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C Program for conversion of infix to postfix and evaluation of postfix
Posted Date: 24 Mar 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: ashish singh Member Level: Diamond
Rating:  Points: 5
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/* Program for conversion of infix to postfix and evaluation of
postfix.
It will take only single digit in expression */
#include
#include
#include
#define Blank ' '
#define Tab '\t'
#define MAX 50
long int pop ();
long int eval_post();
char infix[MAX], postfix[MAX];
long int stack[MAX];
int top;
main()
{
long int value;
char choice='y';
while(choice == 'y')
{
top = 0;
printf("Enter infix : ");
fflush(stdin);
gets(infix);
infix_to_postfix();
printf("Postfix : %s\n",postfix);
value=eval_post();
printf("Value of expression : %ld\n",value);
printf("Want to continue(y/n) : ");
scanf("%c",&choice);
}
}/*End of main()*/
infix_to_postfix()
{
int i,p=0,type,precedence,len;
char next ;
stack[top]='#';
len=strlen(infix);
infix[len]='#';
for(i=0; infix[i]!='#';i++)
{
if( !white_space(infix[i]))
{
switch(infix[i])
{
case '(':
push(infix[i]);
break;
case ')':
while((next = pop()) != '(')
postfix[p++] = next;
break;
case '+':
case '-':
case '*':
case '/':
case '%':
case '^':
precedence = prec(infix[i]);
while(stack[top]!='#' && precedence<= prec(stack[top]))
postfix[p++] = pop();
push(infix[i]);
break;
default: /*if an operand comes */
postfix[p++] = infix[i];
}/*End of switch */
}/*End of if */
}/*End of for */
while(stack[top]!='#')
postfix[p++] = pop();
postfix[p] = '\0' ; /*End postfix with'\0' to make it a string*/
}/*End of infix_to_postfix()*/
/* This function returns the precedence of the operator */
prec(char symbol )
{
switch(symbol)
{
case '(':
return 0;
case '+':
case '-':
return 1;
case '*':
case '/':
case '%':
return 2;
case '^':
return 3;
}/*End of switch*/
}/*End of prec()*/
push(long int symbol)
{
if(top > MAX)
{
printf("Stack overflow\n");
exit(1);
}
else
{
top=top+1;
stack[top] = symbol;
}
}/*End of push()*/
long int pop()
{
if (top == -1 )
{
printf("Stack underflow \n");
exit(2);
}
else
return (stack[top--]);
}/*End of pop()*/
white_space(char symbol)
{
if( symbol == Blank || symbol == Tab || symbol == '\0')
return 1;
else
return 0;
}/*End of white_space()*/
long int eval_post()
{
long int a,b,temp,result,len;
int i;
len=strlen(postfix);
postfix[len]='#';
for(i=0;postfix[i]!='#';i++)
{
if(postfix[i]<='9' && postfix[i]>='0')
push( postfix[i]-48 );
else
{
a=pop();
b=pop();
switch(postfix[i])
{
case '+':
temp=b+a; break;
case '-':
temp=b-a;break;
case '*':
temp=b*a;break;
case '/':
temp=b/a;break;
case '%':
temp=b%a;break;
case '^':
temp=pow(b,a);
}/*End of switch */
push(temp);
}/*End of else*/
}/*End of for */
result=pop();
return result;
}/*End of eval_post */
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