I am learning JS, and I have homework. I am asked to transform array into new array where each item is represented by the running count of element appearances.
For example
[1, 2, 1, 1, 3]
becomes
[1, 1, 2, 3, 1]
I wrote a code which works for numbers, but fails tests with strings:
UPDATE: IT works for some numbers, for others does not :/
function duplicates(arr) {
let i, j, newArr = [],
count = 1;
for (i = 0; i < arr.length; i++) {
for (j = 0; j < arr.length; j++) {
if (i == j) {
continue
}
if (arr[i] === arr[j]) {
newArr.push(count++)
break
}
}
if (j === arr.length) {
newArr.push(1)
}
}
return newArr
}
console.log(duplicates(['a', 'a', 'aa', 'a', 'aa'])) //[ 1, 2, 1, 3, 2] <-- FAILS
console.log(duplicates([1, 2, 1, 2, 3, 1])) //[1, 2, 3, 4, 1, 5] <-- fails
console.log(duplicates([1, 2, 1, 1, 3])) //[ 1, 1, 2, 3, 2, 1 ] <-- MY CODE WORKSCan you give me a hint? :/
Thank you!
One approach is to use .map(), .slice() and .filter()
const duplicates = (nums) =>
nums.map((value, index) => {
const segment = nums.slice(0,index+1);
return segment.filter(v => v === value).length;
});
console.log(duplicates([1, 2, 1, 1, 3]));
console.log(duplicates([1, 2, 1, 2, 3, 1]));
console.log(duplicates(['a', 'a', 'aa', 'a', 'aa']));map creates a new array by iterating through nums and transforming each value via a functionslice is used to create a new array based on nums. In first example, the new array is [1] in first iteration, [1,2] in second, followed by [1,2,1] and so on.filter finds the items in the array from #2 that match the current value.Elaborating on @CRice and @Kaiido idea, let's create an object that is creating the count of the items while you're looping through the array:
function duplicates(arr) {
const obj = {};
let value = 0;
let newArr = [];
for (i = 0; i < arr.length; i++) {
value = arr[i];
if (obj[value]){
obj[value] = obj[value] + 1;
}
else{
obj[value] = 1;
}
newArr.push(obj[value]);
}
return newArr
}
console.log(duplicates(['a', 'a', 'aa', 'a', 'aa'])) //[ 1, 2, 1, 3, 2] <-- FAILS
console.log(duplicates([1, 2, 1, 2, 3, 1])) //[1, 2, 3, 4, 1, 5] <-- fails
console.log(duplicates([1, 2, 1, 1, 3])) //[ 1, 1, 2, 3, 2, 1 ] <-- MY CODE WORKSJS has a nice built-in, reduce, that does so in a simpler way:
const duplicates = (arr) => {
const obj = {}
return arr.reduce ( (acc,cur) => {
obj[cur] = (obj[cur])?obj[cur]+1:1
acc.push(obj[cur])
return acc
}, []);
}
console.log(duplicates(['a', 'a', 'aa', 'a', 'aa'])) //[ 1, 2, 1, 3, 2] <-- FAILS
console.log(duplicates([1, 2, 1, 2, 3, 1])) //[1, 2, 3, 4, 1, 5] <-- fails
console.log(duplicates([1, 2, 1, 1, 3])) //[ 1, 1, 2, 3, 2, 1 ] <-- MY CODE WORKS