# Evaluating prefix/infix/postfix expressions

In yesterday’s post, I talked about three different ways to write expressions: prefix, infix, and postfix expressions. I also promised to write up a web-based example that would show the guts of each algorithm in action. Well, here it is!

Use the three buttons at the top to switch between the different machines. Enter an expression in the box and click run to evaluate it. The only things that are supported at the moment are numbers (integers or floating point) and the operators +, -, *, and /, although the code is extensible enough that adding more shouldn’t be an issue.

Have fun!

# Prefix / Scheme

## Results

 Tick Input Output Rest Error
// known operators
var operators = {
'+': function(a, b) { return a + b; },
'-': function(a, b) { return a - b; },
'*': function(a, b) { return a * b; },
'/': function(a, b) { return a / b; },
};

// process at this level
// return the result of this level and what we didn't use
// return a null value if we fail at any point
function step(current) {
// directly return numbers
if (!isNaN(parseFloat(current[0]))) {
return {
value: parseFloat(current[0]),
rest: current.slice(1)
};
}

// otherwise, we're going to have to recur
else {
var f = operators[current[0]];

// recur for left, use that rest for right
var left = step(current.slice(1));
if (left.value == null) return {value: null, rest: []};
var right = step(left.rest);
if (right.value == null) return {value: null, rest: []};

// return at my level
return {
value: f(left.value, right.value),
rest: right.rest
};
}
}
step(input);


# Infix

## Results

 Tick Command Reducing Error
// known operators
var operators = {
'+': function(a, b) { return a + b; },
'-': function(a, b) { return a - b; },
'*': function(a, b) { return a * b; },
'/': function(a, b) { return a / b; },
};
var precedence = [
['*', '/'],
['+', '-']
]

// process until we are done
while (input.length > 1) {
// find the first operator at the lowest level
var reduceAt = 0;
var found = false;
for (var i = 0; i < precedence.length; i++) {
for (var j = 1; j < input.length - 1; j++) {
if (\$.inArray(input[j], precedence[i]) >= 0) {
reduceAt = j;
found = true;
break;
}
}
if (found) break;
}

// if we didn't find one, bail
if (!found) return;

// otherwise, reduce that operator
var newInput = [];
var f = operators[input[reduceAt]];

for (var i = 0; i < reduceAt - 1; i++)
newInput.push(input[i]);

newInput.push("" + f(
parseFloat(input[reduceAt - 1]),
parseFloat(input[reduceAt + 1])
));

for (var i = reduceAt + 2; i < input.length; i++)
newInput.push(input[i]);

input = newInput;
}


# Postfix / RPN

## Results

 Tick Command Stack Error
// known operators
var operators = {
'+': function(a, b) { return a + b; },
'-': function(a, b) { return a - b; },
'*': function(a, b) { return a * b; },
'/': function(a, b) { return a / b; },
};

// run through all commands in the input
var stack = [];
for (var i = 0; i < input.length; i++) {
var cmd = input[i];

// known operator
if (cmd in operators) {
// get the function
var f = operators[cmd];

// sanity check
if (stack.length < f.length) {
error = 'not enough arguments';
break;
}

// get the correct number of arguments
var args = [];
for (var j = 0; j < f.length; j++)
args.unshift(stack.shift());

// apply and push back onto the stack
// note: the first argument to apply is 'this'
stack.unshift(f.apply(undefined, args));
}

// anything else, push onto the stack as either a number or string
else {
stack.unshift(isNaN(parseFloat(cmd)) ? cmd : parseFloat(cmd));
}
}


Hopefully this helps give some insight into what I was talking about in yesterday’s post. I have to admit, I’m actually starting to like Javascript. It’s a bit strange at times, but it does have a nice functional flavor which is always fun.

If you’d like to download the entire source code, you can do so here: source code