Retrieve the index of the first element using a predicate

Question

I want to change the legacy C-style code by using stl:

for (posEmptyItem = startAt; strlen(collection[posEmptyItem]) > 10; posEmptyItem++) {}
std::cout << posEmptyItem << std::endl;

This code seems a bit hard to read. Anyway to do better?

auto it = std::find_if(collection + startAt, 
 collection + COLLECTION_SIZE, 
 [](const char* line) { return strlen(line) <= 10; });
int idx = std::distance(collection, it); 

Below a complete example:

#include <cstring>
#include <iostream>
#include <algorithm>
#define COLLECTION_SIZE 250
int main()
{
 const char* collection[COLLECTION_SIZE]{ "time11,time2,time3",
 "time12,time2,time3",
 "time13,time2,time3",
 "time14,time2,time3",
 "time15,time2,time3",
 "x\n", 
 "" };
 auto startAt = 2;
 int posEmptyItem;
 // legacy code
 for (posEmptyItem = startAt; strlen(collection[posEmptyItem]) > 10; posEmptyItem++) {}
 std::cout << posEmptyItem << std::endl;
 // replace the loop to search an index by calling to standard library
 auto it = std::find_if(collection + startAt, 
 collection + COLLECTION_SIZE, 
 [](const char* line) { return strlen(line) <= 10; });
 posEmptyItem = std::distance(collection, it); 
 std::cout << posEmptyItem << std::endl;
 return 0;
}

• πάντα ῥεῖ: "This code seems a bit hard to read." Why do you believe it's hard to read actually.
• Less White: To compute the offset from an iterator looks stange to me.
• πάντα ῥεῖ: There's nothing weird about using std::distance (I don't get where you did getting std::dist from. Maybe a MSVC specific typedef??)

Answer 1

For "easier readability" you could extern the lambda expression form the find_if():

auto pred = [](const char* line) { return strlen(line) <= 10; };
auto it = std::find_if(collection + startAt, 
 collection + COLLECTION_SIZE,
 pred);

Also make use of std::begin() and std::end():

auto it = std::find_if(std::begin(collection) + startAt, 
 std::end(collection),
 pred);

At least (but probably not last), don't use raw arrays. Rather change collection to a std::array:

std::array<const char*,COLLECTION_SIZE> collection 
 { "time11,time2,time3"
 , "time12,time2,time3"
 , "time13,time2,time3"
 , "time14,time2,time3"
 , "time15,time2,time3"
 , "x\n"
 , "" 
 };
// Note my formatting style above, which makes it easier to extend the array

Here's the fully refactored code:

#include <cstring>
#include <iostream>
#include <algorithm>
#include <array>
const size_t COLLECTION_SIZE = 250; // rather use a const variable than a macro
int main()
{
 std::array<const char*,COLLECTION_SIZE> collection
 { "time11,time2,time3"
 , "time12,time2,time3"
 , "time13,time2,time3"
 , "time14,time2,time3"
 , "time15,time2,time3"
 , "x\n"
 , "" 
 };
 size_t startAt = 2; // care about the correct type. auto would leave you with int
 // replace the loop to search an index by calling to standard library
 auto pred = [](const char* line) { return strlen(line) <= 10; };
 auto it = std::find_if(std::begin(collection) + startAt, 
 std::end(collection), 
 pred);
 auto posEmptyItem = std::distance(std::begin(collection), it);
 std::cout << posEmptyItem << std::endl;
 return 0;
}

See Live Demo

• Less White: Thanks for your help. I am learning the standard library and want to be check that my code is ok.
• t3chb0t: @LessWhite if you find the answer helpful you can upvote it ;-)
• Incomputable: I don't think that std::array is a good substitute. No type traits work on it, even (ironically) std::is_array.
• πάντα ῥεῖ: @Incomputable Feel free to make a better proposal for that particular case.