How to change a particular element of a C STL vector

Question

vector lt int gt  l  for int i 1 i lt  10 i        l push back i       Now  for example  how do I change the 5th element of the vector to -1   I tried l assign 4  -1   It is not behaving as expected  None of the other vector methods seem to fit   I have used vector as I need random access functionality in my code  using l at i

User · Answer

This should do it   l 4    -1

User · Answer

I prefer  l at 4   -1    while  4  is your index

User · Answer

You can use the subscript operator  l 4    -1

User · Answer

Even though  JamesMcNellis answer is a valid one I would like to explain something about error handling and also the fact that there is another way of doing what you want   You have four ways of accessing a specific item in a vector    Using the    operator Using the member function at      Using an iterator in combination with a given offset Using std  for each from the algorithm header of the standard C   library  This is another way which I can recommend  it uses internally an iterator   You can read more about it for example here    In the following examples I will be using the following vector as a lab rat and explaining the first three methods   static const int arr      1  2  3  4   std  vector lt int gt  v arr  arr sizeof arr  sizeof arr 0       This creates a vector as seen below   1 2 3 4   First let s look at the    way of doing things  It works in pretty much the same way as you expect when working with a normal array  You give an index and possibly you access the item you want  I say possibly because the    operator doesn t check whether the vector actually has that many items  This leads to a silent invalid memory access  Example   v 10    9    This may or may not lead to an instant crash  Worst case is of course is if it doesn t and you actually get what seems to be a valid value  Similar to arrays this may lead to wasted time in trying to find the reason why for example 1000 lines of code later you get a value of 100 instead of 234  which is somewhat connected to that very location where you retrieve an item from you vector   A much better way is to use at       This will automatically check for out of bounds behaviour and break throwing an std  out of range  So in the case when we have  v at 10    9    We will get      terminate called after throwing an instance of  std  out of range    what     vector   M range check    n  which is 10     this- size      which is 4    The third way is similar to the    operator in the sense you can screw things up  A vector just like an array is a sequence of continuous memory blocks containing data of the same type  This means that you can use your starting address by assigning it to an iterator and then just add an offset to this iterator  The offset simply stands for how many items after the first item you want to traverse   std  vector lt int gt   iterator it   v begin       First element of your vector   it 0    9      offest   0 basically means accessing v begin      Now we have 9 2 3 4 instead of 1 2 3 4   it 1    -1     offset   1 means first item of v plus an additional one    Now we have 9 -1 3 4 instead of 9 2 3 4          As you can see we can also do    it 10    9    which is again an invalid memory access  This is basically the same as using at 0   offset  but without the out of bounds error checking   I would advice using at      whenever possible not only because it s more readable compared to the iterator access but because of the error checking for invalid index that I have mentioned above for both the iterator with offset combination and the    operator

User · Answer

at and operator   both return a reference to the indexed element  so you can simply use   l at 4    -1    or   l 4    -1

[c++] How to change a particular element of a C++ STL vector

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