Question

Write pseudocode for the conversion of infix to prefix notation algorithm

Pseudocode may use the following

Input : A * B + C / D

Output : + * A B/ C D

Answer 1

ANSWER:-

PSEUDOCODE:-

arr = infix expression

Set loop_counter to length of the array

do

      char = arr[loop_counter]

       if char is operator

                   push char to prefix stack

       if char is ‘)’

                   push char to stack

       if char is ‘(’

                  do

                        pop char and push it to prefix_stack

                  until char equals ‘(‘

       if char is operator

                 if char precedence is greater or is equal to stack[top] precedence

                              push to stack

                else

                          do

                                pop from stack and push to prefix stack

                          until condition satisfies

          loop_counter--

while loop_counter is not equal to 0

pop out prefix stack and reverse and display each element

CODE IN C:-

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#define max 100
char s[max];
int top=-1;
void push(char);
char pop();
int precede(char);
main(){
   char infix[100],postfix[100],ch,a[100];
   int i=0,j=0,k,l;
   printf("Enter infix expression:");
   scanf("%s",a);
   l=strlen(a);
   infix[l]='\0';
   l--;
   while(a[i]!='\0'){
       if(a[i]=='('){
           infix[l]=')';
       }else if(a[i]==')'){
           infix[l]='(';
       }else{
           infix[l]=a[i];
       }
       l--;
       i++;
   }
   i=0;
   while((ch=infix[i++])!='\0'){
       if(isalnum(ch)){
           postfix[j++]=ch;
       }else if(ch=='('){
           push(ch);
       }else if(ch==')'){
           while(s[top]!='('){
               postfix[j++]=pop();
           }
           top--;
       }else{
           if(top==-1||precede(ch)>=precede(s[top])){
               push(ch);
           }else{
               while(precede(ch)<=precede(s[top])&&top!=-1){
                   postfix[j++]=pop();
               }
               push(ch);
           }
       }
   }
   while(top!=-1){
       postfix[j++]=pop();
   }
   postfix[j]='\0';
   l=strlen(postfix);
   l--;
   i=0;
   while(l>=0){
       a[i]=postfix[l];
       i++;
       l--;
   }
   a[i]='\0';
   printf("Infix expression = %s\nPrefix expression = %s",infix,a);
}
void push(char a)
{
   s[++top]=a;
}
char pop(){
   return s[top--];
}
int precede(char a){
   if(a=='('||a==')'){
       return 1;
   }else if(a=='-'||a=='+'){
       return 2;
   }else if(a=='%'||a=='/'||a=='*'){
       return 3;
   }
}

OUTPUT:-

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