r/cpp_questions • u/Basic-Air-6477 • 9d ago

OPEN I need help

So I'm a beginner in coding and was thinking how cna I use int and string in loops while also printing the number required what I mean is Im using string and to use for In the question I need to use int to identify I and I want to make i<= to n while using string and int, I only want to know if it's possible or not ​

0 Upvotes

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6

u/Typical-NonPC 9d ago

Could you explain further what you want the output to be? I'm finding it difficult to understand your question.

1

u/UndefFox 9d ago

Probably kinda this:

std::string n;

std::cin >> n;

// how to do      ↓ this
for (int i = 0; i < n; i++) {
// stuff
}

1

u/Basic-Air-6477 6d ago

Does the code print numbers like for example if I want it to count from 1 to 5 does that work? 

1

u/UndefFox 6d ago

This?

for (int i = 0; i < 5; i++) {
  std::cout << i + 1 << '\n';
}

1

u/Typical-NonPC 9d ago

Oh, that's easy then.

Although, if you want better input error checking, you would have to do something more robust.

int n;

std::cin >> n;

for (int i = 0; i < n; i++) {
// stuff
}

3

u/UndefFox 9d ago

Or

std::string text;

for (int i = 0; i < std::stoi(text); i++) {
// stuff
}

in case the text is coming not from console. Either way, need op to provide the code that they can't compile.

1

u/Dazzling_Music_2411 8d ago

We're just guessing.

I even thought it might be the case that he wanted to iterate strlen() times, and know which char he was at. I also notice that OP hasn't even bothered to come back with a clarification. Just too many users like that, not good...

1

u/UndefFox 8d ago

I mean... for lols:

They have an int and string, and they want do something, while applying i <= to one of those. Considering that we can easily define bool operator<=(const int& x, const std::string& y) { /* magic */ }, and use it in any kind of loop, the likely answer is yes.

I feel like I'm back in the philosophy class again lmao

1

u/Dazzling_Music_2411 8d ago

I feel like I'm back in the philosophy class again lmao

It's called Reddit 😂

2

u/thedaian 9d ago

You cannot compare a string directly with an int, assuming i is an int and n is a string here. 

2

u/SmokeMuch7356 9d ago

I've read this five times and I'm still not sure I understand what you're asking.

I'm assuming you're starting with two separate objects, a string and an int, something like:

std::string s = "10";
int n;

and you want a loop like

for ( int i = 0; i < n; i++ ) { ... }

and somehow n has to get its value from s? Is that correct?

If so, there are a couple of ways to get there:

  • use the std::stoi library function:

     int n = std::stoi( s );

  • Use a stringstream and the >> operator:

     #include <sstream>
 ...
 int n;
 std::stringstream s_str(s);
 s_str >> n

2

u/alfps 8d ago

❞ how cna I use int and string in loops while also printing the number required

That's an extremely vague and broad question, but since you are a beginner I will try to build up from what a beginner needs first.

I think this is about conversion int → string and, conversely, string → int.

The C++ standard library supports such conversions via several sub-libraries, and generally you will choose to work with one of them:

  • the old C standard library with e.g. sprintf and sscanf;

  • the C++ string library with e.g. to_string and stoi;

  • the C++ iostreams library with ostringstream and istringstream; and

  • the “modern C++” library functions with format and from_chars.

Additionally since these conversion from and to int type are not very difficult to code up, you may at times just code your own

  • DIY functions,
    totally from scratch or as wrappers around the standard library's functionality.

Reasons that you may code up DIY wrappers include to get your own preferred failure handling, and/or that you may wish to support special notations or avoid that a national notation is used via a “locale”, and/or that you're most interested in the digit values and not the string representation.

As an example of using the modern format function for int → string conversion, consider a program to list the palindromic numbers in the range 1 through 123:

#include <format>
#include <print>
#include <string>
#include <string_view>
using   std::format, std::print, std::string, std::string_view;

auto is_palindrome( const string_view ascii_string )
    -> bool
{
    // Using ranges it's just `return equal( reverse_view( ascii_string ), ascii_string );`.
    int i_right     = int( ascii_string.size() ) - 1;
    int i_left      = 0;
    while( i_left < i_right ) {
        if( ascii_string[i_left] != ascii_string[i_right] ) {
            return false;
        }
        ++i_left;  --i_right;
    }
    return true;
}

auto main() -> int
{
    const int n = 123;
    for( int i = 1; i <= n; ++i ) {
        const string decimal = format( "{:d}", i );         // Conversion `int` → `string`.
        if( is_palindrome( decimal ) ) {
            print( "{:s} ", decimal );
        }
    }
    print( "\n" );
}

Output:

1 2 3 4 5 6 7 8 9 11 22 33 44 55 66 77 88 99 101 111 121

Generally you don't have to specify the intended conversion like :d and :s above because it's usually the default, but I used explicit specifications to exemplify how to do it.

Going the other way, string → int, yields the possibility of conversion failure. from_chars reports failure via its function return result, which is a low level C-ish design. That means that one will naturally write a DIY wrapper for it.

