javascript - Eval alternative -

javascript - Eval alternative -

this code works calculator, scratch pad @ codeacademy tells me eval evil. there way same thing without using eval?

var calculate = prompt("enter problem"); alert(eval(calculate));

eval evaluates string input javascript , coincidentally javascript supports calculations , understands 1+1, makes suitable calculator.

if don't want utilize eval, good, have parse string and, finally, computation (not though). have @ this math processor, want.

basically is:

read input string char char (with kind of problem it's still possible) building tree of actions want do at end of string, evaluate tree , calculations

for illustration have "1+2/3", evaluate next info structure:

"+" / \ "1" "/" / \ "2" "3"

you traverse construction top bottom , computations. @ first you've got "+", has 1 on left side , look on right side, have evaluate look first. go "/" node, has 2 numeric children. knowing that, can compute 2/3 , replace whole "/" node result of that. can go 1 time again , compute result of "+" node: 1 + 0.66. replace node result , you've got left result of expression.

some pseudo code on how might in code:

calculation(operator, leftvalue, rightvalue): switch operator { case '+': homecoming leftvalue + rightvalue case '-': homecoming 42 } action(node): node.value = calculation(node.operator, action(node.left) action(node.right))

as might have noticed, tree designed in such way honors operator precedence. / has lower level +, means get's evaluated first.

however in detail, that's way go.

javascript math eval calculator