Here, I am trying to find index of negative numbers in an array in JavaScript? Please help?

 Case1: var arr=[1,-5,-6,7,8,9,10,-14,6,-7];  

/*---------logic to find index of negative numbers in an array?*/

 console.log();

 Case2: var arr=["1","-3","5","-9","-10","11"]
 /*------logic to find index of negative numbers from array of strings(arr)?*/

console.log();

 Case3: var arr=["1","-3M" ,"2P","-8Q","-10.8%","-$8.00M"]

 /*----------How to find index of "-" in an array of strings----------*/

    console.log()

Here, I am trying to find index of negative numbers in an array in JavaScript? Please help?

 Case1: var arr=[1,-5,-6,7,8,9,10,-14,6,-7];  

/*---------logic to find index of negative numbers in an array?*/

 console.log();

 Case2: var arr=["1","-3","5","-9","-10","11"]
 /*------logic to find index of negative numbers from array of strings(arr)?*/

console.log();

 Case3: var arr=["1","-3M" ,"2P","-8Q","-10.8%","-$8.00M"]

 /*----------How to find index of "-" in an array of strings----------*/

    console.log()

• javascript

• 1 Did you try anything ? What approach did you try that did not work ? – G4bri3l Commented Jul 22, 2017 at 17:10
• The indexes of all negatives, or the index of any negative? What do you want to do with them once found? – Paul Commented Jul 22, 2017 at 17:12

7 Answers 7

You could map the indices with the index only for negative numbers and then filter only the ones with valid indices.

var array = [1, -5, -6, 7, 8, 9, 10, -14, 6, -7],
    indices = array
        .map((a, i) => a < 0 ? i : -1)
        .filter(a => a !== -1);

console.log(indices);

Nearly the same for strings and a check if the first character is -.

var array = ["1", "-3M", "2P", "-8Q", "-10.8%", "-$8.00M"],
    indices = array
        .map((a, i) => a[0] === '-' ? i : -1)
        .filter(a => a !== -1);

console.log(indices);

And now a solution for numbers or strings with toString method.

var array = ["1", -3, "2P", "-8Q", "-10.8%", "-$8.00M"],
    indices = array
        .map((a, i) => a.toString()[0] === '-' ? i : -1)
        .filter(a => a !== -1);

console.log(indices);

Use the Array.forEach() looping method and regular expressions.

Here's a single function that will handle all 3 of your scenarios.

var arr = [1,-5,-6,7,8,9,10,-14,6,-7];
var arr2 = ["1","-3","5","-9","-10","11"];
var arr3 = ["1","-3M" ,"2P","-8Q","-10.8%","-$8.00M"];

function getNegatives(arr){
  var results =[];
  arr.forEach(function(item, index){
    // If item is a string, remove all non-numeric characters and then convert to a number
    var newItem = (typeof item === "string") ? parseFloat(item.replace(/[^-\d\.]/g,'')) : item;
    if(newItem < 0){ results.push(index); }
  });
  console.log(results);
}

getNegatives(arr);
getNegatives(arr2);
getNegatives(arr3);

You can do it using for loop and if condition.

var arr = [1,-5,-6,7,8,9,10,-14,6,-7];
var negativeNumbersIndexs = [];
var i; // for the loop
for (i = 0; i < arr.length; i += 1) {
    if (arr[i] < 0){
        negativeNumbersIndexs.push(i)
    }
}

console.log(negativeNumbersIndexs);

You can use for like this.

var i;
for(i = 0; i < arr.length; i++) {
  if(arr[i] < 0) {
    console.log("Index " + i);
  }
} 

Try this code.

var arr = [1,-5,-6,7,8,9,10,-14,6,-7];
for(var i = 0; i < arr.length; i++) {
    if(arr[i] < 0) {
      console.log(arr[i] +  " is at index: " +  i);
    }
}

var arr=[1,-5,-6,7,8,9,10,-14,6,-7];
var res = arr.reduce( function(out, a, i) {
   return a<0 ? out.concat([i]) : out;
},[]);
console.log( res );

Case1: finding index of negative numbers in an array of numbers:

for(var i=0; i < arr.length; i++){
    if(arr[i] < 0){
       console.log(i);
    }
}

Case2: finding index of negative numbers in an array of strings:

for(var i=0; i < arr.length; i++){
    if(Number(arr[i]) < 0){
       console.log(i);
    }
}

Case3: finding index of negative "-" in an array of strings:

for(var i=0; i < arr.length; i++){
    if(arr[i].charAt(0) === "-"){
       console.log(i);
    }
}

Note: you can use solution of case3 in case2 also it will work fine in both cases.