I'm trying to turn a long list of objects (accounts) into smaller arrays of 6 or less. I'm doing this because I have a table in my react component that is listing all the accounts, however, it cannot hold any more than 6 accounts. I created the pagination and everything, and thought that it would work, but the current page doesn't ever seem to update.
const [pageNum, setPageNum] = useState(1);
const [numOfPages, setNumOfPages] = useState(Math.ceil(accounts.length / 6));
const [page, setPage] = useState([]);
const [pages, setPages] = useState([null]);
const onClick = (e) => {
setPageNum(pageNum + 1);
setPage(pages[pageNum]);
};
useEffect(() => {
if (numOfPages > 1) {
for (let i = 0; i < 6; i++) {
setPages(pages.push(accounts.slice(i * 6, i * 6 + 6)));
}
}
console.log(pages[1][1]);
console.log(pages[2][2]);
setPage([pages[1][1], pages[1][2]]);
console.log(page);
}, []);
This isn't the final code, but rather part of my troubleshooting and I also think its where the issue is coming up. When I console log pages[1][1] and pages[1][2], I get two separate objects. So how come after using setPage and placing the two 'objects' inside of an array, does the console log of page come out as an empty array?? I've tried refactoring so many things. I've changed pages to an object instead of an array, i've tried doing it in another method instead of useEffect, all kinds of things. But the root of the issue seems to be that often my 'setPage' and my 'setPageNum' methods fail for seemingly no reason.
Prepare the pages array with useMemo (it will only change when accounts changes).
const { useState, useMemo, useCallback } = React;
const Pager = ({ accounts }) => {
const [pageNum, setPageNum] = useState(0);
const pages = useMemo(() => {
const pages = [];
for (let i = 0; i < accounts.length; i += 6) {
pages.push(accounts.slice(i, i + 6));
}
return pages;
}, [accounts]);
// console.log(pages);
const numOfPages = pages.length;
const page = pages[pageNum];
const next = useCallback(() => {
setPageNum(pageNum => pageNum + 1);
}, []);
const prev = useCallback(() => {
setPageNum(pageNum => pageNum - 1);
}, []);
return (
<div>
<button disabled={pageNum === 0} onClick={prev}>Prev</button>
<button disabled={pageNum === numOfPages - 1} onClick={next}>Next</button>
<ul>
{pages[pageNum].map((s, i) => <li key={i}>{s}</li>)}
</ul>
</div>
);
};
const accounts = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];
ReactDOM.render(
<Pager accounts={accounts} />,
root
)
<script crossorigin src="https://unpkg.com/react#16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#16/umd/react-dom.development.js"></script>
<div id="root"></div>
The main problem in your code is the way you use setPages:
setPages(pages.push(accounts.slice(i * 6, i * 6 + 6)));
The Array.push() returns the new length of the array. So your pages state is actually the last number. It's the last number because setState replaces the state, and doesn't add it.
The term you are looking for is chunk.
Since the data you are chunking comes from props, you don't need chunk inside the useEffect hook. It's not a side effect.
Here is a chunk function I found on the internet:
function chunkArray(myArray, chunk_size){
var index = 0;
var arrayLength = myArray.length;
var tempArray = [];
for (index = 0; index < arrayLength; index += chunk_size) {
myChunk = myArray.slice(index, index+chunk_size);
// Do something if you want with the group
tempArray.push(myChunk);
}
return tempArray;
}
Related
I try to insert a string into a particular index of object if condition is true inside a forloop but its not inserting of some reason. I tried to use push and append and splice but splice just inserting entire string as an new object into the array and i need it to just append to existing object. Any ideas how to make it work?
