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UTK Notes


Clicker Questions - 02-C-Strings

Question 1: What is the output of the following program?



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#include <cstdio>
#include <iostream>
#include <string>
using namespace std;

int main()
{
    string s1, s2;
    const char *s;

    s2 = "Fred";
    s = s2.c_str();
    s1 = s;

    s1[1] = 'y';
    s2[1] = 'z';
    
    cout << s << " "
        << s1 << " "
        << s2 << endl;
    return 0;
}

Multiple Choice Answers:

Simply put the letter of the answer.

(They are in alphabetical order, BTW)

A. Fred Fyed Fyed
B. Fred Fyed Fzed
C. Fred Fzed Fyed
D. Fred Fzed Fzed
E. Fyed Fyed Fyed
F. Fyed Fyed Fzed
G. Fyed Fzed Fyed
H. Fyed Fzed Fzed
I. Fzed Fyed Fyed
J. Fzed Fyed Fzed
K. Fzed Fzed Fyed
L. Fzed Fzed Fzed

Answer The answer is J: Fzed Fyed Fzed 

Fzed Fyed Fzed
Why? You start with s2 being "Fred", and s pointing to the underlying string: 

s2: "Fred"
    ^
    |
s ---
When you assign s1 to s, it makes a copy -- C++ strings always make copies: 

s2: "Fred"
    ^
    |
s ---
s1: "Fred"
Now, each string changes its second character -- s1 changes it to 'y' and s2 changes it to 'z': 

s2: "Fzed"
    ^
    |
s ---
s1: "Fyed"
Thus, the output is "Fzed Fyed Fzed".

Question 2: What is the output of the following program? (Note, it will be a single word composed of numbers and hyphens).



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#include <cstdio>
#include <iostream>
using namespace std;

int main()
{
    int x, y;
    string s;
    
    x = 5;
    y = 10;
    s = "15 Fred";

    sscanf(s.c_str(), "%d %d", &x, &y);
    cout << x << "-" << y << "-";

    s = "Fred 20";

    sscanf(s.c_str(), "%d %d", &x, &y);
    cout << x << "-" << y << endl;

    return 0;
}
Answer The answer is: 

15-10-15-10
Explanation: The first sscanf() correctly reads 15 into x, but fails reading y, because "Fred" is not an integer. So y remains at 10.

The second sscanf() fails reading x, and the sscanf() call exits at that point, so both x and y remain unchanged.

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