Illustrator diagonal lines into straight lines

Question

I have these diagonal lines in my artboard which I want to make vertical lines.

The horizonal lines are easy to make vertical but how can I make the diagonal lines also vertical?

I tried several options but failed, there seems no other topic on here which solves my problem.

Hope there is someone with the knowledge!

EDIT: What I actually mean is this line down here is a diagonal. I want this to be rotated so it gets a straight line (horizontal or vertical).

Answer 1

(updated answer after seeing your question edit)

There's an easy way to rotate one end of a line, while the other end stays in place:

• make sure you have Smart guides turned on
• create a vertical guideline touching one of the anchors
• hit R to rotate and click that very same anchor point (mouse cursor will change). This will make the line rotate around that point (locks the point in place).
• grab the other anchor point and move it towards the guideline until it snaps
• release

Answer 2

If the path you wish to make vertical is part of a larger path, you can align anchors. This only works if all anchors on path are not selected.

• Use the Direct Selection Tool (white arrow)
• Select the top anchor and the bottom anchor
• Click the vertical align options in the Align Panel

Anchors can be aligned if they are individually selected.

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If you aren't concerned with maintaining any specific anchor position, you can use average

• Select the path
• Choose Object > Path > Average
• Tick Vertical and hit OK

This averages the two anchors, aligning them vertically essentially in the middle of where the two anchors started. Sort of the same as "align on center".

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If your path consists of only two anchors, and you want to maintain the existing position of one of the anchors.... see @Lucian's rotate method.