Interview Question : Trim multiple consecutive spaces from a string - c++

Question

This is an interview question
Looking for best optimal solution to trim multiple spaces from a string. This operation should be in-place operation.
input = "I Like StackOverflow a lot"
output = "I Like StackOverflow a lot"
String functions are not allowed, as this is an interview question. Looking for an algorithmic solution of the problem.

Answer 1

Does using <algorithm> qualify as "algorithmic solution"?
#include <iostream>
#include <string>
#include <algorithm>
#include <iterator>
struct BothAre
{
char c;
BothAre(char r) : c(r) {}
bool operator()(char l, char r) const
{
return r == c && l == c;
}
};
int main()
{
std::string str = "I Like StackOverflow a lot";
std::string::iterator i = unique(str.begin(), str.end(), BothAre(' '));
std::copy(str.begin(), i, std::ostream_iterator<char>(std::cout, ""));
std::cout << '\n';
}
test run: https://ideone.com/ITqxB

Answer 2

A c++0x - solution using a lambda instead of a regular function object. Compare to Cubbi's solution.
#include <string>
#include <algorithm>
int main()
{
std::string str = "I Like StackOverflow a lot";
str.erase(std::unique(str.begin(), str.end(),
[](char a, char b) { return a == ' ' && b == ' '; } ), str.end() );
}

Answer 3

Keep two indices: The next available spot to put a letter in (say, i), and the current index you're examining (say, j).
Just loop over all the characters with j, and whenever you see a letter, copy it to index i, then increment i. If you see a space that was not preceded by a space, also copy the space.
I think this would work in-place...

Answer 4

I'd just go with this:
int main(int argc, char* argv[])
{
char *f, *b, arr[] = " This is a test. ";
f = b = arr;
if (f) do
{
while(*f == ' ' && *(f+1) == ' ') f++;
} while (*b++ = *f++);
printf("%s", arr);
return 0;
}

Answer 5

I'd propose a little state machine (just a simple switch statement). Because if the interviewer is anything like me, the first enhancement they'll ask you to do is to fully trim any leading or trailing spaces, so that:
" leading and trailing "
gets transformed to:
"leading and trailing"
instead of:
" leading and trailing "
This is a really simple modification to a state-machine design, and to me it seems easier to understand the state-machine logic in general over a 'straight-forward' coded loop, even if it takes a few more lines of code than a straight-forward loop.
And if you argue that the modifications to the straight forward loop wouldn't be too bad (which can be reasonably argued), then I (as the interviewer) would throw in that I also want leading zeros from numbers to be be trimmed.
On the other hand, a lot of interviewers might actually dislike a state-machine solution as being 'non-optimal'. I guess it depends on what you're trying to optimize.

Answer 6

Here it is using only stdio:
#include <stdio.h>
int main(void){
char str[] = "I Like StackOverflow a lot";
int i, j = 0, lastSpace = 0;
for(i = 0;str[i]; i++){
if(!lastSpace || str[i] != ' '){
str[j] = str[i];
j++;
}
lastSpace = (str[i] == ' ');
}
str[j] = 0;
puts(str);
return 0;
}

Answer 7

Trimming multiple spaces also means a space should always be followed by a non space character.
int pack = 0;
char str[] = "I Like StackOverflow a lot";
for (int iter = 1; iter < strlen(str); iter++)
{
if (str[pack] == ' ' && str[iter] == ' ')
continue;
str[++pack] = str[iter];
}
str[++pack] = NULL;

Answer 8

int j = 0;
int k=0;
char str[] = "I Like StackOverflow a lot";
int length = strlen(str);
char str2[38];
for (int i = 0; i < length; i++)
{
if (str[i] == ' ' && str[i+1] == ' ')
continue;
str2[j] = str[i];
j++;
}
str2[j] =NULL;
cout<<str2;

Answer 9

void trimspaces(char * str){
int i = 0;
while(str[i]!='\0'){
if(str[i]==' '){
for(int j = i + 1; j<strlen(str);j++){
if(str[j]!=' '){
memmove(str + i + 1, str + j, strlen(str)-j+1);
break;
}
}
}
i++;
}
}

Answer 10

Functional variant in Haskell:
import Data.List (intercalate)
trimSpaces :: String -> String
trimSpaces = intercalate " " . words
The algorithm the next:
breaks a string up into a list of words, which were delimited by white space
concatenate the list inserting one space between each element in list

Answer 11

This is a very simple implementation of removing extra whitespaces.
#include <iostream>
std::string trimExtraWhiteSpaces(std::string &str);
int main(){
std::string str = " Apple is a fruit and I like it . ";
str = trimExtraWhiteSpaces(str);
std::cout<<str;
}
std::string trimExtraWhiteSpaces(std::string &str){
std::string s;
bool first = true;
bool space = false;
std::string::iterator iter;
for(iter = str.begin(); iter != str.end(); ++iter){
if(*iter == ' '){
if(first == false){
space = true;
}
}else{
if(*iter != ',' && *iter != '.'){
if(space){
s.push_back(' ');
}
}
s.push_back(*iter);
space = false;
first = false;
}
}
return s;
}

Answer 12

std::string tripString(std::string str) {
std::string result = "";
unsigned previous = 0;
if (str[0] != ' ')
result += str[0];
for (unsigned i = 1; i < str.length()-1; i++) {
if (str[i] == ' ' && str[previous] != ' ')
result += ' ';
else if (str[i] != ' ')
result += str[i];
previous++;
}
if (str[str.length()-1] != ' ')
result += str[str.length()-1];
return result;
}
This may be an implementation of the accepted idea.