Removing Whitespace from a String

Removing all whitespace from a std::string is a common task in string processing, especially when cleaning up user input or formatting data. Here's an efficient way to accomplish this using the standard library:

1#include <iostream>
2#include <string>
3#include <algorithm>
4#include <cctype>
5
6std::string remove_whitespace(std::string str) {
7  str.erase(std::remove_if(
8    str.begin(), str.end(),
9    [](unsigned char c) {
10      return std::isspace(c);
11    }),
12    str.end()
13  );
14  return str;
15}
16
17int main() {
18  std::string text{"  Hello,   World!  \t\n"};
19
20  std::cout << "Original: '" << text << "'\n";
21  std::cout << "Without whitespace: '"
22            << remove_whitespace(text) << "'\n";
23}

1Original: '  Hello,   World!
2'
3Without whitespace: 'Hello,World!'

Let's break down the remove_whitespace() function:

1. We use std::remove_if() to move all non-whitespace characters to the front of the string.
2. The lambda function [](unsigned char c) { return std::isspace(c); } checks if a character is whitespace.
3. std::isspace() from <cctype> checks for any whitespace character (space, tab, newline, etc.).
4. str.erase() then removes all the whitespace characters that were moved to the end of the string.

This approach is known as the erase-remove idiom and is very efficient because it only traverses the string once and minimizes memory allocations.

If you want to modify the string in-place instead of returning a new string, you can do:

1void remove_whitespace(std::string& str) {
2  str.erase(std::remove_if(
3    str.begin(), str.end(),
4    [](unsigned char c) {
5      return std::isspace(c);
6    }),
7    str.end()
8  );
9}

For even more efficiency, especially with longer strings, you can use std::string::reserve() to avoid reallocations:

1std::string remove_whitespace(
2  const std::string& str
3) {
4  std::string result;
5
6  // Pre-allocate space
7  result.reserve(str.length());
8
9  for (char c : str) {
10    if (!std::isspace(
11      static_cast<unsigned char>(c)
12    )) {
13      result += c;
14    }
15  }
16
17  return result;
18}

This method might be slightly faster for very long strings as it avoids moving characters around, but it does create a new string. Choose the method that best fits your specific use case and performance needs.