convert a string to int
c++, performance, string
Solution
I'd use `strtol`. It takes a parameter that it sets to point at the first character it couldn't convert, so you can use that to determine whether the entire string was converted.
Edit: as far as speed goes, I'd expect it to be slightly slower than `atoi`, but faster than the others you tried.
Problem
I have a large file where each line contains space-separated integers. The task is to sparse this file line-by-line. For the string to int conversion I have three solutions: ``` static int stringToIntV1(const string& str) { return (atoi(str.c_str())); } ``` However, if I pass a malformed string, it doesn't produce any error. For instance the string "123error" is converted to 123. Second solution: ``` static int stringToIntV2(const string& str) { int result; istringstream myStream(str); if (myStream >> result) { return result; } // else throw domain_error(str + " is not an int!"); } ``` I have the same problem here, malformed strings don't raise an error. Third solution with Boost (found at Boost Library): ``` static int stringToIntV3(const string& str) { int iResult = 0; try { iResult = lexical_cast<int>(str); } catch(bad_lexical_cast &) { throw domain_error(str + " is not an int!"); } return iResult; } ``` This one gives correct result. However, there is a significant difference in the execution time. Testing on a large text file (32 MB), I got the following times: - (1) with atoi: 4.522s (winner) - (2) with istringstream: 15.303s (very slow) - (3) with lexical_cast: 10.958s (between the two) My question: do you know how to notice malformed strings with atoi? It would give the fastest solution. Or do you know a better solution? Update: Thanks for the answers. Following the tips, I came up with this solution: ``` static int stringToIntV4(const string& str) { char * pEnd; const char * c_str = str.c_str(); int result = strtol(c_str, &pEnd, 10); if (pEnd == c_str+str.length()) { return result; } // else throw domain_error("'" + str + "'" + " is not an int!"); } ``` The good news is that it yields if there is a problem and as efficient as the `atoi` version.