Difference between revisions of "CSC231 Mystery C Program with signed numbers"

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[[CSC231_Class_Page Back]] to weekly schedule
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---
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----
 
=Mystery Program=
 
=Mystery Program=
  
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Figure out what the two variables x and y are initialized with when the program starts.
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* The program below is incomplete.
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* The two values that were used to initialize x and y have been removed.
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<code><pre>
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* We do have the output of the program, though, corresponding to the original x and y values.
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<br />
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;Question 1
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: Figure out the value of x and y in the original program.
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<br />
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;Question 2
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: Find a way to fix the program so that it outputs the correct information
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<br />
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You may use this [http://www.mathsisfun.com/binary-decimal-hexadecimal-converter.html converter] to help you out...
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<br />
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<source lang="cpp">
 
/* mystery.cpp
 
/* mystery.cpp
 
// D. T.
 
// D. T.
Line 26: Line 36:
 
#include <stdio.h>
 
#include <stdio.h>
  
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main() {
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int main() {
  
 
     short int x = ???? ;
 
     short int x = ???? ;
Line 36: Line 46:
 
     x = x+y;
 
     x = x+y;
 
     printf( "%d\n", x );
 
     printf( "%d\n", x );
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    return 0;
 
}
 
}
  
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</pre></code>
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</source>
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<br />
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For reference, 2<sup>15</sup> = 32768, and  2<sup>16</sup> = 65536.
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<br />
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<onlydft>
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=Solution=
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<br />
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;Answer 1
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* First we need to find the unsigned version of -32756, because that is the value we expected, but this value is too large to be represented in 2's complement.
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* The difference between a negative 16-bit number in 2's complement, and its unsigned equivalent is 2<sup>16</sup>, or 65536.
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* So, -32756 which is printed, has an unsigned value of -32756 + 65536 = 32780. 
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* So, x+y  is 32740 and x+y+y = 32780
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* Therefore y = 40
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* Therefore x = 32700
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* Plug these numbers in the program and verify that we get, indeed, the two numbers listed in the comment section of the program.
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<br />
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;Answer 2
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* We could switch from shorts to ints
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* We could use unsigned shorts
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* [[CSC231_Mystery_C_Program_Solution | Solution Program]]
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</onlydft>
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[[Category:CSC231]][[Category:C++]]

Latest revision as of 08:42, 4 November 2015

Mystery Program

  • The program below is incomplete.
  • The two values that were used to initialize x and y have been removed.
  • We do have the output of the program, though, corresponding to the original x and y values.


Question 1
Figure out the value of x and y in the original program.


Question 2
Find a way to fix the program so that it outputs the correct information


You may use this converter to help you out... 

/* mystery.cpp
// D. T.
// mystery program
// To compile and run this program:
//
//     g++ mystery.cpp
//     a.out
//
// The output of the program is the following
//
// 32740
// -32756
//
// With what positive values were x and y initialized
// at the beginning of the program.  Explain why.
// (a short int contains 16 bits)
*/

#include <stdio.h>

int main() {

    short int x = ???? ;
    short int y = ???? ;

    x = x+y;
    printf( "%d\n", x );

    x = x+y;
    printf( "%d\n", x );

    return 0;
}


For reference, 215 = 32768, and 216 = 65536.


...



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