NWEN 241 Systems Programming Exercises (Set 1)

Transcription

1 1. A null statement in C programming is valid but also mandatory in some cases, e.g. if (isalpha(c)) /* true = nonzero, false = zero */ ; /* empty is ok, but ; must be there */ else return(printf("you did not enter an alphabetic character\n")); Instead of using the ; character, what other ways can you define a null statement? Give a simple example like the above. if (isalpha(c)) /* true = nonzero, false = zero */ /* empty is ok, but ; or must be there */ else return(printf("you did not enter an alphabetic character\n")); 2. The return value for printf() is incidental to its main purpose of printing output, and it usually not used. The return type of printf() function is int. Write a simple program to determine the value of printf() and infer its meaning. Under ANSI C, printf() function returns the number of characters it printed. If there is an output error, printf() returns a negative value. The following program illustrates the fact: Output One c = 3 int main(void) int c; c = printf("one"); printf ("\nc = %d",c); 3. What is the output of the following program? Explain. int main(void) int i; for(i=5; --i; ) printf("%d",i); 1

2 What is a more descriptive (understandable) way to write the for loop? Output 4321 For loop starts with i equals to 5, then tests the condition of the loop to execute; as part of the test, the variable i is decremented first, making i contain the value of 4. A true outcome can also be represented by a non-zero value, in this case, i being greater than 0. Execution proceeds into the loop (just one statement) to print i. After printing i = 1, it goes back into the control test which first decrements i to 0; and exits the loop. int main(void) int i; for (i=5; i>0; i--) printf( %d,i); 4. What is the output of the following program? Explain, step-by-step how you got the answer. int i=3; for(i--; i<7; i=7) printf("%d",i++); Output: 2 The for loop starts with i equals 3 and gets decremented by 1. The test for i<7 is true, since i is 2, and execution enters the loop to print the value of i. Then, it executes the expression i=7, and tests the condition i<7. That is false, and execution exits the loop. 5. Explain the effect of the following code: int i; while (i = 2) printf ("Examples of even numbers are: %d %d %d\n", i, i+2, i+4); i = 0; 2

3 Where is the source of the problem? Infinite loop that keeps printing Examples of even numbers are: This is because the while-loop condition is an assignment expression instead of to test, i.e. i==2. 6. In the following code to calculate the area of a circle using a function, will it compile properly? If not, how do you solve the problem? Explain your solution. float AreaOfCircle(float); int main(void) float radius, area; float PI = 22/7.0; printf("radius = "); scanf("%f", &radius); area = AreaOfCircle (radius); printf("area = %f\n", area); float AreaOfCircle (float r) return (PI * r * r); /* area equals Pi times radius squared*/ Compilation Error PI is not visible to the function AreaOfCircle Solution: move declaration float PI = 22/7.0; out of main() function. 7. What is the difference between the declaration and definition of a data object or function? You can use the code example in Q6 to explain. A declaration announces the properties of a data object or a function. The main reason for declaring data objects and functions is type checking. If a variable or function is declared and later reference is made to 3

4 it with data objects that do not match the types in the declaration, the compiler will complain. The purpose of the complaint is to catch type errors at compile time rather than waiting until the program is run, when the results can be more fatal. A definition, on the other hand, actually sets aside storage space (in the case of a data object) or indicates the sequence of statements to be carried out (in the case of a function). 8. Which of the following is an incorrect assignment statement? Explain briefly. (a) n = m = 0; (b) value += 10; (c) mysize = x < y? 9 : 11; (d) testval = (x > 5 x < 0); (e) none of the above (e) none of the above All are valid expressions for an assignment statement. 9. When you compile and execute the following codes, what will be the output? Explain. (a) float c= 3.14; printf("%f", c%2); Output Compiler error Explanation The % operator is applied on a variable of type float. The operands of the % opearator cannot be of float or double. This is why it causes a compiler error. (b) int a=5; a=printf("good")+ printf("boy"); 4

5 printf("%d",a); Output GoodBoy7 Explanation printf() function returns the number of characters printed on the screen. Good and Boy will be printed consecutively. The first printf() returns 4 and the second one returns 3. So = 7 is stored in a. When it is printed, 7 would be printed at the end of GoodBoy. (c) char c = 'A'; printf("%c", c + 10); Output K Explanation The chatracter constant A is stored in the variable c. When 10 is added with c, then 10 is added to the ASCII value of A (i.e. 65), the result is 75. As the result is printed with %c, the character equivalent of 75, which is K, is printed on the screen. (d) int x=10,y=15,a,b; a=x++; b=++y; printf("%d%d\n",a,b); Output 1016 Explanation a = x++ is evaluated as a =x then x = x + 1. So the value of a is 10 and the value of x is 11. The statement b= ++y, ++y is incremented before it is assigned to b. Then b = ++y is evaluated as y = y+1 followed by b = y. So the value of b is 16; hence the output. 5

