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Programming Practicum Lab Exercise 1 Solution
Understanding the difference between the unary * and unary & operators is a truly important skill in C. The * operator has many meanings. It mean multiplication when used as a binary operator (7 * 8), but does “address dereferencing” when used as unary operator. When used in a variable/parameter declaration statement, it means the variable/parameter will store an address location.
Consider the following code (from lab1a.c):
int x = 5;
int *y;
y = &x;
printf ("Print line 1: %d\n", x);
printf ("Print line 2: %p\n", &x);
printf ("Print line 3: %p\n", y);
printf ("Print line 4: %p\n", &y);
printf ("Print line 5: %d\n", *y);
1a. What type of variable is x? What value is stored in x?
1b. What type of variable is y? What value is stored in y?
Will all the above printf( ) statements always display the same results? Why or why not? (I.E. If I ran the above code multiple times on the same or different machines, will I always get the same output?)
What would happen if you could to include the following line to the above program? Why? printf ("Print line 6: %d\n", *x);
In C, we use & and * to simulate pass -by-reference parameters. A pass-by-reference parameter that is changed in a function will reflect that modification at the calling code. Assume we have the function:
void funct1 (int p1, int *p2)
{
printf ("Print line 101: %d\n", p1);
printf ("Print line 102: %p\n", &p1);
printf ("Print line 103: %p\n", p2);
printf ("Print line 104: %p\n", &p2);
printf ("Print line 105: %d\n", *p2);
p1 = p1 + 5;
*p2 = *p2 + 5;
}
Which is called by the following code in main:
int a = 2;
int b = 4;
printf ("Print line 11: For variable a: value: %d, address: %p\n", a, &a); printf ("Print line 12: For variable b: value: %d, address: %p\n", b, &b); funct1 (a, &b);
printf ("Print line 13: For variable a: value: %d, address: %p\n", a, &a);
printf ("Print line 14: For variable b: value: %d, address: %p\n", b, &b);
Do the values associated with variables a and b change because of the call to funct1( )? Why?
Does the address information from Print line 11 through 14 match to any information printed in Print line 101 through 105? If so, describe all the address information that matches between the two sets of Print line statements.
Access the program lab1b.c from the Lab Exercises page. This program is to read in a list of integers until the value of - 999 is input. It then is to print out the number of value read, the total of these values and the average of these values. However, this program does contain some errors.
Fill in the table below with the expected values for the variables count, total and average if the program is run with input of:
2 4 6 8 -999
Compile and run the program. Fill in the table below with the actual values produced by the program?
Expected Value
Actual Value
Value of variable: count
Value of variable: total
Value of variable: average
Does the program produce the expected output? If not, you may need to fix the error before moving to the next question.
Fill in the table below with the expected values for the variables count, total and average if the program is run with input of:
2 3 5 -999
Compile and run the program. Fill in the table below with the actual values produced by the program?
Expected Value
Actual Value
Value of variable: count
Value of variable: total
Value of variable: average
Does the program produce the expected output? If not, you may need to fix the error before moving to the next question.
Fill in the table below with the expected values for the variables count, total and average if the program is run with input of:
-999
Compile and run the program. Fill in the table below with the actual values produced by the program?
Expected Value
Actual Value
Value of variable: count
Value of variable: total
Value of variable: average
Does the program produce the expected output?
What errors contained in the program were exposed in the above test cases?
Give a definition that would define when we can call a program “error free”.
