Difference between revisions of "CSC231 Mystery C Program with signed numbers"
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=Mystery Program= | =Mystery Program= | ||
| − | Figure out | + | * 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. | ||
| + | <br /> | ||
| + | ;Question 1 | ||
| + | : Figure out the value of x and y in the original program. | ||
| + | <br /> | ||
| + | ;Question 2 | ||
| + | : Find a way to fix the program so that it outputs the correct information | ||
| + | <br /> | ||
You may use this [http://www.mathsisfun.com/binary-decimal-hexadecimal-converter.html converter] to help you out... | You may use this [http://www.mathsisfun.com/binary-decimal-hexadecimal-converter.html converter] to help you out... | ||
| − | + | <br /> | |
| − | < | + | <source lang="cpp"> |
/* mystery.cpp | /* mystery.cpp | ||
// D. T. | // D. T. | ||
| Line 28: | Line 36: | ||
#include <stdio.h> | #include <stdio.h> | ||
| − | main() { | + | int main() { |
short int x = ???? ; | short int x = ???? ; | ||
| Line 38: | Line 46: | ||
x = x+y; | x = x+y; | ||
printf( "%d\n", x ); | printf( "%d\n", x ); | ||
| + | |||
| + | return 0; | ||
} | } | ||
| − | </ | + | </source> |
<br /> | <br /> | ||
| − | For reference, 2<sup>15</sup> = 32768. | + | For reference, 2<sup>15</sup> = 32768, and 2<sup>16</sup> = 65536. |
| + | <br /> | ||
| + | <br /> | ||
| + | <onlydft> | ||
<br /> | <br /> | ||
| + | =Solution= | ||
<br /> | <br /> | ||
| + | ;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. | ||
| + | * 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, x+y is 32740 and x+y+y = 32780 | ||
| + | * Therefore y = 40 | ||
| + | * Therefore x = 32700 | ||
| + | * Plug these numbers in the program and verify that we get, indeed, the two numbers listed in the comment section of the program. | ||
<br /> | <br /> | ||
| + | ;Answer 2 | ||
| + | * We could switch from shorts to ints | ||
| + | * We could use unsigned shorts | ||
| + | |||
<br /> | <br /> | ||
| + | * [[CSC231_Mystery_C_Program_Solution | Solution Program]] | ||
| + | <br /> | ||
| + | </onlydft> | ||
<br /> | <br /> | ||
<br /> | <br /> | ||
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<br /> | <br /> | ||
<br /> | <br /> | ||
| − | + | ||
| − | + | ||
| − | + | <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.
