for문은 반복하고자 할 때, 시작점과 시점
ex)학생들의 평균을 구하고자 할 때

if문은 상황에 따라 다른 것을 적용하고자 할 때, 조건

1. for문 기초-1

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.8.2 <0.9.0;

contract FOR {
    function loop1() public pure returns(uint) {
        uint a;

        for(uint i=0; i<5; i++) {
            a += i; //a = a+i
        }

        return a;
    }

    function loop2() public pure returns(uint, uint) {
        uint a;
        uint i;

        for(i=1; i<=6; i++) {
            a += i; //a = a+i
        }

        return (a,i);
    }

    function loop3() public pure returns(uint) {
        uint a;

        for(uint i=1; i<=100; i++) {
            a += i;
        }

        return a;
    }

    function loop4() public pure returns(uint) {
        uint a;

        for(uint i=1; i<=100; i+=2) {
            a += i;
        }

        return a;
    }

    function loop5() public pure returns(uint) {
        uint a;

        for(uint i=1; i<=100; i*=2) {
            a += i;
        }

        return a;
    }
}

2. for문 기초-2

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.8.2 <0.9.0;

contract FOR {
    uint[] numbers;

    function push(uint a) public {
        numbers.push(a);
    }

    function getArray() public view returns(uint[] memory) {
        return numbers;
    }

    function addTwoAll() public {
        for(uint i=0; i<numbers.length; i++) {
            numbers[i] +=2;
        }
    }

    function addTwoAll(uint[] memory _numbers) public pure returns(uint[] memory) {
        for(uint i=0; i< _numbers.length; i++) {
            _numbers[i] +=2;
        }

        return _numbers;
    }
}

3. if문 기초

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.8.2 <0.9.0;

contract IF {
    function grade(uint _score) public pure returns(string memory) {
        if(_score>=90) {
            return "A";
        } else if(_score>=80) {
            return "B";
        } else if(_score >=70) {
            return "C";
        } else {
            return "F";
        }
    }

    function example(uint _number) public pure returns(string memory) {
        if(_number%2==0) {
            return "A";
        } else if(_number%3==0) {
            return "B";
        } else if(_number%5==0) {
            return "C";
        } else {
            return "ETC";
        }
    }

    function example2(uint _number) public pure returns(string memory) {
        if(_number%2==0 && _number >40 || _number <20) { // && || 위치 바꾸는 것 부터 시작하기
            return "A";
        } else if(_number <=40 && _number >=20 && _number%2==0) {
            return "B";
        } else if(_number <=40 && _number >=20 && _number%2!=0) {
            return "C";
        } else {
            return "D";
        }
    }
}

4. if문 복수조건(and, or)

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.8.2 <0.9.0;

contract IF2 {
    function Numbers(uint _number) public pure returns(string memory) {
        if(_number >=40 && _number <=60 || _number >=100 && _number <=150 || _number %5==0) {
            return "A";
        } else if(_number%3==0){
            return "B";
        } else {
            return "C";
        }
    }
}