Data Structures & Algorithms Multiple Choice Questions on “Evaluation of a Prefix Expression”.
1. How many stacks are required for evaluation of prefix expression?
a) one
b) two
c) three
d) four
Answer: b
Clarification: 2 stacks are required for evaluation of prefix expression, one for integers and one for characters.
2. While evaluating a prefix expression, the string is read from?
a) left to right
b) right to left
c) center to right
d) center to left to right
Answer: b
Clarification: The string is read from right to left because a prefix string has operands to its right side.
3. The associativity of an exponentiation operator ^ is right side.
a) True
b) False
Answer: a
Clarification: The associativity of ^ is right side while the rest of the operators like +,-,*,/ has its associativity to its left.
4. How many types of input characters are accepted by this algorithm?
a) one
b) two
c) three
d) four
Answer: c
Clarification: Three kinds of input are accepted by this algorithm- numbers, operators and new line characters.
5. What determines the order of evaluation of a prefix expression?
a) precedence and associativity
b) precedence only
c) associativity only
d) depends on the parser
Answer: a
Clarification: Precedence is a very important factor in determining the order of evaluation. If two operators have the same precedence, associativity comes into action.
6. Find the output of the following prefix expression.
*+2-2 1/-4 2+-5 3 1
a) 2
b) 12
c) 10
d) 4
Answer: a
Clarification: The given prefix expression is evaluated using two stacks and the value is given by (2+2-1)*(4-2)/(5-3+1)= 2.
7. An error is thrown if the character ‘n’ is pushed in to the character stack.
a) true
b) false
Answer: b
Clarification: The input character ‘n’ is accepted as a character by the evaluation of prefix expression algorithm.
8. Using the evaluation of prefix algorithm, evaluate +-9 2 7.
a) 10
b) 4
c) 17
d) 14
Answer: d
Clarification: Using the evaluation of prefix algorithm, +-9 2 7 is evaluated as 9-2+7=14.
9. If -*+abcd = 11, find a, b, c, d using evaluation of prefix algorithm.
a) a=2, b=3, c=5, d=4
b) a=1, b=2, c=5, d=4
c) a=5, b=4, c=7,d=5
d) a=1, b=2, c=3, d=4
Answer: b
Clarification: The given prefix expression is evaluated as ((1+2)*5)-4 = 11 while a=1, b=2, c=5, d=4.
10. In the given C snippet, find the statement number that has error.
//C code to push an element into a stack 1. void push( struct stack *s, int x) 2. { 3. if(s->top==MAX-1) 4. { 5. printf(“stack overflow”); 6. } 7. else 8. { 9. s->items[++s->top]=x; 10. s++; 11. } 12. }
a) 1
b) 9
c) 10
d) 11
Answer: c
Clarification: If the stack is not full then we are correctly incrementing the top of the stack by doing “++s->top” and storing the value of x in it. However, in the next statement “s++”, we are un-necessarily incrementing the stack base pointer which will lead to memory corruption during the next push() operation.