replace_var.cc

// Compile: g++ -std=c++11 -Wall replace_var.cc -o replace_var
// Tested with g++-4.7.2
 
#include <string>
#include <map>
#include <cctype>
#include <iostream>

typedef std::map<std::string, std::string> TVarMap;

using namespace std;

constexpr char kVarBegin[] = "$(";
constexpr char kVarEnd = ')';
constexpr size_t kVarBeginLen = sizeof(kVarBegin) - 1;

/******************************************
 * A helper function to extract the variable 
 * name. Invoked by replaceVars()
 * ****************************************/
std::string extractVar(const std::string &buf, size_t bpos, size_t epos)
{
  bpos += kVarBeginLen;
  --epos;
  while (std::isspace(buf[bpos])) ++bpos;
  while (std::isspace(buf[epos])) --epos;
  if (bpos > epos) return std::string();
  return buf.substr(bpos, epos - bpos + 1);
}

/*************************************************
 * Replace the variables with their values defined
 * in varmap. A variable is identified by $(var).
 * Example:
 *   let varmap = {"x"="1", "y"="2", z="3"}
 *       buf = "$(x) + $(y) = $(z)"
 *   The content in buf at the end of this functions is:
 *       buf = "1 + 2 = 3"
 * Nested variable is allowed. See the example in main().
 *
 * Return number of replacements happened on success, 
 * -1 on failure.
 * ************************************************/
int replaceVars(std::string &buf, const TVarMap &varmap)
{
  int pos = 0;
  int cnt = 0;
  for(auto epos = buf.find(kVarEnd, 0); 
      epos != std::string::npos && pos < (int)buf.size(); 
      epos = buf.find(kVarEnd, pos)) {
    pos = epos + 1;
    if (epos > kVarBeginLen) {
      auto bpos = buf.rfind(kVarBegin, epos - 1);
      if (bpos == std::string::npos) continue;
      auto var = extractVar(buf, bpos, epos);
      auto it = varmap.find(var);
      if (it == varmap.end()) {
        cerr << "variable '" << var << "' not defined" << endl;
        return -1;
      }
      int len = epos - bpos + 1;
      buf.replace(bpos, len, it->second);
      pos += (int)it->second.size() - len;
      ++cnt;
    }
  }
  return cnt;
}


int main(int argc, const char *argv[])
{
  string buf("$( x ) + $( y) = $($(z))");
  TVarMap varmap{{"x", "1"}, {"y", "2"}, {"xy", "3"}, {"z", "xy"}};
  int cnt = replaceVars(buf, varmap);
  cout << cnt << endl;
  cout << buf << endl;
  return 0;
}