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

From dftwiki3
Jump to: navigation, search
 
(3 intermediate revisions by the same user not shown)
Line 53: Line 53:
  
 
<br />
 
<br />
For reference, 2<sup>15</sup> = 32768.
+
For reference, 2<sup>15</sup> = 32768, and  2<sup>16</sup> = 65536.
 
<br />
 
<br />
 
<br />
 
<br />
+
<onlydft>
 +
<br />
 
=Solution=
 
=Solution=
 
<br />
 
<br />
 
;Answer 1
 
;Answer 1
 
* 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.
 
* 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.
* The difference between a negative 16-bit number in 2's complement, and its unsigned equivalent is 2<sup>16</sup>, of 65536.
+
* The difference between a negative 16-bit number in 2's complement, and its unsigned equivalent is 2<sup>16</sup>, or 65536.
 
* So, -32756 which is printed, has an unsigned value of -32756 + 65536 = 32780.   
 
* So, -32756 which is printed, has an unsigned value of -32756 + 65536 = 32780.   
 
* So, x+y  is 32740 and x+y+y = 32780
 
* So, x+y  is 32740 and x+y+y = 32780
Line 71: Line 72:
 
* We could switch from shorts to ints
 
* We could switch from shorts to ints
 
* We could use unsigned shorts
 
* We could use unsigned shorts
</onlydft>
+
 
 
 
<br />
 
<br />
 +
* [[CSC231_Mystery_C_Program_Solution | Solution Program]]
 
<br />
 
<br />
 +
</onlydft>
 
<br />
 
<br />
 
<br />
 
<br />
Line 92: Line 94:
 
<br />
 
<br />
 
   
 
   
[[CSC231_Mystery_C_Program_Solution | Solution]]
+
 
 
  <br />
 
  <br />
 
<br />  
 
<br />  
 
<br />
 
<br />
 
[[Category:CSC231]][[Category:C++]]
 
[[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.


...