Simple Function program, with Strings?

I am using O'reilly's Practical C++ Programming book in its 2nd edition. I have to create a program to complete the following question:

Write a function begins(string1, string2) that returns true if string1 begins string2.

This was confusing to me at first, and I think poorly written. From what I understand, I need to write a program that inputs a first string, and then a second string. Then, I guess it should input a third string - containing both first and second strings in any order - and then output true or false depending on the order..

As an example: Enter your first name, Enter your last name, Enter your Full name (either firstname first or firstname last), and then output true or false depending on the circumstances.., etc. I assume I can use a IF statment to determine the order of the strings..?

Please let me know if my grasp on the question is correct, or if I am way off.

I have the code started, and need some help - but I wanted to make sure I was approaching the question correctly before I flood the thread with questions ;)


Thanks all =)

I think you only need to input two strings.

For example, entering
hello
hell
would return true

Whereas
hello
world
would return false.

Ok, got it. I knew I was interpreting it incorrectly.

What control statment is reccommended to search a string for another string??

Heres where I am at:

Code:

---

#include <iostream>
#include <string>


std::string input1;    //First string
std::string input2;    //Second string

int main()
{

std::string begins(std::string string1, std::string string2);  //Declares Function

std::cout << "Please enter the first string: ";
std::getline(std::cin, input1);        //Gets first string

std::cout << "Please enter the second string: ";
std::getline(std::cin, input2);        //Gets second string


begins(input1, input2);  // Calls Function

void begins(std::string string1, std::string string2)
{
                //Will Contain control statments to determine if input1 contains input2
}


return (0);
}

---

Thanks all :)

I guess this one line is left for you to figure out, as this is the whole point of the exercise. If you want to see what tools are available (string methods), look up a reference.

Actually,

Code:

---

#include <iostream>
#include <string>

std::string begins(std::string string1, std::string string2);  //Declares Function

int main()
{

std::string input1;    //First string
std::string input2;    //Second string

std::cout << "Please enter the first string: ";
std::getline(std::cin, input1);        //Gets first string

std::cout << "Please enter the second string: ";
std::getline(std::cin, input2);        //Gets second string

begins(input1, input2);  // Calls Function

return (0);
}

void begins(std::string string1, std::string string2)
{
                //Will Contain control statments to determine if input1 contains input2
}

---

Now, what will begins() do? Will it modify either of the strings? Right now, when you call your function, it will copy the given strings so that the function has its own special copies. Will that be necessary? I'm willing to say no. So then why not let the function simply refer to the strings you gave it, and in a way where the strings will not be modified?

Code:

---

void begins(const std::string & string1,const std::string & string2)
{
                //Will Contain control statments to determine if input1 contains input2
}

---

Don't forget to update the declaration.

Now, for what to do with the function (which is up to you)... you're going to be finding something... or, maybe considering little strings within the bigger strings...

Big hint: Look at std::equal

First off, thanks everyone for their input thus far..

Heres how I want to handle this..

Two strings are inputted, I need to see if the first string begins the second string (hell, hello = true)

Anyway.. Heres what I think I need to do..

1. Get the length of the first string

Code:

---

length = std::strlen(input1)

---

2. Copy string 2 using only the number determined by length from above.

Code:

---

std::strncpy(string2, string1, length)

---

3. Use strcmp to compare the two inputted strings.

Code:

---

std::strcmp(string, string2)

---

4. Use an IF statement that returns TRUE if the strings match, and FALSE if otherwise..

Is this the correct way of tackling this program??

thx all

That looks like a good way to do it, but you shouldn't be using strlen, strncpy and strcmp since those are for C style strings. Your program above uses the C++ string class, so you would use length(), substr(), and ==.

Also be careful to handle the case when the first string is longer than the second.

Quote:

---

Originally Posted by Daved

That looks like a good way to do it, but you shouldn't be using strlen, strncpy and strcmp since those are for C style strings. Your program above uses the C++ string class, so you would use length(), substr(), and ==.

Also be careful to handle the case when the first string is longer than the second.

---

Heres where I'm at...

Code:

---

#include <iostream>
#include <string>


std::string input1;    //First string
std::string input2;    //Second string
int length;
std::string crop;      //Second string, cropped to same length as first string

int main()
{


std::cout << "Please enter the first string: ";
std::getline(std::cin, input1);        //Gets first string

std::cout << "Please enter the second string: ";
std::getline(std::cin, input2);        //Gets second string



length = input1.length();
input2.copy(crop, length);

std::cout << "Length: " << length << '\n';


return (0);
}

---

Code:

---

input2.copy(crop, length);

---

^^ is an attempt to copy input2 to a new string, crop, but only to the length of input1, which should be defined by the length int variable.

Heres the errors its throwing when compiled:

ex9-2.cpp: In function `int main()':
ex9-2.cpp:24: no matching function for call to `basic_string<char,string_char_traits<char>,__defa ult_alloc_template<false,0> >::copy (string &, int &)'
/usr/include/g++/std/bastring.cc:232: candidates are: size_t basic_string<char,string_char_traits<char>,__defau lt_alloc_template<false,0> >::copy(char *, unsigned int, unsigned int = 0) const



thanks =)

I'd use the substr method rather than copy.

