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Optional parameters in the for loop
2022-07-05 15:19:00 【I'll come to C】
const arr = [1, 2, 3]
for (let i = 0; i < arr.length; i++) {
console.log(i);
}
In daily development , Many times you will use for loop , Do you know when for When the first parameter or the third parameter in the loop is not filled , What will it do ?
Situation 1 The first parameter is not filled in
var longestCommonPrefix = function(strs) {
if (strs.length === 0) return "";
let one = strs[0];
for (let i = 1; i < strs.length; i++) {
let j = 0;
// j To be less than strs The length of the string in the array starting from the second element
// And less than one, That is to say strs The length of the first element string
for (; j < strs[i].length && j < one.length; j++) {
if (strs[i][j] !== one[j]) break;
}
// the second for The first parameter of the loop is not filled , For the second for It can also be used outside the loop j Parameters
one = strs[i].substr(0, j);
if (one === "") return ""
}
return one;
};
for The first parameter in the loop is not filled , In order to in for Outside of the loop , You can also use the first parameter
Situation two The third parameter is not filled in
var romanToInt = function(s) {
const obj = {
I: 1,
IV: 4,
V: 5,
IX: 9,
X: 10,
XL: 40,
L: 50,
XC: 90,
C: 100,
CD: 400,
D: 500,
CM: 900,
M: 1000,
};
let result = 0;
for (let i = 0; i < s.length;) {
if (i + 1 < s.length && obj[s.substring(i, i + 2)]) {
result += obj[s.substring(i, i + 2)];
i += 2;
} else {
result += obj[s.substring(i, i + 1)];
i++;
}
}
return result;
};
for The third parameter in the cycle is not filled , It's for different conditions , by for Cycle through different steps .
expand
substr and substring The difference between
const str = "123456789";
console.log(str.substr(2,5)); // 34567
console.log(str.substring(2,5)); // 345
console.log(str.slice(2, 5)); // 345
console.log(str); // 123456789
substr The second parameter is length
substring The second parameter is the index value
slice Be similar to substring, The second parameter is also the index value
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