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In this article we will analyze the table of symbols ASCII and the ways it can be used. We will also deal with some new functions, the principle of operation of which is based on the peculiarities of the ASCII table, and then we will create a new library, which will include these functions of MQL4 language. They are quite popular in other programming languages, but they are not included into the list of built-in functions. Besides, we will examine in details the basics of working with strings. So, I think you will certainly learn something new about this useful type of data.

What is ASCII ?

ASCII is the American coding standard for information interchange (American Standard Code for Information Interchange). This standard is based on the English alphabet. ASCII codes present a text in computers, communication equipment and other devices, which work with texts. ASCII was created in 1963, but published as a standard first in 1967. The last amendments were made in 1986. More detailed information about ASCII can be found here: http://en.wikipedia.org/wiki/ASCII. Further we will see, how we can display ASCII using MQL4 options, but first let us examine the basis of working with strings.

Principles of Writing a Library

To write a library of this kind, let us understand some essential moments. First let us see how we can go through all symbols, like the procedures with data arrays. Alike code parts will be always repeated in any function, intended for symbolwise processing. As an example, let us write a simple script, which first shows a common string and then a processed one, where each symbol is divided by a space.

//+------------------------------------------------------------------+
//|                                          StringExpereriment1.mq4 |
//|         Copyright © 2007, Antonio Banderass. All rights reserved |
//|                                               banderassa@ukr.net |
//+------------------------------------------------------------------+
#property copyright "Copyright © 2007, Antonio Banderass. All rights reserved"
#property link      "banderassa@ukr.net"
//+------------------------------------------------------------------+
//| start                                                            |
//+------------------------------------------------------------------+
int start()
  {
   string s1 = "Just_a_string", s2, symbol = "s";
   int lenght = StringLen(s1);
   MessageBox(s1);
   for(int x = 0; x < lenght; x++)
     {
       symbol = StringSetChar(symbol, 0, StringGetChar(s1, x));
       s2 = s2 + symbol + " ";
     }
   MessageBox(s2);
   return(0);
  }
//+------------------------------------------------------------------+

Let us analyze the value of each string.

string s1 = "Just_a_string", s2, symbol = "s";

Determine three variables of a string type:

Note, that it is initialized by one symbol. If we do not do this, we will get as a result a string without the first symbol. The matter is, the standard MQL4 function StringGetChar() changes already created symbols, that is why we need at least one symbol for regular operation.

int lenght = StringLen(s1);

MessageBox(s1);

for(int x = 0; x < lenght; x++)

symbol = StringSetChar(symbol, 0, StringGetChar(s1, x));

In this string two standard functions are used: StringSetChar() and StringGetChar(). The first one allows replacing one of the symbols of the string, the second one - getting the symbol code in the indicated position. The function StringSetChar() has three parameters:

The function returns the result in the form of a changed string. One more important function - StringGetChar. It has two parameters:

• string with a symbol, the code of which should be recognized;
• position of the symbol, the code of which should be recognize.

The function returns the symbol code. While the function StringGetChar returns the symbol code, I located its calling in the place of the parameter of the function StringSetChar. Therefore using this function we remember the current symbol for further processing. During the execution of the cycle, each symbol of the string s1 will be assigned to the variable.

s2 = s2 + symbol + " ";

MessageBox(s2);

//+------------------------------------------------------------------+
//|                                            StringExperiment2.mq4 |
//|         Copyright © 2007, Antonio Banderass. All rights reserved |
//|                                               banderassa@ukr.net |
//+------------------------------------------------------------------+
#property copyright "Copyright © 2007, Antonio Banderass. All rights reserved"
#property link      "banderassa@ukr.net"
//+------------------------------------------------------------------+
//| start                                                            |
//+------------------------------------------------------------------+
int start()
  {
   string s1 = "Just_a_string", s2, symbol = "s";
   int lenght = StringLen(s1) - 1;
   MessageBox(s1);
   while(lenght >= 0)
     {
       symbol = StringSetChar(symbol, 0, StringGetChar(s1, lenght));
       s2 = s2 + symbol + " ";
       lenght--;
     }
   MessageBox(s2);
   return(0);
  }
//+------------------------------------------------------------------+

You see, now instead of the cycle 'for' we use 'while', which allows omitting the counter x. For this purpose we use the variable length. Further we will use one of the two templates for writing functions, depending on the fact, whether the processing sequence matters or not. In our case we get the string with the symbols in the reverse order, i.e. here the processing sequence matters much.

