How can I use a for loop to output a bool

Question

I have been trying to use a for loop to check whether an element exists inside an array. I cannot seem to figure out how to do this as all variables declared inside the for loop cannot be accessed from outside its scope. How might I go about doing this properly?

var array = new[] { 3, 4, 2, 6, 3 };
var value = 2;

bool isContained;
for(var i = 0; i < array.Length; i++)
    isContained = (value == array[i]);

if(isContained)
    Console.WriteLine("The value is in the array");
else
    Console.WriteLine("The value is not in the array");

Answer 1

Though you asked for a for loop, an infinite while may be better. Personally, I find the while loop a bit more expressive and explicit. The notion, or premise that while the loop iterates, it will stop once a match has been found.

Where in my mind the for loop can function almost identically, but that explicit while this is being performed, a explicit action will follow. Where the for loop quickly, iterate through a collection, then an action will follow.

Mostly in a phrasing manner, or quick glance, you would quickly assume intent.

int index = 0;
int value = 9;
bool valid = false;

while(collection.length != index)
{
     if(value == collection[index])
     {
          valid = true;
          break;
     }

     index++
}

You could also achieve this with Linq, which is more expressive.

int value = 9;
var valid = collection.Any(item => item == value);

Personally I find the any and while to be more expressive and self documenting for your use case.

Answer 2

I know this has been answered to death... However

Here is a plethora of ways to to achieve what you want, its good to understand all these variants as they will become the bread and butter of just about every algorithm you design in the future

Given these variables

var array = new[] { 3, 4, 2, 6, 3 };
var value = 2;
var isFound = false;

You can use a for loop

for (var i = 0; i < array.Length; i++) // loop to the end of the array
{
    if (array[i] == value) // comparison
    {
        isFound = true;    // we found
        break;             // We can exit the loop early
    }
}

// Write our results
Console.WriteLine($"Found = {isFound}");

---

You can use a foreach loop

foreach (var element in array) // loop to the end of the array
{
    if (element == value) // comparison
    {
        isFound = true;    // we found
        break;             // We can exit the loop early
    }
}

// Write our results
Console.WriteLine($"Found = {isFound}");

---

You can use a while loop

var index = 0;
while (index < array.Length) // loop to the end of the array
{
    if (array[index] == value) // comparison
    {
        isFound = true;        // we found
        break;                 // We can exit the loop early
    }
    index++;                   //++ is just a fancy way of saying Add one now before comparison
}

// Write our results
Console.WriteLine($"Found = {isFound}");

---

You can use a do while loop

var index = 0;
do // loop to the end of the array
{
    if (array[index] == value) // comparison
    {
        isFound = true;        // we found
        break;                 // We can exit the loop early
    }
} while (++index < array.Length);//  ++ is just a fancy way of saying Add one now before comparison

---

You can use Linq Any

// a fancy way of saying, does any element in the array match the value
isFound = array.Any(element => element == value);

// Write our results
Console.WriteLine($"Found = {isFound}");

---

You can use Linq Contains

// a fancy way of saying does the array contain your value
isFound = array.Contains(value);

// Write our results
Console.WriteLine($"Found = {isFound}");

---

Additional Resources

for (C# Reference)

foreach, in (C# Reference)

while (C# Reference)

do (C# Reference)

++ Operator (C# Reference)

Enumerable.Any

Enumerable.Contains

Further Reading about loops

Answer 3

Besides the fact that as it is, your code won't even compile because isContained may never be initialized, the problem is that you would keep overwriting isContained over and over again in the for loop:

bool isContained;
for(var i = 0; i < array.Length; i++)
    isContained = (value == array[i]);

Basically, you need to set it if and only if you've found the correct value. So a basic working version would be:

bool isContained = false;
for(var i = 0; i < array.Length; i++)
{
    if (value == array[i]) isContained = true;
}

Clearly, once a match is encountered, it makes no sense to iterate over the rest of the array. Hence this is slightly better:

bool isContained = false;
for(var i = 0; i < array.Length; i++)
{
    if (value == array[i])
    {
        isContained = true;
        break;
    }
}

This is a good approach to understand the concept. However, as someone pointed out in the comments, a more advanced solution exists that is a 'one-liner':

bool isContained = array.Any(x => x == value);

Answer 4

The problem is that your isContained value gets overwritten inside each iteration. Your code should be written as follows:

Boolean isContained = false;

for (Int32 i = 0; i < array.Length; ++i)
{
    if (value == array[i])
    {
        isContained = true;
        break; // no need further processing, you found it
    }
}

On a side note... why not just using Enumerable.Contains()?

