Program to evaluate simple expressions

You are given a string that represent an expression of digits and operands. E.g. 1+2*3, 1-2+4. You need to evaluate the string or the expression. NO BODMAS is followed. If the expression is of incorrect syntax return -1. 
Test cases: 
a) 1+2*3 will be evaluated to 9. 
b) 4-2+6*3 will be evaluated to 24. 
c) 1++2 will be evaluated to -1(INVALID). 
Also, in the string spaces can occur. For that case we need to ignore the spaces. Like :- 1*2 -1 is equals to 1.
Source: Amazon Interview Question
It is strongly recommend to minimize the browser and try this yourself first.
The idea is simple start from the first character and traverse from left to right and check for errors like two consecutive operators and operands. We also keep track of result and update the result while traversing the expression.
Following is the program to evaluate the given expression. 

C++

// C++ program to evaluate a given expression
#include <iostream>
using namespace std;

// A utility function to check if a given character is operand
bool isOperand(char c) { return (c >= '0' && c <= '9'); }

// utility function to find value of and operand
int value(char c) {  return (c - '0'); }

// This function evaluates simple expressions. It returns -1 if the
// given expression is invalid.
int evaluate(char *exp)
{
    // Base Case: Given expression is empty
    if (*exp == '\0')  return -1;

    // The first character must be an operand, find its value
    int res = value(exp[0]);

    // Traverse the remaining characters in pairs
    for (int i = 1; exp[i]; i += 2)
    {
        // The next character must be an operator, and
        // next to next an operand
        char opr = exp[i], opd = exp[i+1];

        // If next to next character is not an operand
        if (!isOperand(opd))  return -1;

        // Update result according to the operator
        if (opr == '+')       res += value(opd);
        else if (opr == '-')  res -= value(opd);
        else if (opr == '*')  res *= value(opd);
        else if (opr == '/')  res /= value(opd);

        // If not a valid operator
        else                  return -1;
    }
    return res;
}

// Driver program to test above function
int main()
{
    char expr1[] = "1+2*5+3";
    int res = evaluate(expr1);
    (res == -1)? cout << expr1 << " is " << "Invalid\n":
                 cout << "Value of " << expr1 << " is " << res << endl;

    char expr2[] = "1+2*3";
    res = evaluate(expr2);
    (res == -1)? cout << expr2 << " is " << "Invalid\n":
                 cout << "Value of " << expr2 << " is " << res << endl;

    char expr3[] = "4-2+6*3";
    res = evaluate(expr3);
    (res == -1)? cout << expr3 << " is " << "Invalid\n":
                 cout << "Value of " << expr3 << " is " << res << endl;

    char expr4[] = "1++2";
    res = evaluate(expr4);
    (res == -1)? cout << expr4 << " is " << "Invalid\n":
                 cout << "Value of " << expr4 << " is " << res << endl;
    return 0;
}

Java

// Java program to evaluate a given expression

class GFG{
// A utility function to check if
// a given character is operand
static boolean isOperand(char c)
{
    return (c >= '0' && c <= '9');

}

// utility function to find value of and operand
static int value(char c)
{
    return (int)(c - '0');

}

// This function evaluates simple expressions.
// It returns -1 if the given
// expression is invalid.
static int evaluate(String exp)
{
    // Base Case: Given expression is empty
    if (exp.length() == 0) return -1;

    // The first character must be
    // an operand, find its value
    int res = value(exp.charAt(0));

    // Traverse the remaining characters in pairs
    for (int i = 1; i<exp.length(); i += 2)
    {
        // The next character must be an operator, and
        // next to next an operand
        char opr = exp.charAt(i), opd = exp.charAt(i+1);

        // If next to next character is not an operand
        if (isOperand(opd) == false) return -1;

        // Update result according to the operator
        if (opr == '+') res += value(opd);
        else if (opr == '-') res -= value(opd);
        else if (opr == '*') res *= value(opd);
        else if (opr == '/') res /= value(opd);

        // If not a valid operator
        else                 return -1;
    }
    return res;
}

// Driver program to test above function
public static void main(String[] args)
{
    String expr1 = "1+2*5+3";
    int res = evaluate(expr1);
    if(res == -1) System.out.println(expr1+" is Invalid");
    else     System.out.println("Value of "+expr1+" is "+res);

    String expr2 = "1+2*3";
    res = evaluate(expr2);
    if(res == -1) System.out.println(expr2+" is Invalid");
    else         System.out.println("Value of "+expr2+" is "+res);