Show All Symbols ASCII

//+------------------------------------------------------------------+
//|                                            StringExperiment3.mq4 |
//|         Copyright © 2007, Antonio Banderass. All rights reserved |
//|                                               banderassa@ukr.net |
//+------------------------------------------------------------------+
#property copyright "Copyright © 2007, Antonio Banderass. All rights reserved"
#property link      "banderassa@ukr.net"
//+------------------------------------------------------------------+
//| start                                                            |
//+------------------------------------------------------------------+
int start()
  {
   string s, symbol = "s";
   for(int x = 0; x < 256; x++)
    {
      symbol = StringSetChar(symbol, 0, x);
      s = s + x + " = " + symbol + " \t";
      if(x % 10 == 0)
          s = s + " \n";
    }
   MessageBox(s);
   return(0);
  }
//+------------------------------------------------------------------+

The script uses in-built MQL4 functions, and string constants of a new string and tabulation for the visualization of the table. Now compile and start it. You will see the table of ASCII symbols:

Look at it very attentively. You will see absolutely all symbols, which can be needed, from numbers and letters to special symbols, some of them can be new for you. First comes a code, and then after the sign "=" comes the symbol itself. I marked some important sets of symbols:

Pay attention to the arrangement of letters. They are arranged in the alphabetical order. This characteristic will later be used for writing some functions, for example, for transferring letters from the string of the upper case into the lower one and vice versa.

And now let us deal with new functions and in the end create a library based on them.

StringUpperCase and StringLowerCase

These are two very popular functions for transferring a string into the upper or lower case. Their implementation is based on the fact that all codes of the upper case letters symbols are shifted by 32 from the letters of the lower case. It is seen from the table. In practice, if we try to find the codes of symbols 'A', 'Z', 'a', 'z', for example using this code:

string s = "AZaz";
MessageBox("A = " + StringGetChar(s, 0));
MessageBox("Z = " + StringGetChar(s, 1));
MessageBox("a = " + StringGetChar(s, 2));
MessageBox("z = " + StringGetChar(s, 3));

the results will be the following: A= 65, Z= 90, a= 97, z= 122 accordingly. So we should take into account this peculiarity. Let us view the source code of the function:

//+------------------------------------------------------------------+
//| StringUpperCase                                                  |
//+------------------------------------------------------------------+
string StringUpperCase(string str)
  {
   string s = str;
   int lenght = StringLen(str) - 1, symbol;
   while(lenght >= 0)
     {
       symbol = StringGetChar(s, lenght);
       if((symbol > 96 && symbol < 123) || (symbol > 223 && symbol < 256))
           s = StringSetChar(s, lenght, symbol - 32);
       else 
           if(symbol > -33 && symbol < 0)
               s = StringSetChar(s, lenght, symbol + 224);
       lenght--;
     }
   return(s);
  }
//+------------------------------------------------------------------+
//| StringLowerCase                                                  |
//+------------------------------------------------------------------+
string StringLowerCase(string str)
  {
   string s = str;
   int lenght = StringLen(str) - 1, symbol;
   while(lenght >= 0)
     {
       symbol = StringGetChar(s, lenght);
       if((symbol > 64 && symbol < 91) || (symbol > 191 && symbol < 224))
           s = StringSetChar(s, lenght, symbol + 32);
       else 
           if(symbol > -65 && symbol < -32)
               s = StringSetChar(s, lenght, symbol + 288);
       lenght--;
     }
   return(s);
  }

  string s1 = "UPPER_REGISTER_STRING";
  string s2 = "lower_register_string";
   
  MessageBox(StringLowerCase(s1)); // upper_register_string
  MessageBox(StringUpperCase(s2)); // LOWER_REGISTER_STRING

StringCompare

In MQL4 the comparison of strings is performed on the level of operators using "==". Interesting is the fact that comparison is case-dependant, i.e. strings "STRING" and "string" are different:

   if("STRING" == "string") // FALSE
      MessageBox("TRUE");
   else
      MessageBox("FALSE");

If we need to compare two string without ignoring case, use the function StringCompare(). It returns bool type values like the operator of comparison. The implementation is easy. It compares strings that are beforehand transferred into lower case:

//+------------------------------------------------------------------+
//| StringCompare                                                    |
//+------------------------------------------------------------------+
bool StringCompare(string s1, string s2)
  {
    return(StringLowerCase(s1) == StringLowerCase(s2));
  }

Example of use:

   if(StringCompare("STRING", "string")) // TRUE
      MessageBox("TRUE");
   else
      MessageBox("FALSE");

StringIsDigit

This function checks the contents of the string. If the string contains only numbers, it returns true, otherwise - false. The implementation is quite easy and is based on the fact, that the symbols of numbers are in one raw and have codes from 48 to 58.

