Infix Expression :
Any expression in the standard form like "2*3-4/5" is an Infix(Inorder) expression.
Postfix Expression :
The Postfix(Postorder) form of the above expression is "23*45/-".
Infix to Postfix Conversion :
In normal algebra we use the infix notation like a+b*c. The corresponding postfix notation is abc*+. The algorithm for the conversion is as follows :
Example :
Let us see how the above algorithm will be imlemented using an example.
Infix String : a+b*c-d
Initially the Stack is empty and our Postfix string has no characters. Now, the first character scanned is 'a'. 'a' is added to the Postfix string. The next character scanned is '+'. It being an operator, it is pushed to the stack.
Stack
Postfix String
Next character scanned is 'b' which will be placed in the Postfix string. Next character is '*' which is an operator. Now, the top element of the stack is '+' which has lower precedence than '*', so '*' will be pushed to the stack.
Stack
Postfix String
The next character is 'c' which is placed in the Postfix string. Next character scanned is '-'. The topmost character in the stack is '*' which has a higher precedence than '-'. Thus '*' will be popped out from the stack and added to the Postfix string. Even now the stack is not empty. Now the topmost element of the stack is '+' which has equal priority to '-'. So pop the '+' from the stack and add it to the Postfix string. The '-' will be pushed to the stack.
Stack
Postfix String
Next character is 'd' which is added to Postfix string. Now all characters have been scanned so we must pop the remaining elements from the stack and add it to the Postfix string. At this stage we have only a '-' in the stack. It is popped out and added to the Postfix string. So, after all characters are scanned, this is how the stack and Postfix string will be :
Stack
Postfix String
End result :
# include<stdio.h>
main()
{
int x=4, y;
y=++x;
printf("\n %d", x);
printf("\n %d", y);
y=x++;
printf("\n %d", x);
printf("\n %d", y);
getch();
}
infix to postfix 2+3-5/2*(2*15)/100+5
a+v+l
people almost exclusively use infix notation to write mathematical expressions, computer languages almost exclusively allow programmers to use infix notation. However, if a compiler allowed infix expressions into the binary code used in the compiled version of a program, the resulting code would be larger than needed and very inefficient. Because of this, compilers convert infix expressions into postfix notation expressions, which have a much simpler set of rules for expression evaluation. Postfix notation gets its name from the fact that operators in a postfix expression follow the operands that they specify an operation on. Here are some examples of equivalent infix and postfix expressions Infix Notation Postfix Notation 2 + 3 2 3 + 2 + 3 * 6 3 6 * 2 + (2 + 3) * 6 2 3 + 6 * A / (B * C) + D * E - A - C A B C * / D E * + A C * - Where as infix notation expressions need a long list or rules for evaluation, postfix expressions need very few.
infix: old Egyptians/Assirs some thousands year before prefix: Jan Łukasiewicz (Polish Notation) postfix: Burks, Warren, and Wright (Reverse Polish Notation)
/**************************//**********cReDo**********//*****mchinmay@live.com***///C PROGRAM TO CONVERT GIVEN VALID INFIX EXPRESSION INTO POSTFIX EXPRESSION USING STACKS.#include#include#include#define MAX 20char stack[MAX];int top=-1;char pop();void push(char item);int prcd(char symbol){switch(symbol){case '+':case '-':return 2;break;case '*':case '/':return 4;break;case '^':case '$':return 6;break;case '(':case ')':case '#':return 1;break;}}int isoperator(char symbol){switch(symbol){case '+':case '-':case '*':case '/':case '^':case '$':case '(':case ')':return 1;break;default:return 0;}}void convertip(char infix[],char postfix[]){int i,symbol,j=0;stack[++top]='#';for(i=0;iprcd(stack[top]))push(symbol);else{while(prcd(symbol)
/**************************//**********cReDo**********//*****mchinmay@live.com***///C PROGRAM TO CONVERT GIVEN VALID INFIX EXPRESSION INTO POSTFIX EXPRESSION USING STACKS.#include#include#include#define MAX 20char stack[MAX];int top=-1;char pop();void push(char item);int prcd(char symbol){switch(symbol){case '+':case '-':return 2;break;case '*':case '/':return 4;break;case '^':case '$':return 6;break;case '(':case ')':case '#':return 1;break;}}int isoperator(char symbol){switch(symbol){case '+':case '-':case '*':case '/':case '^':case '$':case '(':case ')':return 1;break;default:return 0;}}void convertip(char infix[],char postfix[]){int i,symbol,j=0;stack[++top]='#';for(i=0;iprcd(stack[top]))push(symbol);else{while(prcd(symbol)
Example: prefix: * 2 + 3 4 infix: 2 * (3+4) postfix: 2 3 4 + *
(a + b) * c / ((x - y) * z)
Yes
people almost exclusively use infix notation to write mathematical expressions, computer languages almost exclusively allow programmers to use infix notation. However, if a compiler allowed infix expressions into the binary code used in the compiled version of a program, the resulting code would be larger than needed and very inefficient. Because of this, compilers convert infix expressions into postfix notation expressions, which have a much simpler set of rules for expression evaluation. Postfix notation gets its name from the fact that operators in a postfix expression follow the operands that they specify an operation on. Here are some examples of equivalent infix and postfix expressions Infix Notation Postfix Notation 2 + 3 2 3 + 2 + 3 * 6 3 6 * 2 + (2 + 3) * 6 2 3 + 6 * A / (B * C) + D * E - A - C A B C * / D E * + A C * - Where as infix notation expressions need a long list or rules for evaluation, postfix expressions need very few.
infix: old Egyptians/Assirs some thousands year before prefix: Jan Łukasiewicz (Polish Notation) postfix: Burks, Warren, and Wright (Reverse Polish Notation)
You convert an (infix) expression into a postfix expression as part of the process of generating code to evaluate that expression.
