searching length of sting in arg[]

Question

I'm trying to get string length from char argv array but I don't know how to check this.

I'm writing 3 words to console for example: red blue yellow. Then I want to write out "yellow" but I don't know how to do that. I tried to save the words in a file but I don't have backspaces in the file this looks that: redblueyellow

#include <iostream> 
#include <string> 
#include <fstream> 
using namespace std; 
int main(int argc,char* argv[]) { 
    string temp; 
    fstream save; 
    string tab[argc]; 
    for(int i=1; i<argc; i++) { 
        if(argv[i]==' ') { 
            tab[argv]=tab[i]; 
        }
    } 
    return 0; 
}

Andreas Wenzel · Answer 1 · 2022-02-10T12:01:16.533

The line

string tab[argc];

is not valid ISO C++, because the length of the array must be a compile-time constant. Some compilers may support variable-length arrays as an extension, though.

However, it is possible to convert the C-style argv strings to a std::vector of std::string. See this question for further information:

Convert command line argument to string

In your code, the line

if(argv[i]==' ') {

does not make sense, as you are comparing a pointer to a string with a character literal.

If you call your program like this

myprogramname red blue yellow

then the following will apply:

argc will have the value 4
argv[0] will point to a string containing myprogramname
argv[1] will point to a string containing red
argv[2] will point to a string containing blue
argv[3] will point to a string containing yellow
argv[4] will have the value NULL

Therefore, if you want to print yellow, you can simply write

std::cout << argv[3] << '\n';

If you want to print the length of the string yellow, you can simply write

std::cout << std::strlen( argv[3] ) << '\n';

Note that you must #include <cstring> in order to use std::strlen.

Alternatively, instead of using std::strlen, you can first convert argv[3] to a std::string, before obtaining its length:

std::string str{ argv[3] };
std::cout << str.length() << '\n';

If you want to write to a file, such as to the std::fstream with the name save in your program, then you can replace std::cout with save in the code above. However, you will have to open the file first, before using it. Also, you may want to consider using std::ofstream instead of std::fstream, because you seem to only want to use the file for writing, not reading.

thanks u, but how get longest word from this array? for example I want to print yellow but I don't know how I tried use argv[I].length() and argv[I].size() — Bartek Sacha, Feb 10 '22 at 12:33
@BartekSacha: Calling `length()` or `size()` will not work on a C-style string (i.e. a null-terminated character array). It will only work on C++ strings (i.e. [`std::string`](https://en.cppreference.com/w/cpp/string/basic_string)). Therefore, if you want to use these functions, you will first have to convert the individual C-style strings to `std::string`. In two places of my answer, I showed you how to do that. Alternatively, you can use [`std::strlen`](https://en.cppreference.com/w/cpp/string/byte/strlen) on a C-style string. I also showed you how to do that in my answer. — Andreas Wenzel, Feb 10 '22 at 12:51

searching length of sting in arg[]

1 Answers1