Find the length of string array with strlen()

Question

I have been trying to find the length of string which has an array of chars with strlen() function but it is not working. The code I am using is something like this:

string s[]={"a","b","c"};
int len = strlen(s.c_str());

It produces the following error:

"request for member âc_strâ in âwArrayâ, which is of non-class type"

But when I have used this strlen() function on strings before like this, it worked fine:

fin.open("input.txt");
string tempStr;
getline(fin, tempStr,'\n');
int len = strlen(tempStr.c_str());

What I am missing here? I know I can find the length of string s[] with

int size = sizeof( s ) / sizeof( s[ 0 ] );

But why can't I use strlen(). Can someone explain what is going on?

`s` is an array, it has no `c_str` function. is std::vector an option? — billz, Feb 17 '13 at 08:25
You are missing the fact that `std::string` is not `std::string[]`. — n. m. could be an AI, Feb 17 '13 at 08:27
@n.m. Then why does it work on tempStr. That is a string as well. — DeadCoder, Feb 17 '13 at 08:31

score 8 · Accepted Answer · edited Apr 25 '15 at 18:06

8

Finding the length of a fixed size array of any type is easy enough with a helper function template:

#include <cstddef> // for std::size_t

template< class T, size_t N >
std::size_t length(const T (&)[N] )
{
  return N;
};

string s[]={"a","b","c"};
std::cout << length(s) << std::endl;

In C++11, the function would be constexpr.

Concerning strlen, it counts chars until it finds a null termination character \0. When you call std::string'sc_str() method, you get a pointer to the first char in a null terminated string.

edited Apr 25 '15 at 18:06

Alice Purcell

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answered Feb 17 '13 at 08:30

juanchopanza

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@juanchopanza Thanks.. But i really need to know why strlen() works on tempStr and not string s. (see code for tempStr and s) – DeadCoder Feb 17 '13 at 08:33
@AlexChamberlain I didn't remember off the top of my head which header is required for `std::size_t`. Fixed now. – juanchopanza Feb 17 '13 at 08:33
@DeadCoder I added something about `strlen`. – juanchopanza Feb 17 '13 at 08:36

Simon-Okp · Answer 2 · 2013-02-17T08:49:53.510

1

A C++ way to do it would be using std::vector

vector<string> ss;

ss.push_back("a");
ss.push_back("b");
ss.push_back("c");

cout << ss.size(); // number of strings in your vector : 3

you can also access each string :

cout << ss[0].size(); // 1

edited Feb 17 '13 at 08:49

answered Feb 17 '13 at 08:34

Simon-Okp

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Arpit · Answer 3 · 2013-02-17T08:44:10.790

0

it must be like :

string s[]={"a","b","c"};  //array of string
for(auto i:s){
int len = strlen(i.c_str());  because you need to convert one string to c type string
cout<<len<<" ";
}

to get the array length :

std::array<int> nums {1, 3, 5, 7};

std::cout << "nums contains " << nums.size() << " elements.\n";

http://en.cppreference.com/w/cpp/container/array/size

strlen() accepts ctype strings. and c_str() convert the string(STL) to ctype string(Null terminated) which is accepted by strlen.

edited Feb 17 '13 at 08:44

answered Feb 17 '13 at 08:26

Arpit

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@Arpit of course it is 1, Every element of the array has length 1. – juanchopanza Feb 17 '13 at 08:32
so this is what to be expected. – Arpit Feb 17 '13 at 08:32
@juanchopanza we understand the ques. differently . you work on array and i work on individual string. – Arpit Feb 17 '13 at 08:33
Sorry, that comment was meant to be for @DeadCoder. – juanchopanza Feb 17 '13 at 08:37
@juanchopanza Just last thing. What is difference between ctype string and string ???? – DeadCoder Feb 17 '13 at 08:42
@Arpit same question as asked to Juanchopanza – DeadCoder Feb 17 '13 at 08:42
@DeadCoder "ctype" string would be this null-terminated sequence of `char` I was talking about. `string` would be `std::string`, the standard C++ string class with lots of useful functionality. – juanchopanza Feb 17 '13 at 08:47

score 0 · Answer 4 · answered Feb 17 '13 at 08:47

the define of strlen: size_t strlen ( const char * str ), note the type of input parameter. you define string s[]={"a","b","c"}; which is an array, so the following statement will work: strlen(s[0].c_str()); In addition, to calculate the length of array s, sizeof( s ) / sizeof( s[ 0 ] ) work only when every element of 's' has the same length.

Find the length of string array with strlen()

4 Answers4

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