difference between constant and variable in c | constant and variable in c | constant in c

In mathematics, a constant is a specific number or a symbol that is assigned a fixed value. For example, in the equation below, "y" and "x" are variables, while the numbers 2 and 3 are constants. ... By using constants, programmers can modify multiple instances of a value at one time.
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1). Difference between array and structure.
2).difference between constant and variable.
3).difference between call by value and call by address.
4).difference between while loop and do while loop.
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