區塊鏈入門到實戰（37）之Solidity – 循環語句

與其他語言類似，Solidity語言支持循環結構，Solidity提供以下循環語句。

• while
• do ... while
• for
• 循環控制語句：break、continue。

Solidity – while循環

語法

Solidity 中， while循環的語法如下：

while (表達式) {
   被執行語句(如果表示為真)
}

示例

pragma solidity ^0.5.0;

contract SolidityTest {
   uint storedData; 
   constructor() public{
      storedData = 10;   
   }
   function getResult() public view returns(string memory){
      uint a = 10; 
      uint b = 2;
      uint result = a + b;
      return integerToString(result); 
   }
   function integerToString(uint _i) internal pure 
      returns (string memory) {

      if (_i == 0) {
         return "0";
      }
      uint j = _i;
      uint len;

      while (j != 0) {
         len++;
         j /= 10;
      }
      bytes memory bstr = new bytes(len);
      uint k = len - 1;

      while (_i != 0) { // while 循環
         bstr[k--] = byte(uint8(48 + _i % 10));
         _i /= 10;
      }
      return string(bstr);
   }
}

Solidity – do…while循環

語法

Solidity 中， do…while循環的語法如下：

do {
   被執行語句(如果表示為真)
} while (表達式);

注意: 不要漏掉do后面的分號。

示例

pragma solidity ^0.5.0;

contract SolidityTest {
   uint storedData; 
   constructor() public{
      storedData = 10;   
   }
   function getResult() public view returns(string memory){
      uint a = 10; 
      uint b = 2;
      uint result = a + b;
      return integerToString(result); 
   }
   function integerToString(uint _i) internal pure 
      returns (string memory) {

      if (_i == 0) {
         return "0";
      }
      uint j = _i;
      uint len;

      while (j != 0) {
         len++;
         j /= 10;
      }
      bytes memory bstr = new bytes(len);
      uint k = len - 1;

      do {                   // do while 循環 
         bstr[k--] = byte(uint8(48 + _i % 10));
         _i /= 10;
      }
      while (_i != 0);
      return string(bstr);
   }
}

Solidity – for循環

語法

Solidity 中， for循環的語法如下：

for (初始化; 測試條件; 迭代語句) {
   被執行語句(如果表示為真)
}

示例

pragma solidity ^0.5.0;

contract SolidityTest {
   uint storedData; 
   constructor() public{
      storedData = 10;   
   }

   function getResult() public view returns(string memory){
      uint a = 10; 
      uint b = 2;
      uint result = a + b;
      return integerToString(result); 
   }

   function integerToString(uint _i) internal pure 
      returns (string memory) {
      if (_i == 0) {
         return "0";
      }
      uint j=0;
      uint len;
      for (j = _i; j != 0; j /= 10) {  //for循環的例子
         len++;         
      }
      bytes memory bstr = new bytes(len);
      uint k = len - 1;
      while (_i != 0) {
         bstr[k--] = byte(uint8(48 + _i % 10));
         _i /= 10;
      }
      return string(bstr);//訪問局部變量
   }
}

Solidity – break與contin

• continue – 跳出本次循環，繼續執行接下來的循環
• break – 跳出循環(或跳出代碼塊)

break 示例

pragma solidity ^0.5.0;

contract SolidityTest {
   uint storedData; 
   constructor() public{
      storedData = 10;   
   }
   function getResult() public view returns(string memory){
      uint a = 1; 
      uint b = 2;
      uint result = a + b;
      return integerToString(result); 
   }
   function integerToString(uint _i) internal pure 
      returns (string memory) {

      if (_i == 0) {
         return "0";
      }
      uint j = _i;
      uint len;

      while (true) {
         len++;
         j /= 10;
         if(j==0){
            break;   // break 語句跳出循環
         }
      }
      bytes memory bstr = new bytes(len);
      uint k = len - 1;

      while (_i != 0) {
         bstr[k--] = byte(uint8(48 + _i % 10));
         _i /= 10;
      }
      return string(bstr);
   }
}

輸出

0: string: 3

continue 示例

pragma solidity ^0.5.0;

contract SolidityTest {
   uint storedData; 
   constructor() public{
      storedData = 10;   
   }
   function getResult() public view returns(string memory){
      uint n = 1;
      uint sum = 0;

      while( n < 10){
         n++;
         if(n == 5){
            continue; // 當n的和是5時，跳過n。
         }
         sum = sum + n;
      }
      return integerToString(sum); 
   }
   function integerToString(uint _i) internal pure 
      returns (string memory) {

      if (_i == 0) {
         return "0";
      }
      uint j = _i;
      uint len;

      while (true) {
         len++;
         j /= 10;
         if(j==0){
            break;   // break跳出循環
         }
      }
      bytes memory bstr = new bytes(len);
      uint k = len - 1;

      while (_i != 0) {
         bstr[k--] = byte(uint8(48 + _i % 10));
         _i /= 10;
      }
      return string(bstr);
   }
}

輸出

0: string: 49

參考區塊鏈入門到實戰（31）之Solidity – 第一個程序運行