    String expr3 = "4-2+6*3";
    res = evaluate(expr3);
    if(res == -1) System.out.println(expr3+" is Invalid");
    else         System.out.println("Value of "+expr3+" is "+res);

    String expr4 = "1++2";
    res = evaluate(expr4);
    if(res == -1) System.out.println(expr4+" is Invalid");
    else         System.out.println("Value of "+expr4+" is "+res);
}
}
// This code is contributed by mits

Python3

# Python3 program to evaluate a
# given expression

# A utility function to check if
# a given character is operand
def isOperand(c):

    return (c >= '0' and c <= '9');

# utility function to find
# value of and operand
def value(c):
    return ord(c) - ord('0');

# This function evaluates simple
# expressions. It returns -1 if the
# given expression is invalid.
def evaluate(exp):

    len1 = len(exp);

    # Base Case: Given expression is empty
    if (len1 == 0):
        return -1;

    # The first character must be
    # an operand, find its value
    res = value(exp[0]);

    # Traverse the remaining
    # characters in pairs
    for i in range(1,len1,2):
        # The next character must be
        # an operator, and next to
        # next an operand
        opr = exp[i];
        opd = exp[i + 1];

        # If next to next character
        # is not an operand
        if (isOperand(opd)==False):
            return -1;

        # Update result according
        # to the operator
        if (opr == '+'):
            res += value(opd);
        elif (opr == '-'):
            res -= int(value(opd));
        elif (opr == '*'):
            res *= int(value(opd));
        elif (opr == '/'):
            res /= int(value(opd));

        # If not a valid operator
        else:
            return -1;

    return res;

# Driver Code
expr1 = "1+2*5+3";
res = evaluate(expr1);
print(expr1,"is Invalid") if (res == -1) else print("Value of",expr1,"is",res);

expr2 = "1+2*3";
res = evaluate(expr2);
print(expr2,"is Invalid") if (res == -1) else print("Value of",expr2,"is",res);

expr3 = "4-2+6*3";
res = evaluate(expr3);
print(expr3,"is Invalid") if (res == -1) else print("Value of",expr3,"is",res);

expr4 = "1++2";
res = evaluate(expr4);
print(expr4,"is Invalid") if (res == -1) else print("Value of",expr4,"is",res);

# This code is contributed by mits

C#

// C# program to evaluate a given expression
using System;
class GFG{

// A utility function to check if
// a given character is operand
static bool isOperand(char c) {
    return (c >= '0' && c <= '9');

}

// utility function to find value of and operand
static int value(char c) { return (int)(c - '0'); }

// This function evaluates simple
// expressions. It returns -1 if the
// given expression is invalid.
static int evaluate(string exp)
{
    // Base Case: Given expression is empty
    if (exp.Length == 0) return -1;

    // The first character must be
    // an operand, find its value
    int res = value(exp[0]);

    // Traverse the remaining characters in pairs
    for (int i = 1; i<exp.Length; i += 2)
    {
        // The next character must be an operator, and
        // next to next an operand
        char opr = exp[i], opd = exp[i+1];

        // If next to next character is not an operand
        if (isOperand(opd)==false) return -1;

        // Update result according to the operator
        if (opr == '+') res += value(opd);
        else if (opr == '-') res -= value(opd);
        else if (opr == '*') res *= value(opd);
        else if (opr == '/') res /= value(opd);

        // If not a valid operator
        else                 return -1;
    }
    return res;
}

// Driver program to test above function
static void Main()
{
    string expr1 = "1+2*5+3";
    int res = evaluate(expr1);
    if(res == -1)
    Console.WriteLine(expr1+" is Invalid");
    else
    Console.WriteLine("Value of "+expr1+" is "+res);

    string expr2 = "1+2*3";
    res = evaluate(expr2);
    if(res == -1)
    Console.WriteLine(expr2+" is Invalid");
    else
    Console.WriteLine("Value of "+expr2+" is "+res);

    string expr3 = "4-2+6*3";
    res = evaluate(expr3);
    if(res == -1)
    Console.WriteLine(expr3+" is Invalid");
    else
    Console.WriteLine("Value of "+expr3+" is "+res);

    string expr4 = "1++2";
    res = evaluate(expr4);
    if(res == -1)
    Console.WriteLine(expr4+" is Invalid");
    else
    Console.WriteLine("Value of "+expr4+" is "+res);
}
}
// This code is contributed by mits

PHP

<?php
// PHP program to evaluate a
// given expression

// A utility function to check if
// a given character is operand
function isOperand($c)
{
    return ($c >= '0' && $c <= '9');
}

