Why output is different in both cases????please describe me....

Question

/*to get output

*
**
***
****
*****

*/
//I have written 2 codes in c++

Code 1.

#include <iostream>
#include <string>

using namespace std ;

int main(int argc, char *argv[])
{
string star{"*****"};
for(unsigned i=1; i<=5; i++){
string str{star, i};
cout<<str<<endl;
}

return 0;
}

Code 2.

#include <iostream>
#include <string>

using namespace std ;

int main(int argc, char *argv[])
{
//string star{"*****"};
for(unsigned i=1; i<=5; i++){
string str{"*****", i};
cout<<str<<endl;
}
return 0;
}

Output 1.

****
***
**
*

Output 2.

*
**
***
****
*****
As per expectation is output 2

Answer 1

Ipang is right.

In the line,
`string str{star, i}`
Of rhe 1st code, the constructor being called is
```
basic_string(
const basic_string& other,
size_type pos,
const Allocator& alloc = Allocator() );
```
(number (3) in the link mentioned by Ipang)

If we read the description of this constructor:
"Constructs the string with a substring [pos, pos+count) of other. If count == npos, if count is not specified, or if the requested substring lasts past the end of the string, the resulting substring is [pos, other.size())"

This means that in that line, you are putting substring of `star` from `i` till the end into `str`. When i=1, string "****" (from pos 1 till last) will be created, when i=2, "***" (pos 2 till last) will be created, and so on

[continued in next answer]

Answer 2

I'm thinking perhaps this happens due to the use of different std::string constructor overloads in the two snippets. Have a look at the overloads and find one that matches your constructor argument(s).

https://en.cppreference.com/w/cpp/string/basic_string/basic_string

Answer 3

XXX's answer nailed it well, I was just guessing. Try to read XXX's answers, he had extended it with explanations of the constructor overloads' variety 👍

Answer 4

[continued from previous answer]

In the second code, the constructor being called is
```
basic_string(
const CharT* s,
size_type count,
const Allocator& alloc = Allocator() );
```
(number (4))

Description:
"Constructs the string with the first count characters of character string pointed to by s. s can contain null characters. The length of the string is count"

This means that the second argument you are passing (`i`) is for the length of the string. When i=1, the constructor thinks the length of the string passed is 1, and only creates a string from the 1st character, that is "*". When i=2, a string from the first 2 characters js created, that is "**", and so on

Answer 5

Btw thanks for helping me.....😊

Answer 6

XXX Ipang sorry bros ,but i am unable to get it may be because of I'm beginner right now...........

Answer 7

However if i use different style of array initialization in for loop as below..
string str{star, 0, i};
Or
string str{"*****", 0, i};

No matter whether i am using c++ string or c style literal inside {} ,it gives
gives expected output.......

I observed these things but don't aware about reasons behind these things
..
Please help me to clear these behind the scenes concepts...

Answer 8

I tried but there are so much unknowns to me ....