Develop Reference

How can I get the values new.TITLE['kinds.of'].food

I am new to regex. I have this string
new.TITLE['kinds.of'].food
or
new.TITLE['deep thought'].food
I want to retrieve these tokens:
new, TITLE, kinds.of, food.
or (2nd example)
new, TITLE, deep thought, food.
I can't simply split it with '.' I need regex match to get the values.
How is it done?

When working with tokens a parser (FST - Finite State Machine in this case) should do:
private static IEnumerable<string> ParseIt(string value) {
int lastIndex = 0;
bool inApostroph = false;
for (int i = 0; i < value.Length; ++i) {
char ch = value[i];
if (ch == '\'') {
inApostroph = !inApostroph;
continue;
}
if (inApostroph)
continue;
if (ch == '.' || ch == ']' || ch == '[') {
if (i - lastIndex > 0) {
if (value[lastIndex] != '\'')
yield return value.Substring(lastIndex, i - lastIndex);
else {
string result = value.Substring(lastIndex, i - lastIndex).Replace("''", "'");
yield return result.Substring(1, result.Length - 2);
}
}
lastIndex = i + 1;
}
}
if (lastIndex < value.Length)
yield return value.Substring(lastIndex);
}
Tests:
string test1 = #"new.TITLE['kinds.of'].food";
string test2 = #"new.TITLE['deep thought'].food";
string[] result1 = ParseIt(test1).ToArray();
string[] result2 = ParseIt(test2).ToArray();
Console.WriteLine(string.Join(Environment.NewLine, result1));
Console.WriteLine(string.Join(Environment.NewLine, result2));
Outcome:
new
TITLE
kinds.of
food
new
TITLE
deep thought
food

ByteArrayOutputStream in C#

I have the following code in Java:
public static byte[] hex(String hex) {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
int nexti = 0;
int nextb = 0;
boolean highoc = true;
outer:
while (true)
{
int number = -1;
while (number == -1) {
if (nexti == hex.length()) {
break outer;
}
char chr = hex.charAt(nexti);
if ((chr >= '0') && (chr <= '9'))
number = chr - '0';
else if ((chr >= 'a') && (chr <= 'f'))
number = chr - 'a' + 10;
else if ((chr >= 'A') && (chr <= 'F'))
number = chr - 'A' + 10;
else {
number = -1;
}
nexti++;
}
if (highoc) {
nextb = number << 4;
highoc = false;
} else {
nextb |= number;
highoc = true;
baos.write(nextb);
}
}
label161: return baos.toByteArray();
}
I'm trying to convert it to C#, and failing, because MemoryStream is the only option, and I don't have a buffer.
This is what I have now:
public static byte[] fromString(string hex)
{
MemoryStream baos = new MemoryStream();
int nexti = 0;
int nextb = 0;
bool highoc = true;
for (; ; )
{
int number = -1;
while (number == -1)
{
if (nexti == hex.Length)
{
goto END;
}
char chr = hex.ToCharArray()[nexti];
if (chr >= '0' && chr <= '9')
{
number = chr - '0';
}
else if (chr >= 'a' && chr <= 'f')
{
number = chr - 'a' + 10;
}
else if (chr >= 'A' && chr <= 'F')
{
number = chr - 'A' + 10;
}
else
{
number = -1;
}
nexti++;
}
if (highoc)
{
nextb = number << 4;
highoc = false;
}
else
{
nextb |= number;
highoc = true;
baos.Write(nextb);
}
}
END:
return baos.toByteArray();
}
What else can I do to make it work like the way in Java?.. Thanks.

