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What does it mean when an operand (e.g. a, b, c, d) is evaluated?

It means that each expression is evaluated from whatever is written into a single numeric value. For example if a is the string "sin(45)". then the 45 needs to be converted to an integer, and then the sin function needs to be called on that value to get the 'real' value of a.

- Also what does the order of evaluation of a, b, c, +, /, d, * mean?

It is stack based calculation.

- The values of a,b and c are pushed onto the stack.

- When the '+' is found, b and c (the top two values) are popped off the stack and added together.

- The result (bc) is pushed back. So now you have two values a and sum(bc).

- Next the division operator is examined, and a and bc are poppped off the stack, and a is divided by bc. The result (abc) is pushed onto the stack.

- Next d is pushed onto the stack, so now we have two values: abc and d.

- Next the multiplication operator is examined. The two values are popped off the stack and multiplied together. The final result is abcd.

Try it yourself with some real numbers to see how it works.

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I'm not the one who needs it.