strings

Projects General Guidance
1

I have a data logger and want to add time to the log.

what I do is to watch to see when the motor is running, thereby starting the timer.
once the motor stops, I stop the timer and then take total running seconds.

I create a string in one section of program to log to my SD card, but this time is in a different part.

How can I declare a string to be global so that I can add this to the write string ?

I am thinking it would be less memory intensive to have a string that is

01:22:54

this shows seconds : minutes : hours

than to have each of the three variables as global and adding them to my string for data logging.

2

I am thinking it would be less memory intensive to have a string that is

9 bytes, instead of three scalar variables that are one byte each.

I don't understand that logic.

How can I declare a string to be global

char string[10];

outside of any function.

3

PaulS:
9 bytes, instead of three scalar variables that are one byte each.

I don't understand that logic.

char string[10];

outside of any function.

logic ? who tried to bring logic into this ???
just to understand....
byte second
byte minutes
byte hour
would take less space than

putting these into a string and passing that string globally ?

4

just to understand....
byte second
byte minutes
byte hour
would take less space than

putting these into a string and passing that string globally ?

The second, minutes, and hour variables use three bytes. The string representation of the time uses 9 bytes. In my state, 3 is less than 9. That might be different in New Jersey.

5

Why are you bothered about saving 6 bytes - it hardly matters to the SD Card.

Saving data in human readable format has a lot going for it.

...R

6

I think the 'lesson' is valuable..

but I agree with Robin2.. human readable has it place, and makes it easier for us 'noobs' to understand as well.

7

counting time to data log in one section of code, for use in the data write loop. so I have to use a global in some form to pass the information from where I do the calculating to where it is written to the SD card

I am just starting with time and datalogging so am bumping against the edges of my knowledge and believe me, it is a pretty small box I am working in.

May I offer that I asked because I don't know. Often one can make such an assumption, but it seems that in some cases, that is not the basis of the response.

Besides, if I have 3 times, actually 3 numbers, 2 digits each, that represent times. seconds, minutes, hours, they are not scalar, the are integers. AFAIK

Paul said "9 bytes, instead of three scalar variables that are one byte each. I don't understand that logic."

I guess my question is much deeper. A scalar variable, such as 17:23:03
vs three byte variables 17 , 20 and 03

does the one scalar, when declared as a global, does it use more memory than three variables of all bytes ?

it would seem to me you have the variable name, or address, then the stored value
3 names and 3 stored values vs 1 name and a larger stored value.

May I offer that reading the references has lead me in circles and brought me here.

http://playground.arduino.cc/Code/DatatypePractices
in reading this, it would seem that what Paul is implying is that you take one byte for the address, then one byte for each place, in my case 6 numbers and 2 letters, to have 9 bytes.

8

dave-in-nj:
I guess my question is much deeper. A scalar variable, such as 17:23:03
vs three byte variables 17 , 20 and 03

does the one scalar, when declared as a global, does it use more memory than three variables of all bytes ?

I would deal with the part I high-lighted first by asking "does it matter?"

I then looked at Wikipedia and it seems there are 4 uses of scalar and I can't figure which you mean.

It seems to me that 17:23:03 requires a string variable rather than an integer - and 17,23,03 (i.e. CSV, comma separated variables) would probably be more readily parsed in programs such as Excel.

I would store the time value in SRAM in whatever form made it easiest to develop and maintain the program - including the possibility of converting it to a number of seconds since some base date. A lot depends on what "sums" you want to do with the date/time value. For example if you need to count the number of days or find the last day of a month it is easier if the values are separate.

But I would probably write the data to the SD Card as a string because it would be very easy to check it just by reading the file in a text editor.

...R

9

I guess my question is much deeper. A scalar variable, such as 17:23:03

17:23:00 is not a scalar variable. 17 might be held in a scalar variable, occupying one byte of SRAM. The same applies to 23 and 00, for a total of 3 bytes.

Or, the values could be converted to a string, "17:23:00" (which implies a terminating NULL) that is 8 characters (plus the NULL) long, needing 9 bytes to store it.

does the one scalar, when declared as a global, does it use more memory than three variables of all bytes ?

I hope that it is now clear that this question is meaningless.

10

PaulS:
17:23:00 is not a scalar variable. 17 might be held in a scalar variable, occupying one byte of SRAM. The same applies to 23 and 00, for a total of 3 bytes.

Or, the values could be converted to a string, "17:23:00" (which implies a terminating NULL) that is 8 characters (plus the NULL) long, needing 9 bytes to store it.
I hope that it is now clear that this question is meaningless.

Scalar was defined and a number. it seems that I think 0 to 9. like my minutes and hours.
a colon however, is also a number : 58 in DEC, 072 in OCT, and 00111010 in binary.

seems the word scalar is not self explanatory.

to jump past the confusion I just made sec/min/hr as global and got that to work fine. Added the colon in the string with all the other parts to write to the SD card.