Here is something similar
public static byte[] StringToByteArrayFastest(string hex) {
if (hex.Length % 2 == 1)
throw new Exception("The binary key cannot have an odd number of digits");
byte[] arr = new byte[hex.Length >> 1];
for (int i = 0; i < hex.Length >> 1; ++i)
{
arr[i] = (byte)((GetHexVal(hex[i << 1]) << 4) + (GetHexVal(hex[(i << 1) + 1])));
}
return arr;
}
public static int GetHexVal(char hex) {
int val = (int)hex;
//For uppercase A-F letters:
return val - (val < 58 ? 48 : 55);
//For lowercase a-f letters:
//return val - (val < 58 ? 48 : 87);
//Or the two combined, but a bit slower:
//return val - (val < 58 ? 48 : (val < 97 ? 55 : 87));
}
or
public static byte[] StringToByteArray(String hex)
{
int NumberChars = hex.Length;
byte[] bytes = new byte[NumberChars / 2];
for (int i = 0; i < NumberChars; i += 2)
bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
return bytes;
}
or
private byte[] HexStringToByteArray(string hexString)
{
int hexStringLength = hexString.Length;
byte[] b = new byte[hexStringLength / 2];
for (int i = 0; i < hexStringLength; i += 2)
{
int topChar = (hexString[i] > 0x40 ? hexString[i] - 0x37 : hexString[i] - 0x30) << 4;
int bottomChar = hexString[i + 1] > 0x40 ? hexString[i + 1] - 0x37 : hexString[i + 1] - 0x30;
b[i / 2] = Convert.ToByte(topChar + bottomChar);
}
return b;
}
Here is alot more of them.
How do you convert Byte Array to Hexadecimal String, and vice versa?

Faster parsing of numbers on .NET

I have written two functions that convert a string of whitespace-separated integers into an int array. The first function uses Substring and then applies System.Int32.Parse to convert the substring into an int value:
let intsOfString (s: string) =
let ints = ResizeArray()
let rec inside i j =
if j = s.Length then
ints.Add(s.Substring(i, j-i) |> System.Int32.Parse)
else
let c = s.[j]
if '0' <= c && c <= '9' then
inside i (j+1)
else
ints.Add(s.Substring(i, j-i) |> System.Int32.Parse)
outside (j+1)
and outside i =
if i < s.Length then
let c = s.[i]
if '0' <= c && c <= '9' then
inside i (i+1)
else
outside (i+1)
outside 0
ints.ToArray()
The second function traverses the characters of the string in-place accumulating the integer without creating a temporary substring:
let intsOfString (s: string) =
let ints = ResizeArray()
let rec inside n i =
if i = s.Length then
ints.Add n
else
let c = s.[i]
if '0' <= c && c <= '9' then
inside (10*n + int c - 48) (i+1)
else
ints.Add n
outside(i+1)
and outside i =
if i < s.Length then
let c = s.[i]
if '0' <= c && c <= '9' then
inside (int c - 48) (i+1)
else
outside (i+1)
outside 0
ints.ToArray()
Benchmarking on space-separated integers 1 to 1,000,000, the first version takes 1.5s whereas the second version takes 0.3s.
Parsing such values can be performance critical so leaving 5x performance on the table by using temporary substrings can be undesirable. Parsing integers is easy but parsing other values such as floating point numbers, decimals and dates is considerably harder.
So, are there built-in functions to parse directly from a substring within a string (i.e. using the given start and length of a string) in order to avoid generating a temporary string? If not, are there any libraries that provide efficient functions to do this?

