How do you search a std::string for a substring in C++?

Question

I'm trying to parse a simple string in C++. I know the string contains some text with a colon, followed immediately by a space, then a number. I'd like to extract just the number part of the string. I can't just tokenize on the space (using sstream and <<) because the text in front of the colon may or may not have spaces in it.

Some example strings might be:

Total disk space: 9852465

Free disk space: 6243863

Sectors: 4095

I'd like to use the standard library, but if you have another solution you can post that too, since others with the same question might like to see different solutions.

Brian R. Bondy · Accepted Answer · 2008-12-06 22:13:14Z

14

std::string strInput = "Total disk space: 9852465";
std::string strNumber = "0";
size_t iIndex = strInput.rfind(": ");
if(iIndex != std::string::npos && strInput.length() >= 2)
{
  strNumber = strInput.substr(iIndex + 2, strInput.length() - iIndex - 2)
}

edited Dec 6, 2008 at 22:13

answered Dec 6, 2008 at 22:05

Brian R. Bondy

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8 Comments

Konrad Rudolph Over a year ago

DO NOT use != -1! Use != string::npos instead.

paxdiablo Over a year ago

Why not, Konrad, does it not work, is it inefficient??? Inquiring minds want to know.

dmckee --- ex-moderator kitten Over a year ago

-1 is a implementation detail, string::npos is correct by definition.

Konrad Rudolph Over a year ago

It's not guaranteed to work. The standard says that find returns npos if nothing was found, and it defines that npos = -1. However, npos is of type std::size_t. Some compilers will issue a warning for comparing an unsigned to a signed number, and depending on settings, treat it as an error

paxdiablo Over a year ago

That's better - my personality type likes to see explanations for why to do things a certain way, not just some edict handed down from above :-)

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Adam Rosenfield · Accepted Answer · 2008-12-06 22:09:02Z

8

For completeness, here's a simple solution in C:

int value;
if(sscanf(mystring.c_str(), "%*[^:]:%d", &value) == 1)
    // parsing succeeded
else
    // parsing failed

Explanation: the %*[^:] says to read in as many possible characters that aren't colons, and the * suppresses assignment. Then, the integer is read in, after the colon and any intervening white space.

answered Dec 6, 2008 at 22:09

Adam Rosenfield

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2 Comments

Bill the Lizard Over a year ago

Thanks, I like when people give alternate solutions. I'm sure this will prove helpful to future C programmers.

Johannes Schaub - litb Over a year ago

great personally i like your solution the same like Konrads :) even tho they do not search a substring, they show how to parse it cleanly

Konrad Rudolph · Accepted Answer · 2008-12-06 22:21:28Z

4

I can't just tokenize on the space (using sstream and <<) because the text in front of the colon may or may not have spaces in it.

Right, but you can use std::getline:

string not_number;
int number;
if (not (getline(cin, not_number, ':') and cin >> number)) {
    cerr << "No number found." << endl;
}

edited Dec 6, 2008 at 22:21

answered Dec 6, 2008 at 22:09

Konrad Rudolph

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5 Comments

Adam Rosenfield Over a year ago

That looks for a newline; I'm assuming you intended to use the overload of getline that takes a delimiter as a third parameter and to pass in ':' for that parameter?

Konrad Rudolph Over a year ago

Thanks Adam, I forgot the third argument, which is really what his posting was all about, as you guessed. :-/

Bill the Lizard Over a year ago

Thanks, Konrad. I didn't know about the overloaded getline() function. It seems strange to me to overload the function in this way, since it no longer gets a line.

Bill the Lizard Over a year ago

@Adam: Thanks for explaining this. It had me thoroughly confused. :)

Johannes Schaub - litb Over a year ago

alternatively, if(!(cin.ignore(numeric_limits<streamsize>::max(), ':') >> number)) cout << "no number found" << endl; could be used too i think

Johannes Schaub - litb · Accepted Answer · 2008-12-06 23:29:50Z

3

Similar to Konrads answer, but using istream::ignore:

int number;
std::streamsize max = std::numeric_limits<std::streamsize>::max();
if (!(std::cin.ignore(max, ':') >> number)) {
    std::cerr << "No number found." << std::endl;
} else {
    std::cout << "Number found: " << number << std::endl;
}

answered Dec 6, 2008 at 23:29

Johannes Schaub - litb

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2 Comments

Konrad Rudolph Over a year ago

Yeah, that's actually the better answer. However, IIRC there are some problems with ignore and max on some platforms (probably due to signed/unsigned mismatch). This information might be dated, however.

Johannes Schaub - litb Over a year ago

yeah i read in the std that streamsize must have signed type. i actually just looked up, since i wondered why ppl don't do streamsize(-1) :)

D.Shawley · Accepted Answer · 2008-12-07 02:54:21Z

3

I'm surprised that no one mentioned regular expressions. They were added as part of TR1 and are included in Boost as well. Here's the solution using regex's

typedef std::tr1::match_results<std::string::const_iterator> Results;

std::tr1::regex re(":[[:space:]]+([[:digit:]]+)", std::tr1::regex::extended);
std::string     str("Sectors: 4095");
Results         res;

if (std::tr1::regex_search(str, res, re)) {
    std::cout << "Number found: " << res[1] << std::endl;
} else {
    std::cerr << "No number found." << std::endl;
}

It looks like a lot more work but you get more out of it IMHO.

answered Dec 7, 2008 at 2:54

D.Shawley

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Comments

orip · Accepted Answer · 2008-12-06 22:08:47Z

2

const std::string pattern(": ");
std::string s("Sectors: 4095");
size_t num_start = s.find(pattern) + pattern.size();

answered Dec 6, 2008 at 22:08

orip

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