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... | ||
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| + | <onlydft> | ||
| + | =Solution= | ||
| + | <br /> | ||
| + | * 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. | ||
| + | * 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. | ||
| + | </onlydft> | ||
| + | |||
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[[CSC231_Mystery_C_Program_Solution | Solution]]! | [[CSC231_Mystery_C_Program_Solution | Solution]]! | ||
</onlydft> | </onlydft> | ||
Revision as of 08:13, 13 November 2014
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.
Solution!
</onlydft>