System.Int32.Parse is slowlest, because it used CultureInfo, FormatInfo and etc; and performance reason is not in the temporary strings.
Code from reflection:
private unsafe static bool ParseNumber(ref char* str, NumberStyles options, ref Number.NumberBuffer number, NumberFormatInfo numfmt, bool parseDecimal)
{
number.scale = 0;
number.sign = false;
string text = null;
string text2 = null;
string str2 = null;
string str3 = null;
bool flag = false;
string str4;
string str5;
if ((options & NumberStyles.AllowCurrencySymbol) != NumberStyles.None)
{
text = numfmt.CurrencySymbol;
if (numfmt.ansiCurrencySymbol != null)
{
text2 = numfmt.ansiCurrencySymbol;
}
str2 = numfmt.NumberDecimalSeparator;
str3 = numfmt.NumberGroupSeparator;
str4 = numfmt.CurrencyDecimalSeparator;
str5 = numfmt.CurrencyGroupSeparator;
flag = true;
}
else
{
str4 = numfmt.NumberDecimalSeparator;
str5 = numfmt.NumberGroupSeparator;
}
int num = 0;
char* ptr = str;
char c = *ptr;
while (true)
{
if (!Number.IsWhite(c) || (options & NumberStyles.AllowLeadingWhite) == NumberStyles.None || ((num & 1) != 0 && ((num & 1) == 0 || ((num & 32) == 0 && numfmt.numberNegativePattern != 2))))
{
bool flag2;
char* ptr2;
if ((flag2 = (((options & NumberStyles.AllowLeadingSign) == NumberStyles.None) ? false : ((num & 1) == 0))) && (ptr2 = Number.MatchChars(ptr, numfmt.positiveSign)) != null)
{
num |= 1;
ptr = ptr2 - (IntPtr)2 / 2;
}
else
{
if (flag2 && (ptr2 = Number.MatchChars(ptr, numfmt.negativeSign)) != null)
{
num |= 1;
number.sign = true;
ptr = ptr2 - (IntPtr)2 / 2;
}
else
{
if (c == '(' && (options & NumberStyles.AllowParentheses) != NumberStyles.None && (num & 1) == 0)
{
num |= 3;
number.sign = true;
}
else
{
if ((text == null || (ptr2 = Number.MatchChars(ptr, text)) == null) && (text2 == null || (ptr2 = Number.MatchChars(ptr, text2)) == null))
{
break;
}
num |= 32;
text = null;
text2 = null;
ptr = ptr2 - (IntPtr)2 / 2;
}
}
}
}
c = *(ptr += (IntPtr)2 / 2);
}
int num2 = 0;
int num3 = 0;
while (true)
{
if ((c >= '0' && c <= '9') || ((options & NumberStyles.AllowHexSpecifier) != NumberStyles.None && ((c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F'))))
{
num |= 4;
if (c != '0' || (num & 8) != 0)
{
if (num2 < 50)
{
number.digits[(IntPtr)(num2++)] = c;
if (c != '0' || parseDecimal)
{
num3 = num2;
}
}
if ((num & 16) == 0)
{
number.scale++;
}
num |= 8;
}
else
{
if ((num & 16) != 0)
{
number.scale--;
}
}
}
else
{
char* ptr2;
if ((options & NumberStyles.AllowDecimalPoint) != NumberStyles.None && (num & 16) == 0 && ((ptr2 = Number.MatchChars(ptr, str4)) != null || (flag && (num & 32) == 0 && (ptr2 = Number.MatchChars(ptr, str2)) != null)))
{
num |= 16;
ptr = ptr2 - (IntPtr)2 / 2;
}
else
{
if ((options & NumberStyles.AllowThousands) == NumberStyles.None || (num & 4) == 0 || (num & 16) != 0 || ((ptr2 = Number.MatchChars(ptr, str5)) == null && (!flag || (num & 32) != 0 || (ptr2 = Number.MatchChars(ptr, str3)) == null)))
{
break;
}
ptr = ptr2 - (IntPtr)2 / 2;
}
}
c = *(ptr += (IntPtr)2 / 2);
}
bool flag3 = false;
number.precision = num3;
number.digits[(IntPtr)num3] = '\0';
if ((num & 4) != 0)
{
if ((c == 'E' || c == 'e') && (options & NumberStyles.AllowExponent) != NumberStyles.None)
{
char* ptr3 = ptr;
c = *(ptr += (IntPtr)2 / 2);
char* ptr2;
if ((ptr2 = Number.MatchChars(ptr, numfmt.positiveSign)) != null)
{
c = *(ptr = ptr2);
}
else
{
if ((ptr2 = Number.MatchChars(ptr, numfmt.negativeSign)) != null)
{
c = *(ptr = ptr2);
flag3 = true;
}
}
if (c >= '0' && c <= '9')
{
int num4 = 0;
do
{
num4 = num4 * 10 + (int)(c - '0');
c = *(ptr += (IntPtr)2 / 2);
if (num4 > 1000)
{
num4 = 9999;
while (c >= '0' && c <= '9')
{
c = *(ptr += (IntPtr)2 / 2);
}
}
}
while (c >= '0' && c <= '9');
if (flag3)
{
num4 = -num4;
}
number.scale += num4;
}
else
{
ptr = ptr3;
c = *ptr;
}
}
while (true)
{
if (!Number.IsWhite(c) || (options & NumberStyles.AllowTrailingWhite) == NumberStyles.None)
{
bool flag2;
char* ptr2;
if ((flag2 = (((options & NumberStyles.AllowTrailingSign) == NumberStyles.None) ? false : ((num & 1) == 0))) && (ptr2 = Number.MatchChars(ptr, numfmt.positiveSign)) != null)
{
num |= 1;
ptr = ptr2 - (IntPtr)2 / 2;
}
else
{
if (flag2 && (ptr2 = Number.MatchChars(ptr, numfmt.negativeSign)) != null)
{
num |= 1;
number.sign = true;
ptr = ptr2 - (IntPtr)2 / 2;
}
else
{
if (c == ')' && (num & 2) != 0)
{
num &= -3;
}
else
{
if ((text == null || (ptr2 = Number.MatchChars(ptr, text)) == null) && (text2 == null || (ptr2 = Number.MatchChars(ptr, text2)) == null))
{
break;
}
text = null;
text2 = null;
ptr = ptr2 - (IntPtr)2 / 2;
}
}
}
}
c = *(ptr += (IntPtr)2 / 2);
}
if ((num & 2) == 0)
{
if ((num & 8) == 0)
{
if (!parseDecimal)
{
number.scale = 0;
}
if ((num & 16) == 0)
{
number.sign = false;
}
}
str = ptr;
return true;
}
}
str = ptr;
return false;
}
public static int Parse(string s)
{
return Number.ParseInt32(s, NumberStyles.Integer, NumberFormatInfo.CurrentInfo);
}
internal unsafe static int ParseInt32(string s, NumberStyles style, NumberFormatInfo info)
{
byte* stackBuffer = stackalloc byte[1 * 114 / 1];
Number.NumberBuffer numberBuffer = new Number.NumberBuffer(stackBuffer);
int result = 0;
Number.StringToNumber(s, style, ref numberBuffer, info, false);
if ((style & NumberStyles.AllowHexSpecifier) != NumberStyles.None)
{
if (!Number.HexNumberToInt32(ref numberBuffer, ref result))
{
throw new OverflowException(Environment.GetResourceString("Overflow_Int32"));
}
}
else
{
if (!Number.NumberToInt32(ref numberBuffer, ref result))
{
throw new OverflowException(Environment.GetResourceString("Overflow_Int32"));
}
}
return result;
}
private unsafe static void StringToNumber(string str, NumberStyles options, ref Number.NumberBuffer number, NumberFormatInfo info, bool parseDecimal)
{
if (str == null)
{
throw new ArgumentNullException("String");
}
fixed (char* ptr = str)
{
char* ptr2 = ptr;
if (!Number.ParseNumber(ref ptr2, options, ref number, info, parseDecimal) || ((ptr2 - ptr / 2) / 2 < str.Length && !Number.TrailingZeros(str, (ptr2 - ptr / 2) / 2)))
{
throw new FormatException(Environment.GetResourceString("Format_InvalidString"));
}
}
}

