How can I change the value of the elements in a vector

Question

I have this code  which reads in input from a file and stores it in a vector  So far  I ve gotten it to give me the sum of the values within the vector and give the mean of the values using the sum    What I d like to do now is learn how to access the vector again and subtract a value from each element of the vector and then print it out again  For example  once the sum and mean are calculated  I d like to be able to reprint each value in the terminal minus the mean  Any suggestions examples    include  lt iostream gt   include  lt vector gt   include  lt fstream gt   include  lt cmath gt   using namespace std   int main         fstream input      input open  input txt        double d      vector lt double gt  v      cout  lt  lt   The values in the file input txt are     lt  lt  endl      while  input  gt  gt  d                cout  lt  lt  d  lt  lt  endl          v push back d          double total   0 0  double mean   0 0  double sub   0 0  for  int i   0  i  lt  v size    i          total    v i       mean   total   v size        sub   v i  -  mean    cout  lt  lt   The sum of the values is     lt  lt  total  lt  lt  endl  cout  lt  lt   The mean value is     lt  lt  mean  lt  lt  endl  cout  lt  lt  sub  lt  lt  endl

User · Accepted Answer

You can simply access it like an array i e  v i    v i  - some num

User · Answer

Your code works fine   When I ran it I got the output   The values in the file input txt are  1 2 3 4 5 6 7 8 9 10 The sum of the values is  55 The mean value is  5 5   But it could still be improved   You are iterating over the vector using indexes   This is not the  STL Way  -- you should be using iterators  to wit   typedef vector lt double gt  doubles  for  doubles  const iterator it   v begin    it end   v end    it    it end    it         total     it      mean   total   v size        This is better for a number of reasons discussed here and elsewhere  but here are two main reasons    Every container provides the iterator concept   Not every container provides random-access  eg  indexed access   You can generalize your iteration code    Point number 2 brings up another way you can improve your code   Another thing about your code that isn t very STL-ish is the use of a hand-written loop    lt algorithm gt s were designed for this purpose  and the best code is the code you never write   You can use a loop to compute the total and mean of the vector  through the use of an accumulator    include  lt numeric gt   include  lt functional gt  struct my totals   public std  binary function lt my totals  double  my totals gt        my totals     total  0   count  0          my totals operator  double v  const               my totals ret    this          ret total     v            ret count           return ret            double mean   const   return total  count         double total       unsigned count           and then   my totals ttls   std  accumulate v begin    v end    my totals     cout  lt  lt   The sum of the values is     lt  lt  ttls total   lt  lt  endl  cout  lt  lt   The mean value is     lt  lt  ttls mean    lt  lt  endl    EDIT   If you have the benefit of a C  0x-compliant compiler  this can be made even simpler using std  for each  within  include  lt algorithm gt   and a lambda expression   double total   0  for each  v begin    v end      amp total  double  v    total    v      cout  lt  lt   The sum of the values is     lt  lt  total  lt  lt  endl  cout  lt  lt   The mean value is     lt  lt  total v size    lt  lt  endl

User · Answer

You might want to consider using some algorithms instead      read in the data  std  copy std  istream iterator lt double gt  input              std  istream iterator lt double gt               std  back inserter v     sum   std  accumulate v begin    v end    0   average   sum   v size      You can modify the values with std  transform  though until we get lambda expressions  C  0x  it may be more trouble than it s worth   class difference        double base  public      difference double b    base b         double operator   double v    return v-base        std  transform v begin    v end    v begin    difference average

User · Answer

Well  you could always run a transform over the vector   std  transform v begin    v end    v begin     mean  int i  - gt  int   return i - mean        You could always also devise an iterator adapter that returns the result of an operation applied to the dereference of its component iterator when it s dereferenced   Then you could just copy the vector to the output stream   std  copy adapter v begin     mean  int i  - gt    return i - mean      v end    std  ostream iterator lt int gt  cout    n       Or  you could use a for loop   but that s kind of boring

User · Answer

You can access the values in a vector just as you access any other array   for  int i   0  i  lt  v size    i                 v i  -  1

User · Answer

Just use   for  int i   0  i  lt  v size    i          v i  -  valueToSubstract      Or its equivalent  and more readable     for  int i   0  i  lt  v size    i        v i    v i  - valueToSubstract

User · Answer

int main       using namespace std     fstream input   input txt      if   input  return 1     vector lt double gt  v    for  double d  input  gt  gt  d         v push back d         if  v empty    return 1     double total   std  accumulate v begin    v end    0 0     double mean   total   v size       cout  lt  lt   The values in the file input txt are  n     for  vector lt double gt   const iterator x   v begin    x    v end      x        cout  lt  lt   x  lt  lt    n         cout  lt  lt   The sum of the values is     lt  lt  total  lt  lt    n     cout  lt  lt   The mean value is     lt  lt  mean  lt  lt    n     cout  lt  lt   After subtracting the mean  The values are  n     for  vector lt double gt   const iterator x   v begin    x    v end      x        cout  lt  lt   x - mean  lt  lt    n       outputs without changing      x -  mean      changes the values in the vector        return 0

[c++] How can I change the value of the elements in a vector?

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