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A C SREEDHAR REDDY
A C SREEDHAR REDDY

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Represent pair data structure using functions.

Pair

Pair is a data structure which is used to combine two values.

(1, 4)
("sreedhar", "pavan")
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Javascript does not provide this data structure. So let us implement this.

Represent pairs in JS

Pairs can be represented using an object or an array

//Represent pair using object
function pair(a,b){
    return {first: a, second: b};
}
function first(p){
    return p.first;
}
function second(p){
  return p.second;
}

//use the pair
const p = pair(12, 13)

const f = first(p)
console.log(f) // logs 12

const s = second(p)
console.log(s) // logs 13

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//Represent pair using array.
function pair(a, b) {
  return [a, b];
}
function first(p) {
  return p[0];
}
function second(p) {
  return p[1];
}

//use the pair
const p = pair(12, 13);

const f = first(p);
console.log(f); // logs 12

const s = second(p);
console.log(s); // logs 13
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But is there a way to represent pair in terms of functions without using object or array?

Think for some time before you proceed.
.
.
.
Yes there is a way

Represent pair using functions

Consider our pair function returns a function.

function pair(a, b) {
  return (value) => (value === 0 ? a : b);
}
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Now this returned function when called with 0 returns the first element and when called with 1 returns the second element.

so we can implement first and second functions as

function first(p) {
  return p(0);
}

function second(p) {
  return p(1);
}
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Finally with these functions we can represent pair using functions.

function pair(a, b) {
  return (value) => (value === 0 ? a : b);
}
function first(p) {
  return p(0);
}

function second(p) {
  return p(1);
}

//use the pair
const p = pair(12, 13);

const f = first(p);
console.log(f); // logs 12

const s = second(p);
console.log(s); // logs 13
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Conclusion

I would never implement pair using functions. But it was fun to realise that I can.
I found this in a section in SICP book.

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