I've written this one for doubles, that doesn't create a temporary substring. It's meant to be used inside a JSON parser so it limits itself to how doubles can be represented in JSON according to http://www.json.org/.
It's not optimal yet because it requires you to know where the number begins and ends (begin and end parameters), so you'll have to traverse the length of the number twice to find out where it ends. It's still around 10-15x faster than double.Parse and it could be fairly easily modified that it finds the end inside the function which is then returned as an out parameter to know where you have to resume parsing the main string.
Used like so:
Parsers.TryParseDoubleFastStream("1", 0, 1, out j);
Parsers.TryParseDoubleFastStream("2.0", 0, 3, out j);
Parsers.TryParseDoubleFastStream("3.5", 0, 3, out j);
Parsers.TryParseDoubleFastStream("-4.5", 0, 4, out j);
Parsers.TryParseDoubleFastStream("50.06", 0, 5, out j);
Parsers.TryParseDoubleFastStream("1000.65", 0, 7, out j);
Parsers.TryParseDoubleFastStream("-10000.8600", 0, 11, out j);
Code can be found here:
https://gist.github.com/3010984 (would be too lengthy to post here).
And StandardFunctions.IgnoreChar is for my purpose as simple as:
public static bool IgnoreChar(char c)
{
return c < 33;
}

