looking for an std algorithm to replace a simple for-loop

Question

Is there a standard algorithm in the library that does the job of the following for-loop?

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>


int main( )
{
    const char oldFillCharacter { '-' };
    std::vector<char> vec( 10, oldFillCharacter ); // construct with 10 chars
    // modify some of the elements
    vec[1] = 'e';
    vec[7] = 'x';
    vec[9] = '{';

    const char newFillCharacter { '#' };

    for ( auto& elem : vec ) // change the fill character of the container
    {
        if ( elem == oldFillCharacter )
        {
            elem = newFillCharacter;
        }
    }

    // output to stdout
    std::copy( std::begin( vec ), std::end( vec ),
               std::ostream_iterator<char>( std::cout, " " ) );
    std::cout << '\n';
    /* prints: # e # # # # # x # { */
}

I want to replace the above range-based for-loop with a one-liner if possible. Is there any function that does this? I looked at std::for_each but I guess it's not suitable for such a scenario.

Tell us what you are trying to do, on a high level. In that code, there are several things going on, and what are we supposed to be pinpointing? — PaulMcKenzie
– PaulMcKenzie, Commented Apr 8, 2022 at 7:35
So to boil this down, are you simply trying to replace one value with another in a sequence of values? If so, that's what you should state up front -- no need to show code or other things that may confuse what you are actually trying to accomplish. — PaulMcKenzie
– PaulMcKenzie, Commented Apr 8, 2022 at 7:40
@PaulMcKenzie Yes. Simple as that. I just wanted a one-liner instead of a dummy loop. — digito_evo
– digito_evo, Commented Apr 8, 2022 at 7:42
Well, googling "STL algorithm to replace" would have probably gotten you to std::replace or std::replace_if. Doing a web search for for loops wouldn't take you to what you are looking for. — PaulMcKenzie
– PaulMcKenzie, Commented Apr 8, 2022 at 7:44

Lukas-T · Accepted Answer · 2022-04-08 07:37:32Z

7

This loop will replace every occurrence of oldFillCharacter with newFillCharacter. If you don't want to do something more fancy std::replace looks good:

std::replace(std::begin(vec), std::end(vec), oldFillCharacter, newFillCharacter);

Or a bit simpler with std::ranges::replace:

std::ranges::replace(vec, oldFillCharacter, newFillCharacter);

answered Apr 8, 2022 at 7:37

Lukas-T

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1 Comment

digito_evo Over a year ago

I was totally unaware of this algorithm. Seems like the most suitable one. Also thanks for suggesting the ranges variation!

songyuanyao · Accepted Answer · 2022-04-08 07:36:48Z

3

You can use std::for_each.

std::for_each(vec.begin(), vec.end(), [](char& elem) { 
    if ( elem == oldFillCharacter ) elem = newFillCharacter;
    });

answered Apr 8, 2022 at 7:36

songyuanyao

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Comments

JBahn77 · Accepted Answer · 2022-04-08 07:38:18Z

2

std::replace(vec.begin(), vec.end(), '_', '#');

answered Apr 8, 2022 at 7:38

JBahn77

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