Data looks like that:
const [concerts, setConcerts] = useState([]);
const [tickets, setTickets] = useState([]);
const [limit, setLimit] = useState(25);
const navigate = useNavigate();
const [button, setButton] = useState(false);
const [array, setArray] = useState([]);
//Raw JSON Date example: "2023-02-08T23:15:30.000Z"
let currentDate = new Date().toJSON().slice(0, 10);
const json = { available: "true" };
useEffect(() => {
const loadConcerts = async () => {
const resConcerts = await axios.get("/data/concerts");
const resTickets = await axios.get("/data/tickets");
let table = [];
setTickets(resTickets.data);
// getting all concerts above today
const filteredData = resConcerts.data.filter((concert) => {
return concert.datum >= currentDate;
});
filteredData.forEach((element) => {
table.push(element);
// table.splice(10, 0, { status: "available" });
});
setArray(table);
for (let i = 0; i < resTickets.data.length; i++) {
for (let j = 0; j < filteredData.length; j++) {
if (
resTickets.data[i].concertid == filteredData[j].id &&
resTickets.data[i].booked == 0
) {
table.push({ status: "avaiable" });
// table.splice(10, 0, { status: "available" });
}
}
}
setArray(table);
// filteredData.forEach((concert) => {
// for (const ticket of tickets) {
// if (concert.id == ticket.concertid && ticket.booked == 0) {
// table.push(json);
// }
// }
// });
setConcerts(
filteredData.sort((a, b) =>
a.datum > b.datum ? 1 : a.datum < b.datum ? -1 : 0
)
);
};
console.log("from use effect: " + array.length);
loadConcerts();
}, []);
After using splice method:
Update
Problem is solved. I used Object.assign() helped to append string to existing object in array. Actually i had to insert another object, not a single variable.
The problem is you are trying to push a string "available" into an array-of-objects.
Here you see the object with a property datum:
const filteredData = resConcerts.data.filter((concert) => {
return concert.datum >= currentDate;
});
Yet below when you push, you are not pushing an object into the array which is problematic. It should probably be something like this but you have to verify:
Instead of this:
filteredData.push("available");
Domething like this:
filteredData.push({ datum: '', status: 'available' );
I don't know what your data object is but it's an object not a string you need to add to that array.
The looping twice is likely from React 18 New Strict Mode Behaviors. It intentionally unmounts/remounts components to fire your useEffect calls twice - so that you can identify problematic side effects. If you remove <StrictMode> or run in production that double-looping should not occur.
Problem solved. Push() neither splice() method didn't helped. What helped me to append my object to another object without changing the data was Object.assign() function
for (let i = 0; i < resTickets.data.length; i++) {
for (let j = 0; j < filteredData.length; j++) {
if (
resTickets.data[i].concertid == filteredData[j].id &&
resTickets.data[i].booked == 0
) {
Object.assign(filteredData[j], obj);
}
}
}
const [lnames, setlNames] = React.useState();
const [lnums, setlNums] = React.useState();
React.useEffect(() => {
axios.get("http://localhost:7001/lunch").then(response => {
let arr1 = [];
let arr2 = [];
response.data.forEach(c => {
arr1.push(c.table_id);
arr2.push(c.table_num_count);
});
setlNames(arr1);
setlNums(arr2);
});
}, []);
const [dnums, setdNums] = React.useState();
React.useEffect(() => {
axios.get("http://localhost:7001/dinner").then(response => {
let arr1 = [];
response.data.forEach(c => {
arr1.push(c.table_num_count);
});
setdNums(arr1);
});
}, []);
const [bnums, setbNums] = React.useState();
React.useEffect(() => {
axios.get("http://localhost:7001/breakfast").then(response => {
let arr1 = [];
response.data.forEach(c => {
arr1.push(c.table_num_count);
});
setbNums(arr1);
});
}, []);
const customer_count_breakfast = bnums;
const customer_count_lunch = lnums;
const customer_count_dinner = dnums;
let sumArray = []
if (lnums & lnums.length > 0) {
//sumArray = lnums.map((l, i) => l + bnums[i] + dnums[i]);
for (let i = 0; i < lnums.length; i++) {
sumArray[i] = bnums[i] + lnums[i] + dnums[i];
}
}
// IF COMMENT ABOVE AND UNCOMMENT HERE THE ERROR IS GONE BUT sumArray is not computed
// if (lnums && lnums[0]) {
// //sumArray = lnums.map((l, i) => l + bnums[i] + dnums[i]);
// for (let i = 0; i < lnums.length; i++) {
// sumArray[i] = bnums[i] + lnums[i] + dnums[i];
// }
// }
Hello I have the above code giving me the following error at the if loop condition : Uncaught TypeError: Cannot read properties of undefined (reading '0'). I think I know why the error is being displayed. It is indicating that I am trying to access a property or index of a variable that is undefined, this is most likely due to the useEffect, where when I call for the if condition the value is not set yet, so it is undefined and thus does not have a length. When I try to comment the first if condition and uncomment the second, I get no more errors but the summation does not work, it returns an empty array.