Paste all this code into C# and call Test(). This is as close as you can get to operating directly on the string array to parse numbers using C#. It is built for speed, not elegance. The ParseInt and ParseFloat function were created for an OpenGL graphics engine to import vectors from text-based 3d models. Parsing floats is a significant bottleneck in that process. This was as fast as I could make it.
using System.Diagnostics;
private void Test()
{
Stopwatch sw = new Stopwatch();
StringBuilder sb = new StringBuilder();
int iterations = 1000;
// Build a string of 1000000 space separated numbers
for (var n = 0; n < iterations; n++)
{
if (n > 0)
sb.Append(' ');
sb.Append(n.ToString());
}
string numberString = sb.ToString();
// Time the process
sw.Start();
StringToInts(numberString, iterations);
//StringToFloats(numberString, iterations);
sw.Stop();
long proc1 = sw.ElapsedMilliseconds;
Console.WriteLine("iterations: {0} \t {1}ms", iterations, proc1);
}
private unsafe int[] StringToInts(string s, int length)
{
int[] ints = new int[length];
int index = 0;
int startpos = 0;
fixed (char* pStringBuffer = s)
{
fixed (int* pIntBuffer = ints)
{
for (int n = 0; n < s.Length; n++)
{
if (s[n] == ' ' || n == s.Length - 1)
{
if (n == s.Length - 1)
n++;
// pIntBuffer[index++] = int.Parse(new string(pStringBuffer, startpos, n - startpos));
pIntBuffer[index++] = ParseInt((pStringBuffer + startpos), n - startpos);
startpos = n + 1;
}
}
}
}
return ints;
}
private unsafe float[] StringToFloats(string s, int length)
{
float[] floats = new float[length];
int index = 0;
int startpos = 0;
fixed (char* pStringBuffer = s)
{
fixed (float* pFloatBuffer = floats)
{
for (int n = 0; n < s.Length; n++)
{
if (s[n] == ' ' || n == s.Length - 1)
{
if (n == s.Length - 1)
n++;
pFloatBuffer[index++] = ParseFloat((pStringBuffer + startpos), n - startpos); // int.Parse(new string(pStringBuffer, startpos, n - startpos));
startpos = n + 1;
}
}
}
}
return floats;
}
public static unsafe int ParseInt(char* input, int len)
{
int pos = 0; // read pointer position
int part = 0; // the current part (int, float and sci parts of the number)
bool neg = false; // true if part is a negative number
int* ret = stackalloc int[1];
while (pos < len && (*(input + pos) > '9' || *(input + pos) < '0') && *(input + pos) != '-')
pos++;
// sign
if (*(input + pos) == '-')
{
neg = true;
pos++;
}
// integer part
while (pos < len && !(input[pos] > '9' || input[pos] < '0'))
part = part * 10 + (input[pos++] - '0');
*ret = neg ? (part * -1) : part;
return *ret;
}
public static unsafe float ParseFloat(char* input, int len)
{
//float ret = 0f; // return value
int pos = 0; // read pointer position
int part = 0; // the current part (int, float and sci parts of the number)
bool neg = false; // true if part is a negative number
float* ret = stackalloc float[1];
// find start
while (pos < len && (input[pos] < '0' || input[pos] > '9') && input[pos] != '-' && input[pos] != '.')
pos++;
// sign
if (input[pos] == '-')
{
neg = true;
pos++;
}
// integer part
while (pos < len && !(input[pos] > '9' || input[pos] < '0'))
part = part * 10 + (input[pos++] - '0');
*ret = neg ? (float)(part * -1) : (float)part;
// float part
if (pos < len && input[pos] == '.')
{
pos++;
double mul = 1;
part = 0;
while (pos < len && !(input[pos] > '9' || input[pos] < '0'))
{
part = part * 10 + (input[pos] - '0');
mul *= 10;
pos++;
}
if (neg)
*ret -= (float)part / (float)mul;
else
*ret += (float)part / (float)mul;
}
// scientific part
if (pos < len && (input[pos] == 'e' || input[pos] == 'E'))
{
pos++;
neg = (input[pos] == '-'); pos++;
part = 0;
while (pos < len && !(input[pos] > '9' || input[pos] < '0'))
{
part = part * 10 + (input[pos++] - '0');
}
if (neg)
*ret /= (float)Math.Pow(10d, (double)part);
else
*ret *= (float)Math.Pow(10d, (double)part);
}
return (float)*ret;
}

So, are there built-in functions to parse directly from a substring within a string (i.e.
using the given start and length of a string) in order to avoid generating a temporary
string? If not, are there any libraries that provide efficient functions to do this?
It seems that you want to use a lexing buffer and a lexer, similar to what OCaml can provide with ocamllex and the Lexbuf buffer. (I cannot provide references for F#.)
If your benchmark involving a huge string of integers separated by other tokens is your typical case, it will work well. But in other situations, it could be impractical.

