set var=10 set /a var=%var%+10 echo %var%
The final value of
var is 20.
The second line is not working within a command block used for example on an IF condition or on a FOR loop as delayed expansion would be needed instead of standard environment variable expansion.
Here is another, better way working also in a command block:
set var=10 set /A var+=10 echo %var%
The command prompt environment supports with signed 32-bit integer values:
The Windows command interpreter does not support 64-bit integer values or floating point values in arithmetic expressions.
Note: The operator
% must be written in a batch file as
%% to be interpreted as operator.
In a command prompt window executing the command line
set /A Value=8 % 3 assigns the value
2 to environment variable
Value and additionally outputs
In a batch file must be written
set /A Value=8 %% 3 to assign the value
2 to environment variable
Value and nothing is output respectively written to handle STDOUT (standard output). A line
set /A Value=8 % 3 in a batch file would result in error message Missing operator on execution of the batch file.
The environment requires the switch
/A for arithmetic operations only, not for ordinary string variables.
Every string in the arithmetic expression after
set /A being whether a number nor an operator is automatically interpreted as name of an environment variable.
For that reason referencing the value of a variable with
%variable% or with
!variable! is not necessary when the variable name consists only of word characters (0-9A-Za-z_) with first character not being a digit which is especially helpful within a command block starting with
( and ending with a matching
Numbers are converted from string to integer with C/C++ function strtol with
base being zero which means automatic base determination which can easily result in unexpected results.
set Divided=11 set Divisor=3 set /A Quotient=Divided / Divisor set /A Remainder=Divided %% Divisor echo %Divided% / %Divisor% = %Quotient% echo %Divided% %% %Divisor% = %Remainder% set HexValue1=0x14 set HexValue2=0x0A set /A Result=(HexValue1 + HexValue2) * -3 echo (%HexValue1% + %HexValue2%) * -3 = (20 + 10) * -3 = %Result% set /A Result%%=7 echo -90 %%= 7 = %Result% set OctalValue=020 set DecimalValue=12 set /A Result=OctalValue - DecimalValue echo %OctalValue% - %DecimalValue% = 16 - 12 = %Result%
The output of this example is:
11 / 3 = 3 11 % 3 = 2 (0x14 + 0x0A) * -3 = (20 + 10) * -3 = -90 -90 %= 7 = -6 020 - 12 = 16 - 12 = 4
Variables not defined on evaluation of the arithmetic expression are substituted with value 0.