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You can do this with xacro (a XML macro language): http://wiki.ros.org/xacro

From section 3. Math Expression

Since ROS Jade, Xacro employs python to evaluate expressions enclosed in dollared-braces (${}). This allows for more complex arithmetic expressions. Also, some basic constants, e.g. pi, are already predefined:

<xacro:property name="R" value="2" />
<xacro:property name="alpha" value="${30/180*pi}" />
<circle circumference="${2 * pi * R}" pos="${sin(alpha)} ${cos(alpha)}" />

You can do this with xacro (a XML macro language): http://wiki.ros.org/xacro

From section 3. Math Expression

Since ROS Jade, Xacro employs python to evaluate expressions enclosed in dollared-braces (${}). This allows for more complex arithmetic expressions. Also, some basic constants, e.g. pi, are already predefined:

<xacro:property name="R" value="2" />
<xacro:property name="alpha" value="${30/180*pi}" />
<circle circumference="${2 * pi * R}" pos="${sin(alpha)} ${cos(alpha)}" />