An algorithm can not be written with the following infix expression without knowing what the expression is. Once this information is included a person will be able to know how to write the algorithm.
#include<stdio.h> #include<conio.h> #include<string.h> char symbol,s[10]; int F(symbol) { switch(symbol) { case '+': case '-':return 2; case '*': case '/':return 4; case '^': case '$':return 5; case '(':return 0; case '#':return -1; default :return 8; } } int G(symbol) { switch(symbol) { case '+': case '-':return 1; case '*': case '/':return 3; case '^': case '$':return 6; case '(':return 9; case ')':return 0; default: return 7; } } void infix_to_postfix(char infix[],char postfix[]) { int top=-1,j=0,i,symbol; s[++top]='#'; for(i=0;i<strlen(infix);i++) { symbol=infix[i]; while(F(s[top])>G(symbol)) { postfix[j]=s[top--]; j++; } if(F(s[top])!=G(symbol)) s[++top]=symbol; else top--; } while(s[top]!='#') { postfix[j++]=s[top--]; } postfix[j]='\0'; } void main() { char infix[30],postfix[30]; clrscr(); printf("Enter the valid infix expression\n"); scanf("%s",infix); infix_to_postfix(infix, postfix); printf("postfix expression is \n %s", postfix); getch(); }
stack is the basic data structure needed to convert infix notation to postfix
/**************************//**********cReDo**********//*****mchinmay@live.com***///C PROGRAM TO CONVERT GIVEN VALID INFIX EXPRESSION INTO POSTFIX EXPRESSION USING STACKS.#include#include#include#define MAX 20char stack[MAX];int top=-1;char pop();void push(char item);int prcd(char symbol){switch(symbol){case '+':case '-':return 2;break;case '*':case '/':return 4;break;case '^':case '$':return 6;break;case '(':case ')':case '#':return 1;break;}}int isoperator(char symbol){switch(symbol){case '+':case '-':case '*':case '/':case '^':case '$':case '(':case ')':return 1;break;default:return 0;}}void convertip(char infix[],char postfix[]){int i,symbol,j=0;stack[++top]='#';for(i=0;iprcd(stack[top]))push(symbol);else{while(prcd(symbol)
#include <iostream> #include <cstdlib> #include <cstring> #include <cstdio> #define size 400 using namespace std; char infix[size]="\0",postfix[size]="\0",Stack[size]; int top; int precedence(char ch) { switch(ch) { case '^':return 5; case '/':return 4; case '*':return 4; case '+':return 3; case '-':return 3; default:return 0; } } char Pop() { char ret; if(top!=-1) { ret=Stack[top]; top--; return ret; } else return '#'; } char Topelem() { char ch; if(top!=-1) ch=Stack[top]; else ch='#'; return ch; } void Push(char ch) { if(top!=size-1) { top++; Stack[top]=ch; } } int braces(char* s) { int l,r; l=0;r=0; for(int i=0;s[i];i++) { if(s[i]=='(') l++; if(s[i]==')') r++; } if(l==r) return 0; else if(l<r) return 1; else return -1; } int main() { char ele,elem,st[2]; int T,prep,pre,popped,j=0,chk=0; cin>>T; while(T--) { j=0;chk=0;top=-1; strcpy(postfix," "); cin>>infix; chk=braces(infix); if(chk==0) { for(int i=0;infix[i];i++) { if(infix[i]=='(') { elem=infix[i]; Push(elem); } else if(infix[i]==')') { while((popped=Pop())!='(') { postfix[j++]=popped; } } else if(infix[i]=='^'infix[i]=='/'infix[i]=='*'infix[i]=='+'infix[i]=='-') { elem=infix[i]; pre=precedence(elem); ele=Topelem(); prep=precedence(ele); if(pre>prep) Push(elem); else { while(prep>=pre) { if(ele=='#') break; popped=Pop(); ele=Topelem(); postfix[j++]=popped; prep=precedence(ele); } Push(elem); } } else { postfix[j++]=infix[i]; } } while((popped=Pop())!='#') postfix[j++]=popped; postfix[j]='\0'; cout<<postfix; } } }
/**************************//**********cReDo**********//*****mchinmay@live.com***///C PROGRAM TO CONVERT GIVEN VALID INFIX EXPRESSION INTO POSTFIX EXPRESSION USING STACKS.#include#include#include#define MAX 20char stack[MAX];int top=-1;char pop();void push(char item);int prcd(char symbol){switch(symbol){case '+':case '-':return 2;break;case '*':case '/':return 4;break;case '^':case '$':return 6;break;case '(':case ')':case '#':return 1;break;}}int isoperator(char symbol){switch(symbol){case '+':case '-':case '*':case '/':case '^':case '$':case '(':case ')':return 1;break;default:return 0;}}void convertip(char infix[],char postfix[]){int i,symbol,j=0;stack[++top]='#';for(i=0;iprcd(stack[top]))push(symbol);else{while(prcd(symbol)
Example: prefix: * 2 + 3 4 infix: 2 * (3+4) postfix: 2 3 4 + *