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I'm designing an app where in one screen of the application users can input a single number (and nothing else) by use of a numeric keypad. The number may have a single decimal place.

Depending on the function corresponding to the input field (in other words: depending on the screen the user navigated to), it might be the case that the user can enter only negative numbers into the field.

In this case of solely negative number input, I'd like to ease the input for the user. As no positive values are possible on that particular screen, I have the hunch that users should not be required to always start their input with the minus sign key. Users would save this superflous key press if the input field would just always be pre-filled with a minus sign at the beginning.

Now I'm asking me how the numeric (software) keypad I'm displaying to the user should look and feel. As there is a dedicated place for the minus key in the numpad, which users may know from other input fields on other screens, I'm struggling to decide between the following thee options:

  1. Hide the minus sign softkey from the numeric keypad.
  2. Grey-out the minus sign softkey and make it unusable.
  3. Show the minus sign softkey and allow users to optionally use it as first input character. This way, users may reach their desired negative input value (e.g. "-20.5") on two ways, either by omitting the minus key (here: "20.5", minus is pre-filled by the app) or by explicitly entering the minus key (here: "-20.5"). After entry of the minus key or entry of the first digit, the minus key would be greyed-out.
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    Option 3 is the least restrictive providing the best UX. (if a user types a minus sign multiple times before during or after numeric input it should make a single minus sign appear at the front.) – DaveAlger Jan 13 '15 at 5:13
  • Anytime keys are disabled it causes friction. At least one user will wonder why making them lose focused concentration cycles. – DaveAlger Jan 13 '15 at 5:19
  • @DaveAlger I disagree. IMHO disabling a key makes sense in certain cases. If there is a good reason that a key can't come into effect under a certain condition, and if you can't give users an audible feedback, than disabling the key in that condition is much better than having it enabled while the key does not do anything when being used. This way the user gets visual feedback that he/she can't use the key in that condition. I think its better to give that visual clue instead of no feedback at all. – Tafkadasoh Jan 13 '15 at 8:52
  • If it is a soft numpad for touchscreen use, do not include the minus and plus keys as equals to the digits, since that would only make sense on a calculator. Move the sign switch to the left of the input field and make it sticky in this case. – Crissov Jan 13 '15 at 16:10
  • Good idea Crissov. I prefer the gmail approach which is only show buttons as they make sense and don't bog the user down with inactive buttons. Why show something you can't ever do? Make it a - toggle in front of the input then a - label when it can't be changed – DaveAlger Jan 13 '15 at 16:24
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Of your 3 options, I would prefer the third because one of two things could happen when the user encounters this screen:

Scenario 1:The user assumes that whatever input will be set negative and omit the minus, and that would work.

Scenario 2:The user assumes that needs to add the minus sign, and that would work.

So either way, third option works in both cases, but the first option may confuse the user in scenario 2 and the second may be confusing in scenario 1. With option 3 the user won't even think about the other possibility, just whatever he assumes works.

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A fourth option is to show the minus symbol outside of the form field and have the users input positive numbers.

I'm doing something similar in the app I'm building, like so:

enter image description here

I can't show more of the app, unfortunately! But in context, it is clear that we're asking for a negative number, but having the user input a positive one.

Depending on other factors in your app, this approach may work for you and obviate the need for users to enter the minus symbol entirely.

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    Good addition, thank you very much! That's quite the way how I imagined the visualization of the pre-filled minus after my post here, too (I did a mockup which I can't show unfortunately). However, I don't have a box around the input field, but when the user taps on the previous value to edit it (which will open the numeric keypad), the positive number of the previous value is selected. So the minus is not selected, so it is clear that it won't be overwritten when the user enters the new value. – Tafkadasoh Jan 13 '15 at 8:46
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    I think this is the best approach. One thing to say here is that you should still allow users to include a minus sign (so that the input is effectively --55) and then strip out the additional minus server side (or wherever. – Willl Jan 13 '15 at 11:16
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    Replace the - label here with a button in cases where the user has a choice. The button could change between + and - – DaveAlger Jan 13 '15 at 16:26
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At first I wanted to go for option 1, but then I thought about the possibility that some users might not recognize the pre-filled minus character in the input field. I guess some of these users might be confused that there is no minus sign available to start the input of the negative value they just have in mind ("-20.5"). So in my opinion option 3 is the right way to do it.

However, I'd suggest to color the pre-filled minus sign in front of the input key in a lighter color, in order to be able to give users additional visual feedback. After the first key press (minus key or some digit), the minus sign should change its color to the regular input field color. Without this color-change, users wouldn't see a visual change in the input key after press of the minus key.

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Show the minus button locked in an active/depressed state when the user has no other choice. I'd also include an indication in the numeric display, such as a negative zero as the placeholder prior to entry.

To reinforce this clue (and improve overall interface feedback), I'd do the same for other actions that are active, eg 'plus', 'multiply', 'sq root'.

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