//+------------------------------------------------------------------+
//| StringIsDigit                                                    |
//+------------------------------------------------------------------+
bool StringIsDigit(string str)
  {
   bool result = true;
   string s = str;
   int lenght = StringLen(str) - 1, symbol;
   while(lenght > 0)
     {
       symbol = StringGetChar(s, lenght);
       if(!(symbol > 47 && symbol < 58))
         {
           result = false;
           break;
         }
       lenght--;
     }
   return(result);
  }

Example of use:

  if(StringIsDigit("1234567890")) // TRUE
      MessageBox("TRUE");
  else
      MessageBox("FALSE");
 
  if(StringIsDigit("1234notdigit")) // FALSE
      MessageBox("TRUE");
  else
      MessageBox("FALSE");

StringIsAlpha

This function, like the previous one, allows determining whether the string contains only letters. Its implementation is analogous:

//+------------------------------------------------------------------+
//| StringIsAlpha                                                    |
//+------------------------------------------------------------------+
bool StringIsAlpha(string str)
  {
   bool result = false;
   string s = str;
   int lenght = StringLen(str) - 1, symbol;
   while(lenght > 0)
     {
       symbol = StringGetChar(s, lenght);
       if((symbol > 96  && symbol < 123) || (symbol > 64 && symbol < 91) ||
          (symbol > 191 && symbol < 256) || (symbol > -65 && symbol < 0))
 
         {
           result = true;
           break;
         }
       lenght--;
     }
   return(result);
  }

Example of use:

  if(StringIsAlpha("thereAreSomeLetters")) // TRUE
      MessageBox("TRUE");
  else
      MessageBox("FALSE");
 
  if(StringIsAlpha("thereAreSomeDigits12345")) // FALSE
      MessageBox("TRUE");
  else
      MessageBox("FALSE");

Creating a Library

Now let us gather all these functions into one library. For this purpose in MetaEditor 4 click 'File -> New -> Library -> Next'. In the field name write stringProcess and click OK. Then insert the code of the above described functions and save. Then create a file with the functions prototypes, clicking 'File -> New -> Include(*. MQH) -> OK'. In the field name write stringProcess, -> OK. Now insert the prototypes of all new functions, and indicate the directive for import:

//+------------------------------------------------------------------+ 
//|                                                stringProcess.mqh |
//|                               Antonio Banderass Copyright © 2007 | 
//|                                               banderassa@ukr.net |
//+------------------------------------------------------------------+ 
#property copyright "Antonio Banderass Copyright © 2007"
#property link      "banderassa@ukr.net"
//+--- 
#import "stringProcess.ex4"
//+------------------------------------------------------------------+
//| prototypes                                                       | 
//+------------------------------------------------------------------+
string StringUpperCase(string str); 
string StringLowerCase(string str);
bool StringCompare(string s1, string s2);
bool StringIsDigit(string str);
bool StringIsAlpha(string str);

Using the Library stringProcess

To use the library, include the header file with the prototypes of functions, after that you can call necessary functions. Here is the example of using in a script:

//+------------------------------------------------------------------+
//|                                     stringProcessLibraryTest.mq4 |
//|                               Antonio Banderass Copyright © 2007 |
//|                                               banderassa@ukr.net |
//+------------------------------------------------------------------+
#property copyright "Antonio Banderass Copyright © 2007"
#property link      "banderassa@ukr.net"
 
#include <stringProcess.mqh>
//+------------------------------------------------------------------+
//| script program start function                                    |
//+------------------------------------------------------------------+
int start()
  {
   if(StringIsDigit("1234567890")) // TRUE
       MessageBox("TRUE");
   else
       MessageBox("FALSE");
   if(StringIsDigit("1234notdigit")) // FALSE
       MessageBox("TRUE");
   else
       MessageBox("FALSE");
   return(0);
  }

Conclusion

So, you have learned about the table of ASCII symbols and how you can use the peculiarities of its construction for the design of new functions. You have written new functions, which are very popular in other programming languages, but which MQ4L does not include. Based on them you have created a small library for string processing. I think this library will be used not during trading, but at the receipt of results. For example, if you are developing your own system of reports for your Expert Advisor, then some of the functions will be useful for you. Besides you will find many areas of use, which I may not know. Good luck and profits to you!