Pointers And Structures

Postfix To Prefix Conversion

By now we know how to convert an expression in Infix form to either the Prefix or the Postfix form. Let us now concentrate on other conversions like Postfix to Prefix and Postfix to Infix. Here is a program which carries out the first conversion.

/*Convert Postfix to Prefix */

#define MAX 100

void pop (char *a1);

void push(char *str);

char stack[MAX][MAX];

int top =-1;

main()

{

char s[MAX], str1[MAX], str2[MAX], str[MAX];

char s1[2], temp[2];

int i = 0;

clrscr();

printf("Enter the postfix expression; ");

gets (s);

while(s[i]!=`\0')

{

/*skip whitespace, if any */

if (s[i] == '')

i++;

if(s[i] == `%' || s[i] == `*' || s[i] == `_' || s[i]== `+' || s[i] == `/')

{

pop (str1);

pop (str2);

temp[0] = s[i];

temp[1] = `\0';

strcpy (str, temp);

strcat(str, str2);

strcat(str, str1);

push(str);

}

else

{

temp[0] = s[i];

temp[1] = `\0';

strcpy (s1, temp);

push (s1);

}

i++;

}

printf("\n%s",stack[0]);

}

void pop(char*a1)

{

if(top == -1)

{

printf("\nStack is empty");

exit(1);

}

else

{

strcpy (a1, stack[top]);

top--;

}

}

void push (char *str)

{

if(top == MAX - 1)

printf("\nstack is full");

else

{

top++;

strcpy(stack[top], str);

}

}