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skills/mql5/references/book/01-basis/0088-basis-preprocessor-preprocessor-sharp.md

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+# Special operators '#' and '##' inside #define definitions
+
+Inside macro definitions, two special operators can be used:
+
+- a single hash symbol '#' before the name of a macro parameter turns the contents of that parameter into a string; it is allowed only in function macros;
+- a double hash symbol '##' between two words (tokens) combines them, and if the token is a macro parameter, then its value is substituted, but if the token is a macro name, it is substituted as is, without expanding the macro; if as a result of "gluing" another macro name is obtained, it is expanded;
+
+In the examples in this book, we often used the following macro:
+
+```
+#define PRT(A) Print(#A, "=", (A))
+
+```
+
+It calls the Print function, in which the passed expression is displayed as a string thanks to #A, and after the sign "equal", the actual value of A is printed.
+
+To demonstrate '##', let's consider another macro:
+
+```
+#define COMBINE(A,B,X) A##B(X)
+
+```
+
+With it, we can actually generate a call to the SQN macro defined above:
+
+```
+Print(COMBINE(SQ,N,2)); // 4
+
+```
+
+The literals SQ and N are concatenated, after which the macro SQN expands to ((2)*(2)) and produces the result 4.
+
+The following macro allows you to create a variable definition in code by generating its name given the parameters of the macro:
+
+```
+#define VAR(TYPE,N) TYPE var##N = N
+
+```
+
+Then the line of code:
+
+```
+VAR(int, 3);
+
+```
+
+is equivalent to the following:
+
+```
+int var3 = 3;
+
+```
+
+Concatenation of tokens allows the implementation of a loop shorthand over the array elements using a macro.
+
+```
+#define for_each(I, A) for(int I = 0, max_##I = ArraySize(A); I < max_##I; ++I)
+  
+// describe and somehow fill in the array x
+double x[];
+// ...
+// implement loop through the array
+for_each(i, x)
+{
+   x[i] = i * i;
+}
+
+```