How would I be able to sum the values of .useState() variables in this case?
You need to look into react lifecycle and useMemo.
read about lifecycle: https://www.w3schools.com/react/react_lifecycle.asp
read about useMemo: https://www.w3schools.com/react/react_usememo.asp
Here is a quick example where useEffect puts numbers into an array, useMemo has dependency on those arrays and adds all the numbers together.
import { useEffect, useState, useMemo } from "react";
export default function App() {
const [arr1, setArr1] = useState([]);
const [arr2, setArr2] = useState([]);
// called everytime arr1 or arr2 changes. Which causes react to rerender.
// first run sum = 0
const sum = useMemo(() => {
let res = 0;
if (arr1.length && arr2.length) {
for (let i = 0; i < arr1.length; i++) {
res += arr1[i] + arr2[i];
}
}
return res;
}, [arr1, arr2]);
// called once, updated setArr1 causing sum to be called
useEffect(() => {
setArr1([1,2,3]);
}, []);
// called once, updated setArr2 causing sum to be called
useEffect(() => {
setArr2([4,5,6]);
}, []);
return (
<div>
<p> sum: {sum} </p>
</div>
);
}
Essentially In your code you are calling the arrays before useEffect has finished setting their values.
I solved the issue by adding a try-catch statement, not sure if it is feasible at all
try {
for (let i = 0; i < lnums.length; i++) {
sumArray[i] = bnums[i] + lnums[i] + dnums[i];
}
console.log("Statement was successful");
} catch (error) {
}
I have a simple code which iterates through an array and logs them in the interval of 1000 ms as such :
const arr = [1, 2, 3];
let i = 0;
const choice = () => {
const interval = setInterval(() => {
console.log(arr[i++ % arr.length]);
if (i === 8) {
clearInterval(interval);
}
}, 1000);
};
choice();
Upon introducing React state into the code, the whole thing goes absolutely mad and starts counting out of the interval to a point that I reach almost infinite loop although the simple console.log instead of react state works fine.
const [ele, setEle] = React.useState(null);
const arr = [1, 2, 3];
let i = 0;
const choice = () => {
const interval = setInterval(() => {
setEle(arr[i++ % arr.length]);
if (i === 8) {
clearInterval(interval);
}
}, 1000);
};
choice();
return(
<h1>{ele}</h1>
)
I wonder how I can achieve the effect using the state with the current code.
https://codesandbox.io/s/condescending-dubinsky-kbl5m
With your current implementation, every render of the component is initializing a new interval, so you get lots of intervals running simultaneously, each of which initiate even more intervals. Use setTimeout instead, so that every render only initializes exactly one action to take place in the future.
Since it's the index of the array that changes each time, consider using that as state instead:
const App = () => {
const [i, setI] = React.useState(0);
const arr = [1, 2, 3];
if (i !== 8) {
setTimeout(() => {
setI(i + 1);
}, 1000);
}
return (
<div className="App">
<h1>{arr[i % arr.length]}</h1>
</div>
);
}
ReactDOM.render(
<App />,
document.body
);
<script crossorigin src="https://unpkg.com/react#16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#16/umd/react-dom.development.js"></script>
I found the other answer somewhat unsatisfying because it requires a new timeout to be generated each time. It is possible to set up a single setInterval which requires a useRef to get an up-to-date setI into the timer. The empty useEffect dependencies ensures the setInterval is not re-run on each state update.
const App = () => {
const [i, setI] = React.useState(0);
const timer = React.useRef();
timer.current = () => setI(i+1);
React.useEffect(() => {
const interval = setInterval(() => {
timer.current();
}, 1000);
return () => {
clearInterval(interval)
};
}, [])
return <div>{i}</div>;
}
I am trying to use hooks to add numbers from an array together. Currently it's a voting system. The result of adding all the numbers using a map statement gives me 0. I am pretty sure this has to do with useState not updating in time to add the numbers, therefore it's always giving me zero. I know I could put them in a separate array, and add that, but that seems a little verbose for something that would seem so simple.
