递归中的一元运算符前缀执行未按预期工作

Question

I am trying to count number of combinations in given range and length of combinations. Here is following code.

 function generateCombination() { const perm = []; const permLength = 2; const numberRange = 8; let total = 0; const getCombinations = (result, permLength, numberRange) => { if (result.length === permLength) { //console.log('result: ', result); return 1; } for (var i = 0; i < numberRange; i++) { if (result.indexOf(i) === -1) { result.push(i); total = total + getCombinations(result, permLength, numberRange); result.pop(); } } return 0; } getCombinations(perm, permLength, numberRange); console.log("total: ", total); // expected value "total: 56" } generateCombination();

the console log for total variable always print 0. but following code works as expected with little change in for loop code. I couldn't understand how the prefix is works here (somex = somex + fn()). Can someone please help here?

 // working solution function generateCombination() { const perm = []; const permLength = 2; const numberRange = 8; let total = 0; const getCombinations = (result, permLength, numberRange) => { if (result.length === permLength) { //console.log('result: ', result); return 1; } for (var i = 0; i < numberRange; i++) { if (result.indexOf(i) === -1) { result.push(i); if (getCombinations(result, permLength, numberRange)) { total += 1; } result.pop(); } } return 0; } getCombinations(perm, permLength, numberRange); console.log("total: ", total); // expected value is "total: 56" and working here } generateCombination();

My question is, I don't understand, why solution 1 (top one) is not working as expected (printing total as 0 rather than 56 )?

Thanks

Answer 1

You could move total into getCombinations and return this value on exit.

 function generateCombination() { const perm = []; const permLength = 2; const numberRange = 8; const getCombinations = (result, permLength, numberRange) => { if (result.length === permLength) return 1; let total = 0; for (let i = 0; i < numberRange; i++) { if (result.indexOf(i) === -1) { result.push(i); total += getCombinations(result, permLength, numberRange); result.pop(); } } return total; } console.log("total: ", getCombinations(perm, permLength, numberRange)); // expected value "total: 56" } generateCombination();