+ num + "\n");

}
}
}

Результат:

num: 1
num after turning off bit zero: 0

num: 2
num after turning off bit zero: 0

num: 2
num after turning off bit zero: 2

num: 3
num after turning off bit zero: 2

num: 4
num after turning off bit zero: 4

num: 5
num after turning off bit zero: 4

num: 6
num after turning off bit zero: 6

num: 7
num after turning off bit zero: 6

num: 8
num after turning off bit zero: 8

num: 9
num after turning off bit zero: 8

num: 10
num after turning off bit zero: 10

Анализ:

Вверх

listing 8
// Use bitwise AND to determine if a number is odd.
using System;

class IsOdd {
public static void Main() {
ushort num;

num = 10;

if((num & 1) == 1)
Console.WriteLine("This won't display.");

num = 11;

if((num & 1) == 1)
Console.WriteLine(num + " is odd.");

}
}

Результат:

11 is odd.

Анализ:

Вверх

listing 9
// Display the bits within a byte.
using System;

class ShowBits {
public static void Main() {
int t;
byte val;

val = 123;
for(t=128; t > 0; t = t/2) {

if((val & t) != 0) Console.Write("1 ");
if((val & t) == 0) Console.Write("0 ");
}
}
}

Результат:

0 1 1 1 1 0 1 1

Анализ:

Вверх

listing 10
// Use bitwise OR to make a number odd.
using System;

class MakeOdd {
public static void Main() {
ushort num;
ushort i;

for(i = 1; i <= 10; i++) {

num = i;

Console.WriteLine("num: " + num);

num = (ushort) (num | 1); // num | 0000 0001

Console.WriteLine("num after turning on bit zero: "
+ num + "\n");

}
}
}

Результат:

num: 1
num after turning on bit zero: 1

num: 2
num after turning on bit zero: 3

num: 3
num after turning on bit zero: 3

num: 4
num after turning on bit zero: 5

num: 5
num after turning on bit zero: 5

num: 6
num after turning on bit zero: 7

num: 7
num after turning on bit zero: 7

num: 8
num after turning on bit zero: 9

num: 9
num after turning on bit zero: 9

num: 10
num after turning on bit zero: 11

Анализ:

Вверх

listing 11
// Use XOR to encode and decode a message.

using System;

class Encode {
public static void Main() {
char ch1 = 'H';
char ch2 = 'i';
char ch3 = '!';

int key = 88;

Console.WriteLine("Original message: " +
ch1 + ch2 + ch3);

// encode the message
ch1 = (char) (ch1 ^ key);
ch2 = (char) (ch2 ^ key);
ch3 = (char) (ch3 ^ key);

Console.WriteLine("Encoded message: " +
ch1 + ch2 + ch3);

// decode the message
ch1 = (char) (ch1 ^ key);
ch2 = (char) (ch2 ^ key);
ch3 = (char) (ch3 ^ key);

Console.WriteLine("Decoded message: " +
ch1 + ch2 + ch3);

}
}

Результат:

Original message: Hi!
Encoded message: >1y
Decoded message: Hi!

Анализ:

Вверх

listing 12
// Demonstrate the bitwise NOT.
using System;

class NotDemo {
public static void Main() {
sbyte b = -34;
int t;

for(t=128; t > 0; t = t/2) {

if((b & t) != 0) Console.Write("1 ");
if((b & t) == 0) Console.Write("0 ");
}
Console.WriteLine();

// reverse all bits
b = (sbyte) ~b;

for(t=128; t > 0; t = t/2) {
if((b & t) != 0) Console.Write("1 ");
if((b & t) == 0) Console.Write("0 ");
}
}
}

Результат:

1 1 0 1 1 1 1 0
0 0 1 0 0 0 0 1

Анализ:

Вверх

listing 13
// Demonstrate the shift << and >> operators.
using System;

class ShiftDemo {
public static void Main() {
int val = 1;
int t;
int i;

for(i = 0; i < 8; i++) {

for(t=128; t > 0; t = t/2) {
if((val & t) != 0) Console.Write("1 ");
if((val & t) == 0) Console.Write("0 ");
}
Console.WriteLine();
val = val << 1; // left shift
}
Console.WriteLine();

val = 128;
for(i = 0; i < 8; i++) {
for(t=128; t > 0; t = t/2) {
if((val & t) != 0) Console.Write("1 ");
if((val & t) == 0) Console.Write("0 ");
}
Console.WriteLine();
val = val >> 1; // right shift
}
}
}

Результат:

0 0 0 0 0 0 0 1
0 0 0 0 0 0 1 0
0 0 0 0 0 1 0 0
0 0 0 0 1 0 0 0
0 0 0 1 0 0 0 0
0 0 1 0 0 0 0 0
0 1 0 0 0 0 0 0
1 0 0 0 0 0 0 0

1 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0
0 0 1 0 0 0 0 0
0 0 0 1 0 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 0 1 0 0
0 0 0 0 0 0 1 0
0 0 0 0 0 0 0 1

Анализ:

Вверх

listing 14
// Use the shift operators to multiply and divide by 2.
using System;

class MultDiv {
public static void Main() {
int n;

n = 10;

Console.WriteLine("Value of n: " + n);

// multiply by 2
n = n << 1;
Console.WriteLine("Value of n after n = n * 2: " + n);

// multiply by 4
n = n << 2;
Console.WriteLine("Value of n after n = n * 4: " + n);

// divide by 2
n = n >> 1;
Console.WriteLine("Value of n after n = n / 2: " + n);

// divide by 4
n = n >> 2;
Console.WriteLine("Value of n after n = n / 4: " + n);
Console.WriteLine();

// reset n
n = 10;
Console.WriteLine("Value of n: " + n);

// multiply by 2, 30 times
n = n << 30; // data is lost
Console.WriteLine("Value of n after left-shifting 30 places: " + n);

}
}

Результат:

Value of n: 10
Value of n after n = n * 2: 20
Value of n after n = n * 4: 80
Value of n after n = n / 2: 40
Value of n after n = n / 4: 10

Value of n: 10
Value of n after left-shifting 30 places: -2147483648

Анализ:

Вверх

listing 15
// Prevent a division by zero using the ?.
using System;

class NoZeroDiv {
public static void Main() {
int result;
int i;

for(i = -5; i < 6; i++) {

result = i != 0 ? 100 / i : 0;
if(i != 0)
Console.WriteLine("100 / " + i + " is " + result);
}
}
}

Результат:

100 / -5 is -20
100 / -4 is -25
100 / -3 is -33
100 / -2 is -50
100 / -1 is -100
100 / 1 is 100
100 / 2 is 50
100 / 3 is 33
100 / 4 is 25
100 / 5 is 20

Анализ:

Вверх

listing 16
// Prevent a division by zero using the ?.
using System;

class NoZeroDiv2 {
public static void Main() {
int i;

for(i = -5; i < 6; i++)
if(i != 0 ? true : false)
Console.WriteLine("100 / " + i +
" is " + 100 / i);

}
}

Результат:

100 / -5 is -20
100 / -4 is -25
100 / -3 is -33
100 / -2 is -50
100 / -1 is -100
100 / 1 is 100
100 / 2 is 50
100 / 3 is 33
100 / 4 is 25
100 / 5 is 20

Анализ:


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