Merge pull request div-bargali#214 from ramaadarshh/infix-postfix-exp

Added Infix to Postfix Conversion

Author: jai2dev

C/Data-Structures/Stack/InfixToPostfixConv.c

@@ -0,0 +1,199 @@
+/* This program converts infix expression to postfix expression.
+ * This program assume that there are Five operators: (*, /, +, -,^) 
+in infix expression and operands can be of single-digit only.
+ * This program will not work for fractional numbers.
+ * Further this program does not check whether infix expression is 
+ valid or not in terms of number of operators and operands.*/
+
+#include<stdio.h>
+#include<stdlib.h> /* for exit() */
+#include<ctype.h> /* for isdigit(char ) */
+#include<string.h>
+
+#define SIZE 100
+
+
+/* declared here as global variable because stack[]
+* is used by more than one fucntions */
+char stack[SIZE];
+int top = -1;
+
+/* define push operation */
+
+void push(char item)
+{
+if(top >= SIZE-1)
+{
+printf("\nStack Overflow.");
+}
+else
+{
+top = top+1;
+stack[top] = item;
+}
+}
+
+/* define pop operation */
+char pop()
+{
+char item ;
+
+if(top <0)
+{
+printf("Stack under flow: invalid infix expression");
+getchar();
+/* underflow may occur for invalid expression */
+/* where ( and ) are not matched */
+exit(1);
+}
+else
+{
+item = stack[top];
+top = top-1;
+return(item);
+}
+}
+
+/* define function that is used to determine whether any symbol is operator or not
+(that is symbol is operand)
+* this fucntion returns 1 if symbol is opreator else return 0 */
+
+int is_operator(char symbol)
+{
+if(symbol == '^' || symbol == '*' || symbol == '/' || symbol == '+' || symbol =='-')
+{
+return 1;
+}
+else
+{
+return 0;
+}
+}
+
+/* define fucntion that is used to assign precendence to operator.
+* Here ^ denotes exponent operator.
+* In this fucntion we assume that higher integer value
+* means higher precendence */
+
+int precedence(char symbol)
+{
+if(symbol == '^')/* exponent operator, highest precedence*/
+{
+return(3);
+}
+else if(symbol == '*' || symbol == '/')
+{
+return(2);
+}
+else if(symbol == '+' || symbol == '-') /* lowest precedence */
+{
+return(1);
+}
+else
+{
+return(0);
+}
+}
+
+void InfixToPostfix(char infix_exp[], char postfix_exp[])
+{ 
+int i, j;
+char item;
+char x;
+
+push('('); /* push '(' onto stack */
+strcat(infix_exp,")"); /* add ')' to infix expression */
+
+i=0;
+j=0;
+item=infix_exp[i]; /* initialize before loop*/
+
+while(item != '\0') /* run loop till end of infix expression */
+{
+if(item == '(')
+{
+push(item);
+}
+else if( isdigit(item) || isalpha(item))
+{
+postfix_exp[j] = item; /* add operand symbol to postfix expr */
+j++;
+}
+else if(is_operator(item) == 1) /* means symbol is operator */
+{
+x=pop();
+while(is_operator(x) == 1 && precedence(x)>= precedence(item))
+{
+postfix_exp[j] = x; /* so pop all higher precendence operator and */
+j++;
+x = pop(); /* add them to postfix expresion */
+}
+push(x);
+/* because just above while loop will terminate we have
+oppped one extra item
+for which condition fails and loop terminates, so that one*/
+
+push(item); /* push current oprerator symbol onto stack */
+}
+else if(item == ')') /* if current symbol is ')' then */
+{
+x = pop(); /* pop and keep popping until */
+while(x != '(') /* '(' encounterd */
+{
+postfix_exp[j] = x;
+j++;
+x = pop();
+}
+}
+else
+{ /* if current symbol is neither operand not '(' nor ')' and nor
+operator */
+printf("\nInvalid infix Expression.\n"); /* the it is illegeal symbol */
+getchar();
+exit(1);
+}
+i++;
+
+
+item = infix_exp[i]; /* go to next symbol of infix expression */
+} /* while loop ends here */
+if(top>0)
+{
+printf("\nInvalid infix Expression.\n"); /* the it is illegeal symbol */
+getchar();
+exit(1);
+}
+if(top>0)
+{
+printf("\nInvalid infix Expression.\n"); /* the it is illegeal symbol */
+getchar();
+exit(1);
+}
+
+
+postfix_exp[j] = '\0'; /* add sentinel else puts() fucntion */
+/* will print entire postfix[] array upto SIZE */
+
+}
+
+/* main function begins */
+int main()
+{
+char infix[SIZE], postfix[SIZE]; /* declare infix string and postfix string */
+
+/* why we asked the user to enter infix expression
+* in parentheses ( )
+* What changes are required in porgram to
+* get rid of this restriction since it is not
+* in algorithm
+* */
+printf("ASSUMPTION: The infix expression contains single letter variables and single digit constants only.\n");
+printf("\nEnter Infix expression : ");
+gets(infix);
+
+InfixToPostfix(infix,postfix); /* call to convert */
+printf("Postfix Expression: ");
+puts(postfix); /* print postfix expression */
+
+return 0;
+}