is a before a container the same as moving the index by one?-CodePudding

I am unfamiliar with this syntax: fCount[index]. Where list is another vector.

I was thinking it was the same as below, but its not:

int i = 0;
vector<int> fCount(1001,0);
for(auto index : list)
{
    fCount[i] = index;
    i  ;
}

piece of code:

vector<int> fCount(1001,0);
for(auto index : list)
{
      fCount[index];
}

CodePudding user response：

vector::operator[] returns a reference to an element at a given index.

The increment operator increments the value of a variable.

The two codes examples you have shown are NOT equivalent.

The first code is looping through list, assigning each of its elements as-is to sequential elements of fCount. A range-for loop does not provide access to the indexes of the elements being iterated through, so a separate i variable is being used to index into fCount, where i is initialized to index 0, and i increments the value of i by 1 on each loop iteration. For example:

vector<int> list = {5, 10, 15, 20, ...};

int i = 0;
vector<int> fCount(1001,0);
for(auto index : list)
{
    fCount[i] = index;
    i  ;
}

This is effectively filling fCount like this:

vector<int> fCount(1001,0);
fCount[0] = 5;
fCount[1] = 10;
fCount[2] = 15;
fCount[3] = 20;
...

The second code is using each element of list as an index into fCount, using the operator to increment the value of each indexed element of fCount. This is because fCount[index] is using the prefix increment operator, which has a lower precedence than vector::operator[], so fCount[index] is parsed as (fCount[index]), not as ( fCount)[index] (IOW, fCount is indexed into first, and then the increment is applied to what operator[] returns). And since fCount is initialized with 0s before the loop, after the loop finishes then every element that was indexed will have a value of exactly 1. For example:

vector<int> list = {5, 10, 15, 20, ...};

vector<int> fCount(1001,0);
for(auto index : list)
{
      fCount[index];
}

This is effectively filling fCount like this:

vector<int> fCount(1001,0);
fCount[5]  = 1;
fCount[10]  = 1;
fCount[15]  = 1;
fCount[20]  = 1;
...