Use #define to define an array size

Question

I used to capture 5 seconds of data from an ADC which was sending samples at 125 SPS, this translated to having an array of size 5 * 125. In my code this looked like:

#define SPS 125
#define TIME 5

char Samples[SPS * TIME];

Now, I would like to capture 2.5 seconds of data and increase the ADC sampling rate to 250 SPS. In the code it looks like:

#define SPS 250
#define TIME 2.5

char Samples[SPS * TIME];

But if I do this the compiler throws up an error as:

#901 expression must have integral or enum type main.cpp line 59 C/C++ Problem

I'm able to understand what it is saying.

But what is the best way to overcome this situation keeping the SPS and TIME definition's usability in mind. I mean I've used them in several other places across the project and I wish to keep using them as they are.

Kindly help!

another way is defined separate numerator and denominator for TIME, and declare char Samples[SPS * TIME_NUM / TIME_DENOM]; — phuclv
– phuclv, Commented Apr 28, 2015 at 6:19
The problem is that you've got a "mixed expression": integer * floating point; the final expression resulting in floating point. One solution, as both Cool Guy and Arun A.S correctly suggested, is to "cast" the expression to "int". — paulsm4
– paulsm4, Commented Apr 28, 2015 at 6:23

Arun A S · Accepted Answer · 2015-04-28 06:19:19Z

6

The problem is that the result is converted to a float ( or double ), but the size of an array must be an integral type. The easiest solution would be as suggested by @CoolGuy, you can cast it to an int. Here is an example

#define SPS 125
#define TIME 2.5

char Samples[ (int)(SPS * TIME) ];

answered Apr 28, 2015 at 6:19

Arun A S

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Lundin · Accepted Answer · 2015-04-28 06:35:36Z

5

I'm assuming this is an embedded system, so you probably don't want to use float numbers for this, as doing so typically forces you to link a floating point library to the project.

Float free version:

#define SPS       250ul
#define TIME_MS   2500ul
#define SAMPLES_N ( (SPS * TIME_MS) / 1000ul )

char Samples[SAMPLES_N];

If you insist on using float:

#define SPS        250
#define TIME       2.5
#define SAMPLES_N  ((unsigned long)(SPS * TIME))

char Samples[SAMPLES_N];

(These two snippets will work on all platforms, including small 8/16 bit MCU applications. Unlike all the posted answers using int, which are non-portable.)

answered Apr 28, 2015 at 6:35

Lundin

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Peheje · Accepted Answer · 2015-04-28 06:23:12Z

2

Have you tried casting to unsigned int?

char Samples[(unsigned int)(SPS * TIME)];

answered Apr 28, 2015 at 6:23

Peheje

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