Int32[] array = new Int32[] { 3, 4, 2, 6, 3 };
Int32 value = 2;

if (array.Contains(value))
    Console.WriteLine("The value '{0}' has been found in the array!", value);

Or a LINQ solution as follows:

Int32[] array = new Int32[] { 3, 4, 2, 6, 3 };
Int32 value = 2;

if (array.Any(x => x == value))
    Console.WriteLine("The value '{0}' has been found in the array!", value);

Answer 5

Your code has three problems.

The first is that the compiler can't detect that isContained will definitely be assigned. Consider if the array was empty; then isContained would never be set and it's not legal to use a value that hasn't been set. The compiler has to be relatively local in what it looks at to determine if a value has been set (if it could be cleverer we'd end up with cases that were very hard for us to work out why they were allowed or not), so that's a no-no.

And of course if it did happen that the array was empty, we should have a result anyway, which should be false.

We can solve both of those problems by starting off with the value set to false:

var array = new[] { 3, 4, 2, 6, 3 };
var value = 2;

bool isContained = false;
for (var i = 0; i < array.Length; i++)
    isContained = (value == array[i]);

if (isContained)
    Console.WriteLine("The value is in the array");
else
    Console.WriteLine("The value is not in the array");

And it gives us the wrong answer, revealing the third problem; because isContained keeps being written over it only tells us the last answer. One way to fix this is to use |= so that it can set false to true but not true to false:

var array = new[] { 3, 4, 2, 6, 3 };
var value = 2;

bool isContained = false;
for (var i = 0; i < array.Length; i++)
    isContained |= (value == array[i]);

if (isContained)
    Console.WriteLine("The value is in the array");
else
    Console.WriteLine("The value is not in the array");

This works, but is wasteful. Once we know the value is in the array it doesn't matter if we look at 0, 2, or a billion more values; the answer is going to be true. So we should stop looking:

var array = new[] { 3, 4, 2, 6, 3 };
var value = 2;

bool isContained = false;
for (var i = 0; i < array.Length; i++)
{
    if (value == array[i])
    {
        isContained = true;
        break;
    }
}

if (isContained)
    Console.WriteLine("The value is in the array");
else
    Console.WriteLine("The value is not in the array");

This gives us the correct answer quicker.

Incidentally a simpler version worth knowing about is to use Contains() which is for asking just these sort of questions:

if (array.Contains(value))
    Console.WriteLine("The value is in the array");
else
    Console.WriteLine("The value is not in the array");

Or more generally Any() which can ask more flexible questions about a collection:

if (array.Any(x => x == value))
    Console.WriteLine("The value is in the array");
else
    Console.WriteLine("The value is not in the array");

Being able to loop explicitly yourself though remains a vital technique to know. It's also the fastest, though the difference is so slight as to only matter in tight loops: Continue learning with for loops until you are confident with them, but then use the more concise and self-explanatory Contains or Any when applicable except in performance-critical points.

Answer 6

I cannot seem to figure out how to do this as all variables declared inside the for loop cannot be accessed from outside its scope

What do you mean? Your code has problems, but none involve not being able to access variables declared inside the for loop.

The code you posted should be close to compiling successfully. It does have a couple of problems though:

1. The main one is that you don't initialize isContained. This will cause the compiler to emit CS0165, "Use of unassigned local variable". Declared the variable with an initialization: bool isContained = false;.
2. Also, you keep setting isContained, so what you get at the end is really just whether the last element in the array is what you're looking for. Try isContained |= (value == array[i]); instead, or something like:

if (value == array[i])
{
    isContained = true;
    break;
}

That's a little more verbose than just using the |= operator, but it does terminate the loop early.

Of course, if you just want to get the value, and aren't trying to learn how to write a loop like that, you can use the built-in Enumerable.Contains() method:

bool isContained = array.Contains(value);

Answer 7

You can easily use List instead of int[]

using System;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        List<int> list = new List<int>(){ 2, 3, 7 };

        Console.WriteLine("List size: {0}", list.Count);
        Console.WriteLine("List has '1': {0}", list.Contains(1));
        Console.WriteLine("List has '2': {0}", list.Contains(2));
        Console.WriteLine("List has '3': {0}", list.Contains(3));
    }
}

Answer 8

Try returning from inside the loop.

for(var i = 0; i < array.Length; i++)
{
    if(value == array[i])
    {
        Console.WriteLine("The value is in the array");
        return;
    }
}

Console.WriteLine("The value is not in the array");