Evaluating a string in kotlin - kotlin

Question

I wrote a code in kotlin for evaluating a given string from Divison to subtraction, it works but it gives the wrong answer, for example i got a string x in the first line which have answer -215.96666 whereas the answer from code is -237.366666 idk where the error is. i saw more people using stack for doing it, but i wanted to do this way for evaluating a string
var x : String = "5+4-10x20-40/30x20+34/20"
fun div(x: String): String {
val lis1 = x.split("-").toMutableList()
for (ele1 in lis1) {
if ("/" in ele1) {
val pos1 = lis1.indexOf(ele1)
val lis2 = ele1.split("+").toMutableList()
for (ele2 in lis2) {
if ("/" in ele2) {
val pos3 = lis2.indexOf(ele2)
val lis3 = ele2.split("x").toMutableList()
for (ele3 in lis3) {
if ("/" in ele3) {
val pos4 = lis3.indexOf(ele3)
val lis4 = ele3.split("/")
val div = (lis4[0].toDouble() / lis4[1].toDouble()).toString()
lis3[pos4] = div
}
}
lis2[pos3] = lis3.joinToString("x")
}
}
lis1[pos1] = lis2.joinToString("+")
}
}
return (lis1.joinToString("-"))
}
fun mul(x : String) : String{
val lis1 = x.split("-").toMutableList()
for (ele1 in lis1) {
if("x" in ele1){
val pos1 = lis1.indexOf(ele1)
val lis2 = ele1.split("+").toMutableList()
for(ele2 in lis2){
if("x" in ele2){
val pos2 = lis2.indexOf(ele2)
val lis3 = ele2.split("x")
val mul = (lis3[0].toDouble() * lis3[1].toDouble()).toString()
lis2[pos2] = mul
}
}
lis1[pos1] = lis2.joinToString("+")
}
}
return (lis1.joinToString("-"))
}
fun add(x : String): String {
val lis1 = x.split("-").toMutableList()
for(ele1 in lis1){
if("+" in ele1){
val pos1 = lis1.indexOf(ele1)
val lis2 = ele1.split("+")
val add = (lis2[0].toDouble() + lis2[1].toDouble()).toString()
lis1[pos1] = add
}
}
return (lis1.joinToString("-"))
}
fun sub(x : String) : String{
val lis1 = x.split("-").toMutableList()
var sub = 0.0
for(ele1 in lis1){
sub -= ele1.toDouble()
}
return (sub.toString())
}
fun eval(x: String): String {
val divanswer = div(x)
val mulanswer = mul(divanswer)
val addanswer = add(mulanswer)
return sub(addanswer)
}
fun main(args: Array<String>){
println(eval(x))
}
i wanted the answer as -215.96666 but the answer given by code is -237.366666

Answer 1

In the case of this specific input, there are two bugs involved.
First bug is related to the fact you first process + and then -, but in mathematics we should process them together, from left to right. As a result, your algorithm processes this: 1-1+1 (correct answer: 1) as: 1-(1+1) (answer: -1). Alternatively, I believe handling - first and then + should work properly.
Second bug is that in sub() you start from 0 and subtract even the first number, but the first number should be actually added or used as an initial value.
I believe after fixing these two bugs, it should provide a correct answer for this specific input, but there are probably more bugs like these. For example, you incorrectly handle chains of the same operator, you only care about the first pair, so 1+1+1 becomes 2 (and then it is turned to -2 due to earlier mentioned bug).
As a general advice, learn to use a debugger and analyze how your data changes while your code is processing it. Even if you simply print your intermediate results: divanswer, mulanswer and addanswer and then run them through Google for processing, you will notice there is a problem somewhere at the add() step. Then rinse and repeat until everything works correctly.

Answer 2

Thanks to #broot for spotting this, the problem was with the addition and subtraction part, now that i merged them into one and got the correct answers for strings i input, here is the full code corrected.
var x : String = "5+4-10x20-40/30x20+34/20"
fun div(x: String): String {
val lis1 = x.split("-").toMutableList()
for (ele1 in lis1) {
if ("/" in ele1) {
val pos1 = lis1.indexOf(ele1)
val lis2 = ele1.split("+").toMutableList()
for (ele2 in lis2) {
if ("/" in ele2) {
val pos3 = lis2.indexOf(ele2)
val lis3 = ele2.split("x").toMutableList()
for (ele3 in lis3) {
if ("/" in ele3) {
val pos4 = lis3.indexOf(ele3)
val lis4 = ele3.split("/")
val div = (lis4[0].toDouble() / lis4[1].toDouble()).toString()
lis3[pos4] = div
}
}
lis2[pos3] = lis3.joinToString("x")
}
}
lis1[pos1] = lis2.joinToString("+")
}
}
return (lis1.joinToString("-"))
}
fun mul(x : String) : String{
val lis1 = x.split("-").toMutableList()
for (ele1 in lis1) {
if("x" in ele1){
val pos1 = lis1.indexOf(ele1)
val lis2 = ele1.split("+").toMutableList()
for(ele2 in lis2){
if("x" in ele2){
val pos2 = lis2.indexOf(ele2)
val lis3 = ele2.split("x")
val mul = (lis3[0].toDouble() * lis3[1].toDouble()).toString()
lis2[pos2] = mul
}
}
lis1[pos1] = lis2.joinToString("+")
}
}
return (lis1.joinToString("-"))
}
fun final(x: String): Double {
val substr = StringBuffer()
var answer = 0.0
var opr = '+'
for(ele in x.indices){
var current = x[ele]
if(current in '0'..'9' || current == '.'){
substr.append(current)
}
if(current == '-' || current == '+'){
var operand = substr.toString().toDouble()
substr.setLength(0)
when (opr){
'+' -> answer += operand
'-' -> answer -= operand
}
opr = current
}
}
val num = substr.toString().toDouble()
when(opr){
'+' -> answer += num
'-' -> answer -= num
}
return answer
}
fun eval(x: String): Double {
val divanswer = div(x)
return final(mul(divanswer))
}
fun main(args: Array<String>){
println(eval(x))
}