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Solidity Syntax - Version, Import, Types, Control Structures

Solidity is a contract-oriented programming language used for writing smart contracts on the Ethereum blockchain. In this tutorial, we will discuss the syntax of Solidity.

Version Pragma​

Version pragma specifies the version of the compiler that should be used to compile the contract. It is recommended to use the latest version available.

Example:

pragma solidity ^0.5.2;

The above code will compile with a compiler version >= 0.5.2 and < 0.6.0.

Import Files​

Import statements allow us to use code from other Solidity files. There are three types of import statements:

  1. Import all symbols from a file:
import "filename"
  1. Import all symbols from a file and assign a symbol to the file:
import * as symbolName from "filename";

or

import "filename" as symbolName;
  1. Import only selected symbols from a file:
import {symbol1 as alias, symbol2} from "filename";

Types​

Solidity supports several data types, which include:

Boolean​

The bool type in Solidity can have the values true or false.

Operators:

  • Logical: ! (logical negation), && (AND), || (OR)
  • Comparisons: == (equality), != (inequality)

Example:

bool x = true;
bool y = false;

Integer​

Solidity supports both unsigned and signed integers.

Unsigned: uint8, uint16, uint32, uint64, uint128, uint256.

Signed: int8, int16, int32, int64, int128, int256.

Operators:

  • Comparisons: <=, <, ==, !=, >= and >
  • Bit operators: &, |, ^ (bitwise exclusive or) and ~ (bitwise negation)
  • Arithmetic operators: +, **, unary **, unary +, ``, /, %, * (exponentiation), << (left shift) and >> (right shift)

Example:

uint256 a = 10;
int256 b = -20;

Address​

The address type in Solidity holds an Ethereum address (20-byte value).

The address payable type is similar to address but includes additional methods transfer and send.

Operators:

  • Comparisons: <=, <, ==, !=, >= and >

Methods:

  • balance: address.balance returns the balance of the address in Wei.
  • transfer: address.transfer(amount) sends the given amount of Wei to the address and throws on failure.
  • send: address.send(amount) returns (bool) sends the given amount of Wei to the address and returns false on failure.
  • call: address.call(...) issues a low-level CALL and returns false on failure.
  • delegatecall: address.delegatecall(...) issues a low-level DELEGATECALL and returns false on failure.

Example:

address payable x = address(0x1234567890123456789012345678901234567890);

Array​

Solidity supports both dynamic and fixed-sized arrays.

Example:

uint[] dynamicSizeArray;
uint[7] fixedSizeArray;

Fixed Byte Arrays​

Fixed byte arrays are declared using the syntax bytesI, where I is an integer between 1 and 32, inclusive. They are used to store binary data of a fixed length. The byte array can be accessed using its index, which starts from 0.

bytes1 myByte;
bytes32 myBytes;

myByte[0] = 0x12;
myBytes[31] = 0xff;

Dynamic Byte Arrays​

Dynamic byte arrays are declared using the bytes keyword. They are used to store binary data of a variable length. The byte array can be accessed using its index, which starts from 0. It is similar to the byte[] type.

bytes myBytes;
myBytes.push(0x12);
myBytes.push(0xff);

Enum​

An enum is a custom data type that allows a developer to define a set of named values. It is declared using the enum keyword followed by the name of the enum and the values.

enum ActionChoices {
  GoLeft,
  GoRight,
  GoStraight,
  SitStill
}

ActionChoices choice = ActionChoices.GoStraight;

Struct​

A struct is a user-defined data type that groups together variables of different data types. It is declared using the struct keyword, followed by the name of the struct and the variables.

struct Funder {
    address addr;
    uint amount;
}

Funder funders;
funders.addr = 0x123;
funders.amount = 1000;

Mapping​

A mapping is a data structure that maps a key to a value. It is declared using the mapping keyword, followed by the data type of the key and the data type of the value.

mapping(address => uint) balances;

balances[0x123] = 1000;

Control Structures​

Solidity supports most of the control structures from JavaScript, such as if, else, while, do, for, break, continue, return, and the ternary operator ?:. However, the switch and goto statements are not supported.

if (x > 10) {
  // do something
} else {
  // do something else
}

while (i < 10) {
  // do something
  i++;
}

for (uint i = 0; i < 10; i++) {
  // do something
}

return 42;