javascript - Test If The Array Index Equals The Array Value - Stack Overflow

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The code is to return the lowest index in an array when the value in the array is the same as the index. If there are no matches i should return -1. For example:

indexEqualsValue([-8,0,2,5])
output: 2 //array[2] == 2

indexEqualsValue([-1,0,3,6])
output: -1  //no matches

The code works when there are no matches or if the length of the array is zero, but not at other times. I think the problem is the first condition in my if statement. I don't necessarily want an answer, more tips on what I should check/rewrite.

Thanks!

function indexEqualsValue(a) {
    return a.reduce((acc, currV, currI) => {
      if (currI === currV) {
        return currV;
      }
      return -1;
  }, 0);
}

The code is to return the lowest index in an array when the value in the array is the same as the index. If there are no matches i should return -1. For example:

indexEqualsValue([-8,0,2,5])
output: 2 //array[2] == 2

indexEqualsValue([-1,0,3,6])
output: -1  //no matches

The code works when there are no matches or if the length of the array is zero, but not at other times. I think the problem is the first condition in my if statement. I don't necessarily want an answer, more tips on what I should check/rewrite.

Thanks!

function indexEqualsValue(a) {
    return a.reduce((acc, currV, currI) => {
      if (currI === currV) {
        return currV;
      }
      return -1;
  }, 0);
}

• javascript
• arrays
• undefined
• reduce

Share Improve this question Follow edited Sep 5, 2018 at 10:33 shokha 3,19944 gold badges2727 silver badges3838 bronze badges asked Aug 17, 2018 at 12:02 ValerieValerie 11111 silver badge88 bronze badges

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3 Answers 3

Sorted by: Reset to default 9

You could just find the index with Array#findIndex.

const indexEqualsValue = array => array.findIndex((v, i) => v === i);

console.log(indexEqualsValue([-8, 0, 2, 5])); //  2
console.log(indexEqualsValue([-1, 0, 3, 6])); // -1

some exits when it matches, so you can use it to quickly find what you need:

const indexEqualsValue = array => {
  let match;
  
  const didMatch = array.some((v, i) => {
    match = i;
    return v === i;
  })
  
  return didMatch ? match : -1;
}

console.log(indexEqualsValue([-8,0,2,5]))
console.log(indexEqualsValue([-8,0,2,5,0]))
console.log(indexEqualsValue([-1,0,3,6]))

---

nina-scholz's answer is better, the only advantage of using some over findIndex is that some is supported in ie whereas it seems findIndex is not.

for(i = 0,c=0;i < arr.length; i++ ) { 
    if(arr[i] == i) {
        c = 1;
        break; 
    }
}   
if( c == 0 ) {  
    print(c);  
} else {  
    print (i);  
}

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