China DOS Union

General type:

@echo off

set var1=abc

set var2=123

echo Before exchange: var1=%var1% var2=%var2%

set var1=%var2%& set var2=%var1%

echo After exchange: var1=%var1% var2=%var2%

pause

If the values of the two variables to be exchanged are numbers, the following two schemes can also be used:

Scheme one:

@echo off

set /a num1=123,num2=456

echo Before exchange: num1=%num1% num2=%num2%

set /a num1=%num2%,num2=%num1%

:: set num1=%num2%& set num2=%num1% is also acceptable

echo After exchange: num1=%num1% num2=%num2%

pause

Scheme two:

@echo off

:: There will be a limit on the value range, that is, the sum of num_a and num_b must be within the range of 2^-31 to 2^31-1

set /a num_a=123,num_b=456

echo Before exchange: num_a=%num_a% num_b=%num_b%

set /a num_a=%num_a%+%num_b%-%num_a%,num_b=%num_a%+%num_b%-%num_b%

echo After exchange: num_a=%num_a% num_b=%num_b%

pause

Record it for future use!~

Hehe, tried it, really, ... still thinking about it...

Not bad, making clever use of the feature of environment variable expansion. Actually, these three examples should all be of the same meaning, right?

Originally posted by 0401 at 2007-1-24 20:02:
Not bad, cleverly using the characteristics of environment variable expansion. In fact, these three examples should all be of the same meaning, right?

　　If you have carefully looked at the code, you will feel that they are not the same idea: the universal code uses & to exchange the values of two variables. Solution 1 uses some characteristics of set /a. On the basis of using the characteristics of set /a, Solution 2 also uses the formula A+B-A=B, A+B-B=A to exchange the values of variables.

　　In fact, these几段demonstration codes also lead to a relatively interesting topic: set var1=%var2% has assigned the value of var2 to var1. According to reason, at this time the value of var1 has changed, but when using & to lead to another assignment statement set var2=%var1%, why can var2 still get the value before var1 changed? Does the variable var1 can store more than one value at the same time? set /a num1=num2%,num2=%num1% also has a similar phenomenon.

The algorithm to swap the values of two variables is very simple.



Suppose there are two cups: A and B. To swap the water in cups A and B, a third cup C is needed.

1. Pour the water from cup A into cup C;

2. Pour the water from cup B into cup A;

3. Pour the water from cup C into cup A;



In this way, the water in the two cups is swapped.

Among the three methods given by brother namejm, the first one uses the pipe character to swap variables, and the latter two are similar in meaning, using the comma operator to swap variables.

In fact, this method is realized by the particularity of the script. If you are familiar with macros in the C language, it is not difficult to understand. The script is executed by the interpreter. When the batch script interpreter is interpreting, it must first perform macro expansion, and in general, it is interpreted line by line.

set x=2

set y=3

set x=%y%&set y=%x%



Here, the last line is macro-expanded to set x=3&set y=2, and the values of x and y are automatically expanded into the assignment statement, so the values of the variables are swapped without an intermediate variable. Here, the interpreter acts as the intermediate variable.

In fact, there are also methods to swap the values of two variables without using an intermediate variable in binary programs.

That is to use bit operations to achieve it, hehe~ Everyone think about it?

Teaching Cases of C Language Textbooks

set /a "a=b^a"

set /a "b=b^a"

set /a "a=b^a"

In addition, four arithmetic operations can also be implemented

Just not as flexible and unrestricted as XOR operation

set /a a=a+b

set /a b=a-b

set /a a=a-b

Yes, in terms of both efficiency and scope of use, the four arithmetic operations are not as good as bitwise operations.

@echo off

set a=2

set b=3

echo a=%a%  b=%b%

set /a "a=b^a"

set /a "b=b^a"

set /a "a=b^a"

echo a=%a%  b=%b%

set /a a=a+b

set /a b=a-b

set /a a=a-b

echo a=%a%  b=%b%

pause

In a single statement, the assignment of a variable cannot be expanded (referenced) immediately; it can only be expanded in the next statement. A single statement refers to if, for statements, and statements connected using parentheses && & ||, etc. Unless environment variable expansion delay is enabled and "!" is used instead of "%" to expand the variable value. So that's why I said that Brother namejm's three examples are similar in meaning...

Originally posted by pengfei at 2007-1-24 21:25:Scripts are executed by an interpreter. When a batch script is interpreted, macro expansion must be performed first, and in general, it is interpreted line by line.

　　Hehe, reminds me of this is the preprocessing mechanism of batch processing ^_^, the accurate statement seems to be executed line by line.

Yes, generally speaking, it is explained line by line. It should be changed to executed sentence by sentence. For compound statements in parentheses, the interpreter also executes them at one time.

Brother lxmxn, using call %%var1%% doesn't actually expand, just like !var!, which is another way of variable delay. Therefore, variable swapping can't be achieved either.