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louiscklaw
2025-01-31 21:27:38 +08:00
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commit ca0eb416dd
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123
marissa.sam/task2/temp/_draft/helloworld copy 2.cpp Normal file
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#include <iostream>
#include <stack>
#include <cctype>
#include <cstring>
// 1. how to read and store the input?
std::string readInput()
{
std::string input;
char c;
while (std::cin.get(c)) {
if (c == '\n')
break;
input += c;
}
return input;
}
// 2. pseudo code of the algorithm
// convert to postfix expression use stack,
// 1. if it is an operand, push it into stack,
// 2. if it is an operator, pop the top two elements and push the operator result,
// 3. if it is a bracket, pop until the matched bracket, then push it into stack,
// 4. if the input is end, pop all the remaining elements,
// 5. if the last element is a bracket, return error,
// else return the result
std::string infixToPostfix(const std::string& input)
{
std::stack<char> stack;
std::string postfix;
const char* precedence = "+-*/()";
for (const auto& c : input) {
if (std::isalpha(c) || std::isdigit(c)) {
postfix += c;
} else if (c == '(') {
stack.push(c);
} else if (c == ')') {
while (!stack.empty() && stack.top() != '(') {
postfix += stack.top();
stack.pop();
}
if (!stack.empty()) {
stack.pop();
}
} else {
while (!stack.empty() && precedence[c] <= precedence[stack.top()]) {
postfix += stack.top();
stack.pop();
}
stack.push(c);
}
}
while (!stack.empty()) {
postfix += stack.top();
stack.pop();
}
return postfix;
}
// 3. data structures used in the algorithm
// stack
// string
// 4. time complexity
// O(n), where n is the length of input
// 5. space complexity
// O(n), where n is the length of input
// 6. how to store the postfix expression?
// using string
int main()
{
std::string input = readInput();
std::string postfix = infixToPostfix(input);
std::cout << postfix << std::endl;
// 1. how to read the postfix expression
// read postfix expression from standard input stream and store them into string
// 2. pseudo code of the algorithm
// evaluate a postfix expression use stack,
// if it is an operand, push it into stack,
// if it is an operator, pop the top two elements, apply the operator, and push the result,
// if it is a bracket, pop until the matched bracket, then push it into stack,
// if the input is end, return the last element in stack,
// else error
std::stack<int> stack;
const char* operators = "+-*/";
for (const auto& c : postfix) {
if (std::isalpha(c) || std::isdigit(c)) {
stack.push(c - '0');
} else if (std::strchr(operators, c)) {
int operand2 = stack.top();
stack.pop();
int operand1 = stack.top();
stack.pop();
switch (c) {
case '+':
stack.push(operand1 + operand2);
break;
case '-':
stack.push(operand1 - operand2);
break;
case '*':
stack.push(operand1 * operand2);
break;
case '/':
stack.push(operand1 / operand2);
break;
default:
return -1;
}
} else {
return -1;
}
}
std::cout << stack.top() << std::endl;
return 0;
}

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marissa.sam/task2/temp/_draft/helloworld copy 3.cpp Normal file
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#include <iostream>
#include <stack>
#include <string>
#include <cctype>
#include <map>
// Function to convert infix expression to postfix expression
std::string infixToPostfix(const std::string &infix)
{
std::stack<char> s;
std::string postfix;
std::map<char, int> precedence = {{'+', 1}, {'-', 1}, {'*', 2}, {'/', 2}, {'(', 0}};
for (char c : infix)
{
if (std::isalnum(c))
{ // 1. if operand, add to postfix
postfix += c;
}
else if (c == '(')
{ // 2. if '(', push to stack
s.push(c);
}
else if (c == ')')
{ // 3. if ')', pop until '('
while (!s.empty() && s.top() != '(')