// utility function to find
// value of and operand
function value($c)
{
    return ($c - '0');
}

// This function evaluates simple
// expressions. It returns -1 if the
// given expression is invalid.
function evaluate($exp)
{
    $len = strlen($exp);

    // Base Case: Given expression is empty
    if ($len == 0) return -1;

    // The first character must be
    // an operand, find its value
    $res = (int)(value($exp[0]));

    // Traverse the remaining
    // characters in pairs
    for ($i = 1; $i < $len; $i += 2)
    {
        // The next character must be
        // an operator, and next to
        // next an operand
        $opr = $exp[$i];
        $opd = $exp[$i + 1];

        // If next to next character
        // is not an operand
        if (!isOperand($opd))
        return -1;

        // Update result according
        // to the operator
        if ($opr == '+')
        $res += value($opd);
        else if ($opr == '-')
            $res -= (int)(value($opd));
        else if ($opr == '*')
            $res *= (int)(value($opd));
        else if ($opr == '/')
            $res /= (int)(value($opd));

        // If not a valid operator
        else
        return -1;
    }
    return $res;
}

// Driver Code
$expr1 = "1+2*5+3";
$res = evaluate($expr1);
($res == -1) ? print($expr1." is Invalid\n"):
               print("Value of " . $expr1 .
                     " is " . $res . "\n");

$expr2 = "1+2*3";
$res = evaluate($expr2);
($res == -1) ? print($expr2." is Invalid\n"):
               print("Value of " . $expr2 .
                     " is " . $res . "\n");

$expr3 = "4-2+6*3";
$res = evaluate($expr3);
($res == -1) ? print($expr3." is Invalid\n"):
               print("Value of " . $expr3 .
                     " is " . $res . "\n");

$expr4 = "1++2";
$res = evaluate($expr4);
($res == -1) ? print($expr4." is Invalid\n"):
               print("Value of " . $expr4 .
                     " is " . $res . "\n");

// This code is contributed by mits
?>

Javascript

<script>
// Javascript program to evaluate a given expression

// A utility function to check if
// a given character is operand
function isOperand(c)
{
    return (c.charCodeAt(0) >= '0'.charCodeAt(0) && c.charCodeAt(0) <= '9'.charCodeAt(0));
}

// utility function to find value of and operand
function value(c)
{
    return (c.charCodeAt(0) - '0'.charCodeAt(0));
}

// This function evaluates simple expressions.
// It returns -1 if the given
// expression is invalid.
function evaluate(exp)
{
    // Base Case: Given expression is empty
    if (exp.length == 0) return -1;

    // The first character must be
    // an operand, find its value
    let res = value(exp[0]);

    // Traverse the remaining characters in pairs
    for (let i = 1; i<exp.length; i += 2)
    {
        // The next character must be an operator, and
        // next to next an operand
        let opr = exp[i], opd = exp[i+1];

        // If next to next character is not an operand
        if (isOperand(opd) == false) return -1;

        // Update result according to the operator
        if (opr == '+') res += value(opd);
        else if (opr == '-') res -= value(opd);
        else if (opr == '*') res *= value(opd);
        else if (opr == '/') res /= value(opd);

        // If not a valid operator
        else                 return -1;
    }
    return res;
}

// Driver program to test above function
let expr1 = "1+2*5+3";
let res = evaluate(expr1);
if(res == -1) document.write(expr1+" is Invalid<br>");
else     document.write("Value of "+expr1+" is "+res+"<br>");

let expr2 = "1+2*3";
res = evaluate(expr2);
if(res == -1) document.write(expr2+" is Invalid<br>");
else         document.write("Value of "+expr2+" is "+res+"<br>");

let expr3 = "4-2+6*3";
res = evaluate(expr3);
if(res == -1) document.write(expr3+" is Invalid<br>");
else         document.write("Value of "+expr3+" is "+res+"<br>");

let expr4 = "1++2";
res = evaluate(expr4);
if(res == -1) document.write(expr4+" is Invalid<br>");
else         document.write("Value of "+expr4+" is "+res+"<br>");


// This code is contributed by avanitrachhadiya2155
</script>

Output:

Value of 1+2*5+3 is 18
Value of 1+2*3 is 9
Value of 4-2+6*3 is 24
1++2 is Invalid

Time Complexity: O(|exp|)

Auxiliary Space: O(1)

The above code doesn’t handle spaces. We can handle spaces by first removing all spaces from the given string. A better solution is to handle spaces in single traversal. This is left as an exercise.
Time Complexity is O(n) where n is length of the given expression.
This article is contributed by Abhishek. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above