Not sure if this is any good, but have you tried something like:
var stringValues = input.split(" ");
var intValues = Array.ConvertAll(stringValues, s => int.Parse(s));

Create all permutations of a string incrementally c#

I am trying to create a function that will create all permutations of a string in an incremental fashion. I would like to start at:
AAAAA
...
AAAAB
...
ACCCC
...
...
ZZZZZ
I have looked around, and can't seem to find anything of that sort. I tried to create it, but it wasn't incrementally.
Any suggestions?

The "permutation" you are describing is better known as the Cartesian product. If you have an arbitrary number of sequences that you need to form the Cartesian product of, see my answer to this question on the subject:
Generating all Possible Combinations

Normally I wouldn't help these brute force type results... but seeing how many useless result you will get out of the set I figured I'd just toss this in.
var query = from c0 in Enumerable.Range(0, 26)
from c1 in Enumerable.Range(0, 26)
from c2 in Enumerable.Range(0, 26)
from c3 in Enumerable.Range(0, 26)
from c4 in Enumerable.Range(0, 26)
select new string(
new [] {
(char)('A' + c0),
(char)('A' + c1),
(char)('A' + c2),
(char)('A' + c3),
(char)('A' + c4),
}
);
BTW... if you just want the next value you can do something like this...
public static string Increment(string input)
{
var array = input.ToCharArray();
if (array.Any(c => c < 'A' || c > 'Z'))
throw new InvalidOperationException();
for (var i = array.Length-1; i >= 0; i--)
{
array[i] = (char)(array[i] + 1);
if (array[i] > 'Z')
{
array[i] = 'A';
if (i == 0)
return 'A' + new string(array);
}
else
break;
}
return new string(array);
}

A different variant where I had the idea of using modulo arithmetic. Note that I lowered the character to {A,B,C} to test it, since going up to Z for 5 letters is a lot of strings.
public IEnumerable<char[]> AlphaCombinations(int length = 5, char startChar = 'A', char endChar = 'C')
{
int numChars = endChar - startChar + 1;
var s = new String(startChar, length).ToCharArray();
for (int it = 1; it <= Math.Pow(numChars, length); ++it)
{
yield return s;
for (int ix = 0; ix < s.Length; ++ix)
if (ix == 0 || it % Math.Pow(numChars, ix) == 0)
s[s.Length - 1 - ix] = (char)(startChar + (s[s.Length - 1 - ix] - startChar + 1) % numChars);
}
}
...
foreach (var s in AlphaCombinations(5))
{
Console.WriteLine(s);
}

Bashed out quickly - I expect this could be done better:
public static IEnumerable<string> GenerateStrings(int length = 5)
{
var buffer = new char[length];
for (int i = 0; i < length; ++i)
{
buffer[i] = 'A';
}
for(;;)
{
yield return new string(buffer);
int cursor = length;
for(;;)
{
--cursor;
if (cursor < 0)
{
yield break;
}
char c = buffer[cursor];
++c;
if (c <= 'Z')
{
buffer[cursor] = c;
break;
}
else
{
buffer[cursor] = 'A';
}
}
}
}

Here is the LINQPad friendly code and it uses lambda expression.
void Main()
{
var chars = Enumerable.Range(65, 26);
var strings = chars.SelectMany (a =>
{
return chars.SelectMany (b => chars.SelectMany (c =>
{
return chars.SelectMany (d =>
{
return chars.Select (e => {return new string(new char[] {(char)a, (char)b, (char)c, (char)d, (char)e});});
});
}));
});
strings.Dump();
}