{
postfix += s.top();
s.pop();
}
if (!s.empty())
s.pop(); // pop '('
}
else
{ // 4. if operator, pop operators with greater or equal precedence
while (!s.empty() && precedence[c] <= precedence[s.top()])
{
postfix += s.top();
s.pop();
}
s.push(c); // push current operator
}
}
// 5. pop remaining operators
while (!s.empty())
{
postfix += s.top();
s.pop();
}
return postfix;
}
// Function to evaluate postfix expression
int evaluatePostfix(const std::string &postfix)
{
std::stack<int> s;
for (char c : postfix)
{
if (std::isdigit(c))
{ // 1. if operand, push to stack
s.push(c - '0');
}
else
{ // 2. if operator, pop two operands, evaluate and push result
int op2 = s.top();
s.pop();
int op1 = s.top();
s.pop();
switch (c)
{
case '+':
s.push(op1 + op2);
break;
case '-':
s.push(op1 - op2);
break;
case '*':
s.push(op1 * op2);
break;
case '/':
s.push(op1 / op2);
break;
}
}
}
return s.top(); // 3. result is the only element in stack
}
int main()
{
std::string infix;
// std::cout << "Enter an arithmetic expression: ";
// std::getline(std::cin, infix);
std::cout << "e.g. 1 1+4*5" << std::endl;
infix = "1+4*5";
std::string postfix = infixToPostfix(infix);
std::cout << "Postfix expression: " << postfix << std::endl;
int result = evaluatePostfix(postfix);
std::cout << "Result: " << result << std::endl;
std::cout << "e.g. 2 (1+4)*5" << std::endl;
infix = "(1+4)*5";
postfix = infixToPostfix(infix);
std::cout << "Postfix expression: " << postfix << std::endl;
result = evaluatePostfix(postfix);
std::cout << "Result: " << result << std::endl;
std::cout << "e.g. 3 1+4-5" << std::endl;
infix = "1+4-5";
postfix = infixToPostfix(infix);
std::cout << "Postfix expression: " << postfix << std::endl;
result = evaluatePostfix(postfix);
std::cout << "Result: " << result << std::endl;
return 0;
}

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marissa.sam/task2/temp/_draft/helloworld copy.cpp Normal file
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#include <iostream>
#include <stack>
#include <string>
#include <cctype>
#include <map>
// Function to convert infix expression to postfix expression
std::string infixToPostfix(const std::string &infix)
{
std::stack<char> s;
std::string postfix;
std::map<char, int> precedence = {{'+', 1}, {'-', 1}, {'*', 2}, {'/', 2}, {'(', 0}};
for (char c : infix)
{
if (std::isalnum(c))
{ // 1. if operand, add to postfix
postfix += c;
}
else if (c == '(')
{ // 2. if '(', push to stack
s.push(c);
}
else if (c == ')')
{ // 3. if ')', pop until '('
while (!s.empty() && s.top() != '(')
{
postfix += s.top();
s.pop();
}
if (!s.empty())
s.pop(); // pop '('
}
else
{ // 4. if operator, pop operators with greater or equal precedence
while (!s.empty() && precedence[c] <= precedence[s.top()])
{
postfix += s.top();
s.pop();
}
s.push(c); // push current operator
}
}
// 5. pop remaining operators
while (!s.empty())
{
postfix += s.top();
s.pop();
}
return postfix;
}
// Function to evaluate postfix expression
int evaluatePostfix(const std::string &postfix)
{
std::stack<int> s;
for (char c : postfix)
{
if (std::isdigit(c))
{ // 1. if operand, push to stack
s.push(c - '0');
}
else
{ // 2. if operator, pop two operands, evaluate and push result
int op2 = s.top();
s.pop();
int op1 = s.top();
s.pop();
switch (c)
{
case '+':
s.push(op1 + op2);
break;
case '-':
s.push(op1 - op2);
break;
case '*':
s.push(op1 * op2);
break;
case '/':
s.push(op1 / op2);
break;
}
}
}
return s.top(); // 3. result is the only element in stack
}
int main()
{
std::string infix;
std::cout << "Enter an arithmetic expression: ";
std::getline(std::cin, infix);
std::string postfix = infixToPostfix(infix);
std::cout << "Postfix expression: " << postfix << std::endl;
int result = evaluatePostfix(postfix);
std::cout << "Result: " << result << std::endl;
return 0;
}