Here is the code I have that produces 0
const PollResultsContainer = (props) => {
const option = props.option
const [totalVotes, setTotalVotes] = useState(0)
useEffect(() => {
let newVote
if (option.length > 0) {
option.map(opt => {
newVote = opt.optionVotes + totalVotes
})
}
setTotalVotes(newVote)
}, [totalVotes])
console.log(totalVotes)
return (
<>
<div className='poll-results-div'>
<TitleCardNS title={`${totalVotes}`} size='2.5rem' align='center' color='white' />
</div>
There is not need to store it in a state.
const PollResultsContainer = ({option}) => {
let totalVotes = option.reduce((acc, {optionVotes}) => acc + optionVotes, 0);
console.log(totalVotes);
};
I don't think you need any state or effect in this component :
const PollResultsContainer = (props) => {
const option = props.option
let totalVotes = 0;
if (option.length > 0) {
option.forEach(opt => {
totalVotes += opt.optionVotes
})
}
console.log(totalVotes)
I added another variable into the equation. This probably solved the problem of useState not updating in time.
const PollResultsContainer = (props) => {
const option = props.option
const [totalVotes, setTotalVotes] = useState(0)
let newVote = 0
useEffect(() => {
if (option.length > 0) {
option.map(opt => {
newVote = opt.optionVotes + newVote
console.log(newVote)
})
}
console.log(newVote)
setTotalVotes(newVote)
}, [totalVotes])
console.log(totalVotes)
I am building a react project for visualizing insertion sort using redux. I am using react-redux to create and handle actions. However, the problem is that in my insertionSort algorithm, I dispatch an updateArray action every time the array being sorted changes. I put print statements inside the reducer and saw that the state was in fact changing and the action was being dispatched correctly, however, my actual array does not re-render. I put prints inside the relevant UI component's render() function and saw that it was only being called once or twice rather than every time the reducer receives the action. I tried restructuring my code multiple times and reading about similar problems that people have had but their answers did not help me.
Am I just structuring this the wrong way? Should I not be using dispatches every second or so to update my array?
I have a main.js file which is used to render the UI components including my array:
class Main extends React.Component {
setArray = () => {
this.props.setArray(50, window.innerHeight / 1.4)
startSort = () => {
this.props.startSorting(this.props.algorithm, this.props.array)
}
render() {
let { array} = this.props
return (
<div>
<Navbar
startSort={this.startSort}
setArray={this.setArray}
/>
<MainWrapper>
<Row />
</MainWrapper>
</div>
)
}
}
const mapStateToProps = state => {
return {
array: state.array,
}
}
const mapDispatchToProps = dispatch => {
return {
setArray: (length, height) => {
let array = Array.from({ length: length }, () =>
Math.floor(Math.random() * height))
dispatch(setArray(array))
},
startSorting: (algorithm, array) => {
var doSort
if (algorithm == 'insertionSort') {
doSort = insertionSort
}
doSort(array, dispatch)
}
}
}
My actual array is generated with Row.js
class Row extends React.Component {
generateNodes(array) {
var elements = []
array.forEach((value, index) => {
elements.push(
<CenteredColumn>
<ArrayNode idx={index} value={value} />
</CenteredColumn>
)
})
return elements
}
render() {
let { array } = this.props
console.log('UPDATED ARRAY: ' + array)
var arrayElements = this.generateNodes(array)
return <RowWrapper>{arrayElements}</RowWrapper>
}
}
const mapStateToProps = state => {
return {
array: state.array
}
}
And finally, my actual algoritm is in insertionSort.js in which I import my actions from their reducers and pass in a dispatch function from main.js:
function delayedInsertion(array, dispatch) {
let n = array.length
var i = 0
function loop() {
setTimeout(function() {
var temp = array[i]
var j = i - 1
while (j >= 0 && array[j] > temp) {
array[j + 1] = array[j]
j--
}
array[j + 1] = temp
// console.log('ARRAY: ' + array)
dispatch(updateArray(array))
i++
if (i < n) {
loop()
}
}, 200)
}
loop()
console.log('DONE')
}
It seems that you are mutating your state.
You are passing this.props.array to your doSort action and as I understand your idea correctly, you are just calling delayedInsertion from that action (you did not post source code of that action).
But in delayedInsertion you are mutating the passed array when you are changing positions of you items, here:
while (j >= 0 && array[j] > temp) {
array[j + 1] = array[j]
j--
}
array[j + 1] = temp
You need to perform immutable change of the array.