Remove vowels from string C++

Question

So I am new to C++ and tried to create a function that would remove vowels from a string, but I am failing at it horribly, Here is my code so far :

#include <string>
using namespace std;
string remove(string st) {
    for(int i = 0; st[i] != '\0'; i++) 
        st[i] = st[i] == 'a' || st[i] == 'e' || st[i] == 'i' || st[i] == 'o' || st[i] ==
        'u' || st[i] == 'A' || st[i] == 'E' || st[i] == 'I' || st[i] == 'O' || st[i] ==
        'U' ? '' : st[i];
    }
return st;

This seems to throw an error ? any idea what I am doing wrong

The error I get is :

main.cpp:10:16: error: expected expression 'U' ? '' : Z[i];

And running on another interpreter :

   .code.tio.cpp:7:14: error: incompatible operand types ('const char *' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' (aka 'char'))
            'U' ? "" : Z[i];
                ^ ~~   ~~~~

`''` is not a valid character value. Maybe you want to use a space character or something instead, but it won't "remove" that character from the string, just replace it's value. — François Andrieux, Jun 13 '18 at 20:16
Seems like [`std::remove_if`](http://en.cppreference.com/w/cpp/algorithm/remove) might be a better tool for this job. — François Andrieux, Jun 13 '18 at 20:18
Please [edit] your question and replace "seems to throw an error" with an accurate description of what you're observing. — Drew Dormann, Jun 13 '18 at 20:18
please make sure the error is from the code you show, the error says `""` but code says `''` — 463035818_is_not_an_ai, Jun 13 '18 at 20:19
@user463035818 I interpreted that as `''` with extra quotes introduced by the error message's format, indicating the unexpected token. — François Andrieux, Jun 13 '18 at 20:20
@MuhammadSalman -- As pointed out, `std::remove_if` (and `std::erase`) makes this a one or two line function. Is your goal to get the final results correct, or to keep the code you have now and attempt to fix it? — PaulMcKenzie, Jun 13 '18 at 20:22
It's a shame the c++ community is so quick to downvote-and-forget. The question's problems were fixed quickly after being pointed out. — François Andrieux, Jun 13 '18 at 20:25

François Andrieux · Accepted Answer · 2018-06-14T00:39:56.237

The canonical way to remove elements from a sequential containers according to a predicate (a condition) is to use std::remove_if. Unlike it's name implies, this standard algorithm doesn't quite remove the elements, it moves them to the back of the container so they are easy to erase, leaving the other elements intact and in the same order. It returns an iterator that indicates the beginning of the portion of the container which contains the "removed" elements. Since the standard algorithms cannot change the size the containers they operate on, these elements must then be removed using the container's appropriate remove method. In the case of std::string, that's std::string::erase.

std::remove_if accepts a pair of iterators which define the range of elements to inspect, as well as a predicate that is used to determine which elements to remove. Elements for which the predicate is true are removed.

#include <algorithm>
#include <iostream>
#include <string>

// Returns true if p_char is a vowel
bool is_vowel(const char p_char)
{

    constexpr char vowels[] = { 'a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U' };
    return std::find(std::begin(vowels), std::end(vowels), p_char) != std::end(vowels);
}

std::string remove_vowel(std::string st) 
{
    // Moves all the characters for which `is_vowel` is true to the back
    //  and returns an iterator to the first such character
    auto to_erase = std::remove_if(st.begin(), st.end(), is_vowel);

    // Actually remove the unwanted characters from the string
    st.erase(to_erase, st.end());
    return st;
}

int main()
{
    std::cout << remove_vowel("Hello, World!");
}

The mandatory STL-heavy response in any StackOverflow C++ post :) — Kostas, Jun 13 '18 at 20:40
Note that I've renamed the function to `remove_vowel` as `remove` was conflicting with [`remove`](http://en.cppreference.com/w/c/io/remove), though I don't know how it gets included. — François Andrieux, Jun 13 '18 at 20:41
@MuhammadSalman -- Also, note that the function is named exactly as described. In other words, any STL knowledgeable person can take one look at the code and know what it does and that it works, all without having to debug or step through the code. On the other hand, hand-coded loops have to be 1) figured out what is being done and 2) is it doing it correctly. — PaulMcKenzie, Jun 13 '18 at 20:43
I rather like this code ... and I find the description of remove_if (in cppreference.com) unreadable. Your description here is great. Thanks. — 2785528, Jun 13 '18 at 21:17

Aconcagua · Answer 2 · 2018-06-13T20:51:41.823

2

'' is not a valid character, you cannot place an "empty" one (such thing does not exist in C++) into the string to remove content...

What you can do is moving the non-vowels, i. e. the consonants, to front, skipping the vowels, and afterwards erase the surplus characters at the end:

auto pos = st.begin();
for(auto c : st)
{
    if(isConsonant(c))
        *pos++ = c;
}
st.erase(pos, st.end());

Edit: As François (correctly) denotes: There is no need to re-invent the wheel (provided you are not disallowed to use the standard library):

st.erase(std::remove_if(st.begin(), st.end(), [](char c) { return isConsonant(c); }), st.end());

Be aware that std::remove_if (as well as std::remove) "remove" by just moving the elements to remain to front and returns an iterator to the new end of data – but does not really remove the elements "behind" the new end. So it is necessary to explicitly erase them as shown above.

edited Jun 13 '18 at 20:51

answered Jun 13 '18 at 20:22

Aconcagua

24,880
4
34
59

Ahh, I see. so I cannot just put an empty string. My bad used to JS , python. Thanks I will see what this is – Muhammad Salman Jun 13 '18 at 20:23
I am assuming that `isConstant` function is not part of C++ ? – Muhammad Salman Jun 13 '18 at 20:26
@FrançoisAndrieux : Can you write an answer with `std::remove_if` ? My C++ is really bad ? otherwise a link that explains that ? – Muhammad Salman Jun 13 '18 at 20:27
@MuhammadSalman Right, otherwise it would reside in namespace `std` (and likely be all small letters). Just used a custom function name to shorten the code, use the test you had in your original question instead... – Aconcagua Jun 13 '18 at 20:27
1

@MuhammadSalman: See the example code [here](https://en.cppreference.com/w/cpp/algorithm/remove). – Fred Larson Jun 13 '18 at 20:29
@Aconcagua : Thanks – Muhammad Salman Jun 13 '18 at 20:29
@FredLarson : Thanks – Muhammad Salman Jun 13 '18 at 20:30

Remove vowels from string C++

2 Answers2

Linked