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Description
html markup
shown to student

Postfix notation transforms mathematical expressions by putting
each operator after its arguments. So <tt>1 + 2</tt> becomes
<tt>1 2 +</tt> and <tt>5 * 2</tt> becomes <tt>5 2 *</tt>.
An advantage
over normal (infix) expressions is that parentheses are not needed.
For example <tt>4 3 * 2 1 * +</tt> is the prefix version of <tt>4*3+2*1</tt>
while <tt>4 3 2 + * 1 *</tt> is the prefix version of <tt>4*(3+2)*1</tt>.
Write an evaluator for prefix notation (just the <tt>*</tt> and <tt>+</tt>
operators) by using a vector as a stack.
Each time you read a number, place it at the end of the stack. Each time
you read an operator, combine the last two elements of the stack.

Epilogue
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Remarks
Comments, history, license, etc.

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Engine
Choices
json list of string, bool pairs
e.g. [["good", true], ["bad", false]]

Correct answer
Template / Reference solution

Template / Reference solution

#include <vector>
#include <iostream>
#include <string>
#include <cstdlib>
using namespace std;

int main() {
vector<int> memory;
string input;
// read each input
while (cin >> input) {
// if you read an operator, combine the last two elements of the stack
if (input == "+" || input == "*") {
\[
int v1 = memory.back();
memory.pop_back();
int v2 = memory.back();
memory.pop_back();
]\
if (input == "+"){
\[memory.push_back(v1+v2);]\
}
else {
\[memory.push_back(v1*v2);]\
}
}
// if you read a number, place it at the end of the stack
else {
memory.push_back(stoi(input));
}
}
cout << memory.back();
}

Java test suite
See manual

C++ test suite
json list of stdin/args tests
e.g. [{"stdin":"hi", "args":["4", "5"]}, {"stdin":"noargs"}]
to just run once with no input use [{}]

[
{"stdin": "1 2 +"},
{"stdin": "5 2 *"},
{"stdin": "4 3 * 2 1 * +"},
{"stdin": "4 3 2 + * 1 *"},
{"stdin": "5 4 3 2 1 * * * *"},
{"stdin": "5 4 3 2 1 + + + +"},
{"stdin": "1 2 + 3 4 * 5 6 + 7 8 + 9 * + + *"}
]

C++ test suite
See manual

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json list of strings
e.g. ["for","while"]

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Java imports
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e.g. ["java.util.*"]

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websheet name
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Dependent on other websheets?
json list of websheet names in this folder
e.g. ["DataStructure"]

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Is example?
i.e., just a demo
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Add compiler flag(s)
json list of flags
e.g. ["-Wno-unused-variable"]

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Remove default compiler flag(s)
json list of flags
see default_cppflags
e.g. ["-Wno-write-strings"]

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Property	Value
Description html markup shown to student	<i>Postfix notation</i> transforms mathematical expressions by putting each operator <i>after</i> its arguments. So <tt>1 + 2</tt> becomes <tt>1 2 +</tt> and <tt>5 * 2</tt> becomes <tt>5 2 </tt>. <p>An advantage over normal (infix) expressions is that parentheses are not needed. For example <tt>4 3 2 1 * +</tt> is the prefix version of <tt>43+21</tt> while <tt>4 3 2 + * 1 </tt> is the prefix version of <tt>4(3+2)1</tt>. <p>Write an evaluator for prefix notation (just the <tt></tt> and <tt>+</tt> operators) by using a vector as a stack. Each time you read a number, place it at the end of the stack. Each time you read an operator, combine the last two elements of the stack.
Epilogue html markup shown when solved	remove
Public permissions	remove
Remarks Comments, history, license, etc.	remove
Engine
Choices json list of string, bool pairs e.g. `[["good", true], ["bad", false]]`
Correct answer
Template / Reference solution
Template / Reference solution	#include <vector> #include <iostream> #include <string> #include <cstdlib> using namespace std; int main() { vector<int> memory; string input; // read each input while (cin >> input) { // if you read an operator, combine the last two elements of the stack if (input == "+" \|\| input == "") { \[ int v1 = memory.back(); memory.pop_back(); int v2 = memory.back(); memory.pop_back(); ]\ if (input == "+"){ \[memory.push_back(v1+v2);]\ } else { \[memory.push_back(v1v2);]\ } } // if you read a number, place it at the end of the stack else { memory.push_back(stoi(input)); } } cout << memory.back(); }
Java test suite See manual
C++ test suite json list of stdin/args tests e.g. `[{"stdin":"hi", "args":["4", "5"]}, {"stdin":"noargs"}]` to just run once with no input use `[{}]`	[ {"stdin": "1 2 +"}, {"stdin": "5 2 "}, {"stdin": "4 3 2 1 * +"}, {"stdin": "4 3 2 + * 1 "}, {"stdin": "5 4 3 2 1 * * "}, {"stdin": "5 4 3 2 1 + + + +"}, {"stdin": "1 2 + 3 4 5 6 + 7 8 + 9 * + + *"} ]
C++ test suite See manual
Forbidden substrings json list of strings e.g. `["for","while"]`	remove
Solution visibility	remove
Java imports json list of importables e.g. `["java.util.*"]`	remove
Override Java classname else defaults to websheet name	remove
Dependent on other websheets? json list of websheet names in this folder e.g. `["DataStructure"]`	remove
Is example? i.e., just a demo	remove
Add compiler flag(s) json list of flags e.g. `["-Wno-unused-variable"]`	remove
Remove default compiler flag(s) json list of flags see default_cppflags e.g. `["-Wno-write-strings"]`	remove

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