JS string learning: Calculate all occurrence positions of a given character

In the previous article "How does js know whether a given substring exists", we introduced the method of obtaining the first or last occurrence position of the substring in the string. This time we increase the difficulty and see how to get all occurrences of a substring in a string.

We learned in the previous article that the indexOf() and lastIndexOf() functions can be used to obtain the first occurrence position and the last occurrence position of the substring; if the substring is not found, -1 is returned. .

Both functions can receive the optional second parameter start, the value can only be an integer, indicating the position in the string to start searching, and the value range It’s 0 ~ length-1. lastIndexOf() If the start parameter is specified, searches from back to front at the specified position in a string.

Using this optional second parameter start and the loop statement, we can calculate all occurrences of a given substring (containing one or more characters).

First look at how to use indexOf() to count all occurrences of a given character in a string

var stringValue = "Lorem ipsum dolor sit amet, consectetur adipisicing elit";
var positions = new Array();
var pos = stringValue.indexOf("e");

while(pos > -1){
	positions.push(pos);  //将出现位置赋给数组
	pos = stringValue.indexOf("e",pos + 1);  //从给定字符出现位置的后一位开始查找
}

console.log(positions);//"3,24,32,35,52"

Analysis: First use indexOf("e") to obtain The first occurrence position is assigned to the variable pos; then the while statement is used to call indexOf() through a loop, and the search starting position is constantly set to "pos 1", starting from the end of the given character position pos Start searching with one bit and get the next occurrence position; until the search cannot be found, -1 is returned. Therefore, the output result is:

It can be seen that the given character "e" appears 5 times in the string stringValue, because the string position It starts at 0, not 1, so the occurrence positions are 3, 24, 32, 35, and 52 respectively.

Then let’s take a look at how to use lastIndexOf() to count all occurrences of a given character in a string

var stringValue = "Lorem ipsum dolor sit amet, consectetur adipisicing elit";
var positions = new Array();
var pos = stringValue.lastIndexOf("e");

while(pos > -1){
	positions.push(pos);
	pos = stringValue.lastIndexOf("e",pos - 1);
}
console.log(positions);//"3,24,32,35,52"

Analysis: Using lastIndexOf() to obtain all occurrences is actually the same as using indexOf( ) is almost the same, except that lastIndexOf() returns the last occurrence position, so every time we loop, we have to set the starting position of the search to "pos - 1".

Look at the output results:

It can be seen that the obtained given character "e" is in reverse order in the string stringValue. , the order is: 52, 35, 32, 24, 3.

Okay, that’s all. If you need it, you can read: javascript advanced tutorial

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