Question

what does return 4 return 2 return 6 in this code really returning is not making sense to me may someone explain to me what they return,i saw this code on stack flow someone wanted an explanation on infix and prefix convertion

#include<stdio.h> 
#include<conio.h> 
#include<string.h> 
#define MAX 20 

char stack[MAX]; 
int top = -1; 
char pop(); 
void push(char item); 

int prcd(char symbol) 
{ 
    switch(symbol) 
    { 
    case '+': 
    case '-': 
        return 2; 
    case '*': 
    case '/': 
        return 4; 
    case '^': 
    case '$': 
        return 6; 
    case '(': 
    case ')': 
    case '#': 
        return 1; 
    } 
} 

int isoperator(char symbol) 
{ 
    switch(symbol) 
    {
    case '+': 
    case '-': 
    case '*': 
    case '/': 
    case '^': 
    case '$': 
    case '(': 
    case ')': 
        return 1; 
    default: 
        return 0; 
    } 
} 

void convertip(char infix[],char prefix[]) 
{ 
    int i,symbol,j=0; 
    char test[MAX]; 

    infix=strrev(infix); 
    stack[++top]='#'; 

    for(i=0;i<strlen(infix);i++) 
    { 
        symbol=infix[i]; 
        if(isoperator(symbol)==0) 
        { 
            prefix[j]=symbol; 
            j++; 
        }
        else 
        { 
            if(symbol==')') 
            { 
                push(symbol); 
            } 
            else if(symbol=='(') 
            {    
                while(stack[top]!=')') 
                { 
                    prefix[j]=pop(); 
                    j++; 
                }    

                pop();//pop out (. 
            } 
            else 
            { 
                if(prcd(symbol)>prcd(stack[top])) 
                { 
                    push(symbol); 
                }
                else 
                { 
                    while(prcd(symbol)<=prcd(stack[top])) 
                    { 
                        prefix[j]=pop(); 
                        j++; 
                    } 
                    push(symbol); 
                }//end of else. 
            }//end of else. 
        }//end of else. 
    }//end of for. 

    while(stack[top]!='#') 
    { 
        prefix[j]=pop(); 
        j++; 
    } 

    prefix[j]='\0';//null terminate string. 
    prefix=strrev(prefix); 

} 

int main() 
{ 
    char infix[20],prefix[20]; 
    //clrscr(); 
    printf("Enter the valid infix string:\n"); 
    gets(infix); 
    convertip(infix,prefix); 
    printf("The corresponding prefix string is:\n"); 
    puts(prefix); 
    getch(); 

    return 0; 
} 

void push(char item)
{ 
    top++; 
    stack[top]=item; 
} 

char pop() 
{ 
    char a; 
    a=stack[top]; 
    top--; 
    return a; 
} 

Answer 1

This code might interpret numerical terms, like. 17 + 3 * 8. To calculate this properly, the code must determine to first take the * and then the +. The order of evaluation is set by the precedence rules: * and / come before + and -.

The return statements look like some precedence code.

• (, ), #: return 1
• +, -: return 2
• *, /: return 4
• ^, $: return 6

(, ) and # have lowest precedence. After that + and – have next lowest precedence. Then follows * and /. Highest precedence is for ^ and $.