Difference between _variablename and variablename

Question

contract Student{
    struct stu{
        string name;
        uint age;
        bool tookTest;
    }
    mapping(uint => stu) public studentNames;
    function addStudent (uint ID, string _name, uint _age) {
        studentNames[ID] = stu(_name, _age, false);
    }
    function updateStudent (uint ID) {
        studentNames[ID].tookTest = true;
    }
}

Answer 1

There is no difference between both of that. Generally, developers use it to differentiate global variables and function arguments (variablename for global variable and _variablename for function argument).

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