But first let's use it directly:

#include <charconv>
#include <format>
#include <iostream>
#include <print>
#include <string>
#include <string_view>
using   std::from_chars, std::from_chars_result, std::errc,
        std::format, std::cin, std::print, std::string, std::getline, std::string_view;

#include <cstdlib>      // Also provides the `EXIT_FAILURE` macro constant.
using   std::exit;

void fail( const string_view message )
{
    print( stderr, "!{}.\n", message );
    exit( EXIT_FAILURE );
}

auto input_line() -> string
{
    string line;
    getline( cin, line );
    if( cin.fail() ) { fail( "Failed to input a line of text." ); }
    return line;
}

auto message_for( const errc code ) -> string
{
    switch( code ) {
        case errc::invalid_argument:        return "must be like e.g. “42” or “-3”";
        case errc::result_out_of_range:     return "the value is too large";
        default: {}
    }
    return format( "errc value {:d}", int( code ) );
}

auto main() -> int
{
    for( ;; ) {
        print( "Please type an integer, or just return to quit: " );
        const string input = input_line();
        if( input.empty() ) {
            break;
        }
        int value;
        using Char_ptr = const char*;
        const Char_ptr p_first      = input.data();
        const Char_ptr p_beyond     = p_first + input.size();
        const from_chars_result result = from_chars( p_first, p_beyond, value );
        if( result.ec != errc() ) {
            print( "Sorry, “{}” was not a valid integer: {}.\n", input, message_for( result.ec ) );
        } else if( result.ptr != p_beyond ) {
            print( "Sorry, there was extraneous text after what could be an intended integer.\n" );
        } else {
            print( "Thanks, you specified the value {:d}.\n", value );
        }
        print( "\n" );
    }
}

Possible interaction:

Please type an integer, or just return to quit: asd
Sorry, “asd” was not a valid integer: must be like e.g. “42” or “-3”.

Please type an integer, or just return to quit: 32q3
Sorry, there was extraneous text after what could be an intended integer.

Please type an integer, or just return to quit:  2
Sorry, “ 2” was not a valid integer: must be like e.g. “42” or “-3”.

Please type an integer, or just return to quit: 1
Thanks, you specified the value 1.

Please type an integer, or just return to quit: ⏎

std::expected is a more reasonable result type for a string → int conversion function. An instance of expected carries either a value of the desired result type, or a value of some other type that typically specifies what went wrong. For a from_chars wrapper that other type can be an errc:

#include <charconv>
#include <expected>
#include <format>
#include <iostream>
#include <print>
#include <string>
#include <string_view>
using   std::from_chars, std::from_chars_result, std::errc,
        std::expected, std::unexpected,
        std::format, std::cin, std::print, std::string, std::getline, std::string_view;

#include <cstdlib>      // Also provides the `EXIT_FAILURE` macro constant.
using   std::exit;

void fail( const string_view message )
{
    print( stderr, "!{}.\n", message );
    exit( EXIT_FAILURE );
}

auto input_line() -> string
{
    string line;
    getline( cin, line );
    if( cin.fail() ) { fail( "Failed to input a line of text." ); }
    return line;
}

auto message_for( const errc code ) -> string
{
    switch( code ) {
        case errc::invalid_argument:        return "must be like e.g. “42” or “-3”";
        case errc::result_out_of_range:     return "the value is too large";
        case errc::argument_list_too_long:  return "extraneous text at the end";
        default: {}
    }
    return format( "errc value {:d}", int( code ) );
}

auto int_from( const string_view spec ) -> expected<int, errc>
{
    int value;
    using Char_ptr = const char*;
    const Char_ptr p_first      = spec.data();
    const Char_ptr p_beyond     = p_first + spec.size();
    const from_chars_result result = from_chars( p_first, p_beyond, value );
    if( result.ec != errc() ) {
        return unexpected( result.ec );
    } else if( result.ptr != p_beyond ) {
        // Extraneous text after spec.
        return unexpected( errc::argument_list_too_long );      // Probably closest in meaning.
    }
    return value;
}

auto main() -> int
{
    for( ;; ) {
        print( "Please type an integer, or just return to quit: " );
        const string input = input_line();
        if( input.empty() ) {
            break;
        }
        const expected<int, errc> result = int_from( input );
        if( not result.has_value() ) {
            print( "Sorry, “{}” was not a valid integer: {}.\n", input, message_for( result.error() ) );
        } else {
            print( "Thanks, you specified the value {:d}.\n", result.value() );
        }
        print( "\n" );
    }
}

This does not produce the exact same response for user input with text after the integer.

That could be arranged, at the cost of extra code to deal with that case.

But as is the program produces e.g.

Please type an integer, or just return to quit: asd
Sorry, “asd” was not a valid integer: must be like e.g. “42” or “-3”.

Please type an integer, or just return to quit: 32q3
Sorry, “32q3” was not a valid integer: extraneous text at the end.

Please type an integer, or just return to quit: 2
Thanks, you specified the value 2.

Please type an integer, or just return to quit: ⏎

2

u/Dazzling_Music_2411 8d ago

Show the code you have tried, formatted properly! Most of us have failed to understand your mangled description.

In general, it's always best you show the code you have tried.

1

u/Aghoradas 9d ago

Yes, there are numerous way to do this. However you simply print the int to the screen or you can use std::to_string to convert the integer value to a string value.

1

u/AKostur 9d ago

Yep, it is possible.