# python 3 – x for x for loops – how do they work?

## python 3 – x for x for loops – how do they work?

The reason for the apparently redundant extra mention of the variable `x`

when writing `x for x`

is that the first `x`

does not need to be `x`

. It just happens to be in the examples you give. Here are a few more examples which should clarify the difference between the first and second `x`

in your question:

```
ones = [1 for x in range(10)]
```

This simply gives a list of 10 ones, the same as `[1] * 10`

.

```
squares = [x*x for x in range(10)]
```

This gives `x`

squared for each `x`

in the specified range.

In your example, the second `x`

is the variable used by the for loop, and the first `x`

is simply an expression, which happens in your case to be just `x`

. The expression can be whatever you like, and does not need to be in terms of `x`

.

```
results = [expression for x in range(10)]
```

`expression`

can include anything you like – a string, a calculation, a function – whatever you choose. If the expression happens to be just `x`

then it looks unusual if you are not used to it, but its the same as the following:

```
results = []
for x in range(10):
results.append(expression)
```

The good way to understand it is to read it a bit different. So if we take your piece of code:

```
primes = [x for x in range(2, 50) if x not in noprimes]
```

We can read is as:

Primes = All x for which x in range(2,50) but only if x not in noprimes

I hope this helps you understand the functionality better.

#### python 3 – x for x for loops – how do they work?

It is a placeholder for a transformation of the element.

Suppose you wanted a list of primes multiplied by 2. Then you could replace the first `x`

with `x*2`

.

```
primes = [x*2 for x in range(2, 50) if x not in noprimes]
```

You write the following to say that you do not want a transformation, and just original `x`

.

```
primes = [x for x in range(2, 50) if x not in noprimes]
```

**Another perspective**: Consider the `x`

in the statement `primes.append(x)`

. *That* is the same as the first `x`

in the loop in your question. In fact, this is the **exact same loop** as the one above.

```
primes = []
for x in range(2,50):
if x not in noprimes:
primes.append(x)
```

Here is the other example, with `x*2`

.

```
primes = []
for x in range(2,50):
if x not in noprimes:
primes.append(x*2)
```