DS – Evaluation of Prefix Expression

Evaluation of Prefix Expression:

  • Prefix and Postfix expressions can be evaluated faster than infix expressions.
  • No need to process any brackets or follow operator precedence rule.
  • In Postfix or Prefix expressions, which ever the operator comes before will be evaluated first irrespective to priority.
  • These expressions not having brackets to evaluate.

Algorithm: To Evaluate Prefix(String)

  1. Put a pointer ‘p’ at the end of String(come in backward direction)
  2. If character at ‘p’ is an operand, push it on to Stack.
  3. If the character at ‘p’ is an operator, POP two elements from the Stack. Operate these elements according to the operator, and PUSH result back to Stack.
  4. Decrement ‘p’ by 1 and go to step 2 as long as there are characters left in the expression.
  5. The RESULT is stored at the top of the Stack, return it.
  6. End.
int isOperand(char);
int evaluate(char[]);
void push(int);
int pop();
int stack[20];
int top=-1;
int main(){
            char pre[20];
            int res;
            printf(“Enter Prefix expression : “);
            printf(“Input expression : %s \n”, pre);
            res = evaluate(pre);
            printf(“Result is : %d \n”, res);
            return 0;          
int evaluate(char exp[]){
            int j;
            for(j=strlen(exp)-1 ; j>=0 ; j–){
                                    int x,y;
                                    x = pop();
                                    y = pop();
                                                case ‘+’ :           push(x+y);
                                                case ‘-‘             :           push(x-y);
                                                case ‘*’             :           push(x*y);
                                                case ‘/’             :           push(x/y);
            return pop();
int isOperand(char ch)
            if(ch>=48 && ch<=57)
                        return 1;
                        return 0;          
void push(int val)
            stack[++top] = val;
int pop()
            return stack[top–];
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