Question

Write a C program which evaluate and calculate prefix expression notation (normal polish notation). In this notation, the operators come first, so ++ 2 3 4 should evaluate as 9. Instead of using any loop, write it using recursion.

Answer 1

#include <cstdio>

int eval(const char *s, const char **outptr)
{
int a, b, y;
const char *out;

switch (*s) {
case '+':
a = eval(s + 1, &out);
b = eval(out, &out);
y = a + b;
*outptr = out;
break;
case '-':
a = eval(s + 1, &out);
b = eval(out, &out);
y = a - b;
*outptr = out;
break;
case '*':
a = eval(s + 1, &out);
b = eval(out, &out);
y = a * b;
*outptr = out;
break;
case '/':
a = eval(s + 1, &out);
b = eval(out, &out);
y = a / b;
*outptr = out;
break;
default: /* '0'...'9'assumed */
y = *s - '0';
*outptr = s + 1;
break;
}

return y;
}

int main(int argc, char *argv[])
{
const char *end;
int x;
   printf("Enter without spaces :");
   char c[100];
   scanf("%s",c);
x = eval(c, &end);
printf("%d\n", x);

return 0;
}

output:

Enter without spaces :++234
9

Process exited normally.
Press any key to continue . . .

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