Question: ```
string[] groups;
int groupCount;
double[] grades;
int gradeCount;

```

So the `groups` and `grades` are in two separate arrays and I need to combine them as *one* `string` and add them to a new array.

```
string[] test = new string[groupCount];
for (int i = 0; i < groupCount; i++)
{
   test[i] = ("{0}:   {1}", groups[i], Math.Round(grades[i],2));              
   Console.WriteLine("{0}", test[i]);
}

```

How do i do it ?

Answer: 
You forgot `string.Format()`

It should be,

```
string.Format("{0}:   {1}", groups[i], Math.Round(grades[i], 2));

```

Hope helps,
C# 6.0 *string interpolation* (please, notice `$` before the string):

```
test[i] = $"{groups[i]}:   {Math.Round(grades[i],2)}"; 

```

Another possibility is *Linq* (in order to output the *entire* collection in *one go*):

```
string[] groups;
double[] grades;

...

var test = groups
  .Zip(grades, (group, grade) => $"{group}:    {Math.Round(grade, 2)}")
  .ToArray(); // array materialization (if you want just to output you don't need it)

Console.Write(String.Join(Environemnt.NewLine, test)); 

```