How can I extract pairs of values from a string in C++ - c++

Question

I have a string with this format:
"name1":1234 " name2 " : 23456 "name3" : 12345
and so on...
I have tried using nested while loops and two integers to store the position and length to use in string::substr, but I can't find a proper way to get it (most of the time I end up out of the string).
The values don't need to be stored, because I can call a function to process them as soon as I get them.
This is what I've done so far:
void SomeClass::processProducts(std::string str) {
unsigned int i = 0;
std::string name;
while (i < str.length()) {
if (str[i] == '\"') {
int j = 1;
while (str[i + j] != '\"') {
j++;
}
name = str.substr(i + 1, j - 1);
i += j;
}
else if (str[i] >= '0' && str[i] <= '9') {
int j = 1;
while (str[i + j] >= '0' && str[i + j] <= '9') {
j++;
}
//This is just processes the values
std::stringstream ss;
std::string num = str.substr(i, j);
ss.str(num);
int products = 0;
ss >> products;
if (products == 0) {
Util::error(ERR_WRONG_PRODUCTS);
}
int pos = getFieldPos(name);
if (pos == -1) {
Util::error(ERR_WRONG_NAME);
}
else {
fields[pos].addProducts(products);
}
i += j;
}
i++;
}
}
Thanks in advance.

Answer 1

Unfortunately, C++ doesn't have strong string parsing abilities out of the box. That's why there are many ways of doing these kinds of tasks.
However, there C++ does provide tools to help. So we can use them and at least avoid manual loops.
Before we begin I want to draw attention to the fact that when we are dealing with user input we must take extra care to validate the input.
The blocks that we need for the solution I chose are:
matching the format (with "name" : value). For this I chose std::find. Regex can also be used.
parsing the value into a number. For this we could use std::stoi. See bellow why it's not enough.
always make sure we are getting the input we expect. This adds some boilerplate code, but that's the price we have to pay. Also here we have a problem with std::stoi as it happily accepts trailing non whitespace without a fuss. So for instance 123 invalid would be parsed to 123. This is the reason I use a small wrapper around it parse_string_to_int
Ok, on we go:
Little helper:
auto parse_string_to_int(const std::string& str)
{
std::size_t num_processed = 0;
int val = std::stoi(str, &num_processed, 10);
auto next_non_space = std::find_if(str.begin() + num_processed, str.end(),
[](char ch) { return !std::isspace(ch); });
if (next_non_space != str.end())
throw std::invalid_argument{"extra trailing characters in parse_string_to_int"};
return val;
}
struct Product_token
{
std::string name;
int value;
};
auto get_next_product(std::string::const_iterator& begin, std::string::const_iterator end)
-> Product_token
{
// match `"name" : value "`
auto name_open_quote = std::find(begin, end, '\"');
auto name_close_quote = std::find(name_open_quote + 1, end, '\"');
auto colon = std::find(name_close_quote, end, ':');
auto next_token_open_quote = std::find(colon, end, '\"');
if (name_close_quote == end || name_close_quote == end || colon == end)
{
// feel free to add more information regarding the error.
// this is just the bare minimum to accept/reject the input
throw std::invalid_argument{"syntax error on parsing product"};
}
// advance to next token
begin = next_token_open_quote;
return Product_token{{name_open_quote + 1, name_close_quote},
parse_string_to_int({colon + 1, next_token_open_quote})};
}
auto process_products(const std::string& str)
{
auto begin = str.begin();
while (begin != str.end())
{
auto product = get_next_product(begin, str.end());
cout << '"' << product.name << "\" = " << product.value << endl;
}
}
int main()
{
auto str = R"("name1":1234 " name2 " : 23456 "name3" : 12345)"s;
try
{
process_products(str);
}
catch (std::exception& e)
{
cerr << e.what() << endl;
}
}
See full code in action on ideone

Answer 2

As long as you know the format, then extracting data is rather easy. First remove any quote or colon from the string and replace them with space. Now the string is delimited by space.
#include <iostream>
#include <iterator>
#include <string>
#include <algorithm>
#include <vector>
#include <sstream>
using namespace std;
int main()
{
string str("\"name1\":1234 \" name2 \" : 23456 \"name3\" : 12345");
cout << str << endl;
// remove ':' and '"' and replace them by space
std::replace_if(str.begin(), str.end(), ispunct, ' ');
istringstream ss(str);
vector<string> words;
// store data as name and number in vector<string>
copy(istream_iterator<string>(ss),istream_iterator<string>(),back_inserter(words));
for (int i(0); i < words.size(); i+=2)
cout << "name: " << words[i] << " number: " << words[i+1] << endl;
return 0;
}
The result is
"name1":1234 " name2 " : 23456 "name3" : 12345
name: name1 number: 1234
name: name2 number: 23456
name: name3 number: 12345