Ok, when I try to use substr, heres what I get:

Code:

---

#include <iostream>
#include <string>


std::string input1;    //First string
std::string input2;    //Second string
int length;
std::string crop;      //Second string, cropped to same length as first string

int main()
{


std::cout << "Please enter the first string: ";
std::getline(std::cin, input1);        //Gets first string

std::cout << "Please enter the second string: ";
std::getline(std::cin, input2);        //Gets second string



length = input1.length();
crop = input2.substr(length);

std::cout << "Length: " << length << '\n';
std::cout << "input 2 cropped = " << crop << '\n';

return (0);
}

---

The program DOES compile correctly without errors. For the first string, I enter "hell", and for the second string I enter "hello".. It says the first string is length 4 (correct), but then it outputs "o". Its simply taking 4 chars off the beginning of the second string. I need it to crop them from the end.

If the first strings 4 long, I need to make string 2 (no matter how long) just the first 4 chars. From here I will use a == IF statement to compare.

How do I use substr to remove all chars except the defined beginning length?

thank you =)

The first parameter of substr is the the index of the first character you want to include in your substring. So when you used 4 on the string hello, you start with the letter 'o' ('h' is index 0, 'e' is 1, 'l' is 2, 'l' is 3 and 'o' is 4).

So change that first parameter to the index of the first character you want to include.

It is the second parameter to substr that is the number of characters you want.

This should take a single line of code, people:

Code:

---

if(string1.size() <= string2.size() && std::equal(string1.begin(), string1.end(), string2.begin()))
    // it matches

---

This is an exercise to see if the OP can figure out how to solve the problem, not to see what the "best way" is.

Quote:

---

Originally Posted by Daved

This is an exercise to see if the OP can figure out how to solve the problem, not to see what the "best way" is.

---

If the intention is to comprehend how to manipulate the individual characters of strings, then I think the exercise is quite bizarre, since the entire point of std::string is to insulate you from the representation of the string as a sequence of characters.

In other words, making people "look inside" a std::string only teaches them that that's the right way to deal with strings, and it's not (at least in C++).

I think the benefit of the exercise is come up with an algorithm that solves the problem given your available skillset. If the OP understands algorithms and was able to find equal and use it, then I'd say great. But otherwise, identifying a series of steps to accomplish the task is what programming is all about.

The string is just the vehicle. It's pretty irrelevant to the task in my eyes.

The begins() in the code above still returns void. Not good.

Quote:

---

Originally Posted by brewbuck

This should take a single line of code, people:

Code:

---

if(string1.size() <= string2.size() && std::equal(string1.begin(), string1.end(), string2.begin()))
    // it matches

---

---

Code:

---

#include <iostream>
#include <string>


std::string string1;    //First string
std::string string2;    //Second string

int main()
{


std::cout << "Please enter the first string: ";
std::getline(std::cin, string1);                //Gets first string

std::cout << "Please enter the second string: ";
std::getline(std::cin, string2);                //Gets second string

if(string1.size() <= string2.size() && std::equal(string1.begin(), string1.end(), string2.begin()))
        {
        std::cout << "It matches";
        }
else
        {
        std::cout << "It does not match.";
        }


return (0);
}

---

Throws an error saying:

ex9-2.cpp: In function `int main()':
ex9-2.cpp:19: `::equal' undeclared (first use here)


It says ::equal is undeclared. That to me means that the compiler is interpreting equal as a variable...? Isn't it a string function method? I can't a type, is that is the problem..

If you want to use equal, you have to #include the proper header that includes it. You should be able to find the proper header in your reference book/website.

A way I thought of (though, again, not the best way) is

Code:

---

std::string s1("hello"), s2("hell");
if(s1.find(s2) == 0)  std::cout << "Yup." << std::endl;
else  std::cout << "Nope." << std::endl;

---

CodeMonkey that is correct to a certain extent, but it does not correctly answer the entire question. The question as if string one starts with string two. Although that does give a relatively decent answer. I dont know why not just get a substring of string1 as the length of string2, and compare string1.substr(0, string2.length()) == string2. Then again that may be incorrect.
[EDIT]
it does work. here ya go. simple as hell really.

Code:

---

int main()
{
    string one, two;
    cout << "Enter string1: ";
    getline(cin, one);
    cout << "Enter string2: ";
    getline(cin, two);
    cout << '\n' << endl;
    if ( two.length() < one.length() && one.substr(0, two.length()).compare(two) == 0 )
    {
        cout << "String 1 starts with string 2." << endl;
    }
    else
    {
        cout << "String 1 does not start with string 2." << endl;
    }
    if ( one.length() < two.length() && two.substr(0, one.length()).compare(one) == 0 )
    {
        cout << "String 2 starts with string 1." << endl;
    }
    else
    {
        cout << "String 2 does not start with string 1." << endl;
    }
}

---

[/EDIT]

Quote:

---

Originally Posted by Raigne

CodeMonkey that is correct to a certain extent, but it does not correctly answer the entire question. The question as if string one starts with string two. Although that does give a relatively decent answer.

---

Since the question has been answered many times over, the point is moot. However, I do not see how my method does not entirely answer the question.

s1.find(std::string& str) returns the index of the first occurrence of str within the invoking string (s1). If s1 does not contain str, then npos is returned. Thus, if the function returns the value 0, then s1 contains str, and it is at the beginning of s1.

Correct me if I misunderstand the function. I'm using Schildt as a quick reference... I know, I know....

The method is correct. It's just inefficient for long strings, because it keeps searching even after it's clear that s2 doesn't start s1.
All methods involving substrings are also inefficient, because I know of no implementation of std::string that doesn't allocate a new string for substrings. (Although with small buffer optimization, this doesn't mean much as long as the substring is short.)

The most efficient way is indeed using std::equal.

Code:

---

#include <algorithm>
#include <string>

bool begins(const std::string s1, const std::string s2)
{
  return s1.length() < s2.length() &&
    std::equals(s1.begin(), s1.end(), s2.begin(), s2.begin()+s1.length());
}

---