Code Smell in Dynamic Expression

This code solves an interesting puzzle outlined in http://www.programgood.net/2011/01/13/DynamicOperatorsGuernseyChallenge.aspx
Problem: There seems to be lots of repeating here.. DRY (Don't Repeat Yourself) principle springs to mind here. Anyone see a refactor?
string opZ = "";
string opA = "";
string opB = "";
string opC = "";
string opD = "";
for (int h = 1; h <= 2; h++) // making the first number positive or negative
{
if (h == 1) opZ = "";
if (h == 2) opZ = "-";
for (int i = 1; i <= 4; i++)
{
if (i == 1) opA = "*";
if (i == 2) opA = "/";
if (i == 3) opA = "+";
if (i == 4) opA = "-";
for (int j = 1; j <= 4; j++)
{
if (j == 1) opB = "*";
if (j == 2) opB = "/";
if (j == 3) opB = "+";
if (j == 4) opB = "-";
for (int k = 1; k <= 4; k++)
{
if (k == 1) opC = "*";
if (k == 2) opC = "/";
if (k == 3) opC = "+";
if (k == 4) opC = "-";
for (int l = 1; l <= 4; l++)
{
if (l == 1) opD = "*";
if (l == 2) opD = "/";
if (l == 3) opD = "+";
if (l == 4) opD = "-";
string expression = opZ + 1 + opA + 3 + opB + 5 + opC + 7 + opD + 9;
DataTable dummy = new DataTable();
double result = Convert.ToDouble(dummy.Compute(expression, string.Empty));
if (result == 3)
Debug.WriteLine(expression + " = 3");
if (result == 47)
Debug.WriteLine(expression + " = 47");
if (result == 18)
Debug.WriteLine(expression + " = 18");
}
}
}
}
}

Well, the first obvious refactoring would be to have an array of operators:
String[] operators = { null, "*", "/", "+", "-" };
Then use:
opC = operators[j]; // etc
(Personally I'd use loops going from 0 to 3 instead of 1 to 4 - that's more idiomatic IMO, but that's a different matter.)
Then there's the way of building the permutations. I'd actually use LINQ for this:
string[] prefixes = { "", "-" };
string[] operators = { "*", "/", "+", "-" };
var expressions = from prefix in prefixes
from opA in operators
from opB in operators
from opC in operators
from opD in operators
select prefix + 1 + opA + 3 + opB + 5 + opC + 7 + opD + 9;
foreach (string expression in expressions)
{
...
}

char[] ops = new [] {'*','/','+','-'};
foreach(string opA in ops)
foreach(string opB in ops)
foreach(string opC in ops)
foreach(string opD in ops)
foreach(string opZ in new []{'-',' '}) {
string expression = opZ + 1 + opA + 3 + opB + 5 + opC + 7 + opD + 9;
DataTable dummy = new DataTable();
double result = Convert.ToDouble(dummy.Compute(expression, string.Empty));
if (result == 3)
Debug.WriteLine(expression + " = 3");
if (result == 47)
Debug.WriteLine(expression + " = 47");
if (result == 18)
Debug.WriteLine(expression + " = 18");
}

I suppose there's no real point doing this while using DataTable.Compute, but
var calculator = new DataTable () ;
var operators = "*/+-" ;
for (int i = 0 ; i < 0x200 ; ++i)
{
var value = calculator.Compute (String.Format ("{0}1{1}3{2}5{3}7{4}9",
(i & 0x100) != 0 ? "-" : "",
operators[(i >> 0) & 3],
operators[(i >> 2) & 3],
operators[(i >> 4) & 3],
operators[(i >> 6) & 3]), String.Empty) ;
...
}
Otherwise, this will definitely be faster if somewhat more abstruse:
var opstrings = "+-*/" ;
var operators = new Func<int, int, int>[] {
(a, b) => a + b,
(a, b) => a - b,
(a, b) => a * b,
(a, b) => a / b, } ;
for (int i = 0 ; i < 0x200 ; ++i)
{
var stack = 0 ; // seed value
var last = 0 ; // imitate + for lowest precedence
var value =(i & 0x100) != 0 ? -1 : 1 ;
for (int j = 0 ; j < 5 ; ++j) // extra item to force last reduction
{
var oper = (i >> j * 2) & 3 ; // "input" operator
var input = 3 + j * 2 ; // "input" number
if (oper / 2 <= last / 2) // reduce below top?
{
stack = operators[last] (stack, value) ;
last = oper ; // shift operator
value = input ; // shift number
}
else // reduce top
value = operators[oper] (value, input) ;
}
var result = stack ;
if (result == 3 || result == 47 || result == 18)
Debug.WriteLine ("{0}1{1}3{2}5{3}7{4}9 = {5}",
(i & 0x100) != 0 ? "-" : "",
opstrings[(i >> 0) & 3],
opstrings[(i >> 2) & 3],
opstrings[(i >> 4) & 3],
opstrings[(i >> 6) & 3],
result) ;
}