import std.experimental.all;

​

auto splitLowerAndFilter(string input)

{

    return input.take(1).asLowerCase.chain(input.drop(1).filter!(c => c != ' ')).array;

}

​

auto filterAndMapWithIndex(string input)

{

    auto index=0;

    return input.filter!(c => c != ' ')

            .map!((a){if(index++==0){return std.uni.toLower(a);}else{return a;}});

}

​

auto filterEnumerateAndMap(string input)

{

    return input.filter!(c => c != ' ')

            .enumerate

            .map!((a){if(a[0]==0){return std.uni.toLower(a[1]);}else{return a[1];}});

}

​

string filterAndLowerFirstAsSlice(string input)

{

    char[] output = uninitializedArray!(char[])(input.length);

    size_t out_idx = 0;

    foreach(in_idx, char c; input)

    {

        if(c == ' ')

            continue;

        if(in_idx == 0)

            c = std.ascii.toLower(c);

        output[out_idx++] = c;

    }

    return cast(string) output[0..out_idx];

}

​

void main()

{

    string input = "My Capital String";

    auto lower1 = input.splitLowerAndFilter.array;

    writeln(lower1);

    assert(lower1 == "myCapitalString");

    auto lower2 = input.filterAndMapWithIndex.array;

    writeln(lower2);

    assert(lower2 == "myCapitalString");

    auto lower3 = input.filterEnumerateAndMap.array;

    writeln(lower3);

    assert(lower3 == "myCapitalString");

    auto lower4 = input.filterAndLowerFirstAsSlice;

    writeln(lower4);

    assert(lower4 == "myCapitalString");

}