**Problem :**

I have a list

```
a = [49, 51, 53, 56]
```

How do I subtract 13 from each integer value in the list?

Solution :

With a list comprehension:

```
a = [x - 13 for x in a]
```

If are you working with numbers a lot, you might want to take a look at NumPy. It lets you perform all kinds of operation directly on numerical arrays. For example:

```
>>> import numpy
>>> array = numpy.array([49, 51, 53, 56])
>>> array - 13
array([36, 38, 40, 43])
```

You can use map() function:

```
a = list(map(lambda x: x - 13, a))
```

To clarify an already posted solution due to questions in the comments

```
import numpy
array = numpy.array([49, 51, 53, 56])
array = array - 13
```

will output:

array([36, 38, 40, 43])

This will work:

```
for i in range(len(a)):
a[i] -= 13
```