6 10. When you compile and execute the following codes, what will be the output? (a) int x = 3, y = 5, z = 7, w = 9; w += x; printf("w = %d\n", w); w -= y; printf("w = %d\n", w); x *= z; printf("x = %d\n", x); w += x + y - (z -= w); printf("w = %d, z = %d\n", w, z); w += x -= y %= z; printf("w = %d, x = %d, y = %d\n", w, x, y); w *= x / (y += (z += y)); printf("w = %d, y = %d, z = %d\n", w, y, z); w /= 2 + (w %= (x += y - (z -= -w))); printf("w = %d, x = %d, z = %d\n", w, x, z); Output w = 12 w = 7 x = 21 w = 33, z = 0 Floating exception Explanation w = 12: w += x is w = w + x = = 12 Note that after the above assignment, w is 12. w = 7: w =- y is w = w y = 12 5 = 7 Note that after the above assignment, w is 7. x = 21: x *= z is x = x * z = 3 * 7 = 21 Note that after the above assignment, x is 21. w = 33, z = 0: w += x + y - (z -= w) is evaluated as w = w + (x + y - (z -= w)) = (w + ((x + y) - (z = (z w)))) (z = (z w)) evaluates to 0 (z = (7-7)), with z being assigned 0 6

7 Therefore: w = (w + ((x + y) - (z = (z w)))) = (7 + ((21 + 5) - (0))) = 33 Floating exception: w += x -= y %= z is evaluated as (w = (w + (x = (x - (y = (y % z)))))) y % z is evaluated first. Since z is 0, the modulus operator will fail because of the attempt to divide by 0. This causes the exception. (b) int x = 7, y = -7, z = 11, w =- 11, s = 9, t = 10; x += (y -= (z *= (w /= (s %= t)))); printf("x = %d, y = %d, z = %d, w = %d,\ s = %d, t = %d\n", x, y, z, w, s, t); t += s -= w *= z *= y %= x; printf("x = %d, y %d, z = %d, w = %d,\ s = %d, t = %d\n", x, y, z, w, s, t); Output x = 11, y = 4, z = -11, w = -1, s = 9, t = 10 x = 11, y 4, z = -44, w = 44, s = -35, t = -25 Explanation x = 11, y = 4, z = -11, w = -1, s = 9, t = 10: The expression x += (y -= (z *= (w /= (s %= t)))) is evaluated as x = (x + (y = (y -(z = (z * (w = (w / (s = (s % t))))))))) = (7 + (y = (-7 -(z = (11 * (w = (-11 / (s = (9 % 10))))))))) = (7 + (y = (-7 -(z = (11 * (w = (-11 / (s = 9)))))))) = (7 + (y = (-7 -(z = (11 * (w = (-11 / 9))))))) = (7 + (y = (-7 -(z = (11 * (w = -1)))))) = (7 + (y = (-7 -(z = (11 * -1))))) = (7 + (y = (-7 -(z = -11)))) = (7 + (y = ( ))) = (7 + (y = 4)) = (7 + 4) x = 11 x = 11, y 4, z = -44, w = 44, s = -35, t = -25: The expression t += s -= w *= z *= y %= x is evaluated as t += s -= w *= z *= (y = (y % x)) t += s -= w *= (z = (z*(y = (y % x)))) 7

8 t += s -= (w = (w*(z = (z*(y = (y % x)))))) t += (s = (s - (w = (w*(z = (z*(y = (y % x)))))))) t = (t + (s = (s - (w = (w*(z = (z*(y = (y % x))))))))) = (10 + (s = (9 - (w = (-1*(z = (-11*(y = (4 % 11)))))))) = (10 + (s = (9 - (w = (-1*(z = (-11*(y = 4))))))) = (10 + (s = (9 - (w = (-1*(z = (-11*4)))))) = (10 + (s = (9 - (w = (-1*(z = -44)))))) = (10 + (s = (9 - (w = (-1*-44))))) = (10 + (s = (9 - (w = 44)))) = (10 + (s = (9-44))) = (10 + (s = -35)) = ( ) t = -25 8