Reading versus writing APIs in C

const in the signature says whether a function will change what you hand it.

typedef struct {
    int values[8];
    size_t count;
} Series;

double series_mean(const Series *s) {
    long total = 0;
    for (size_t i = 0; i < s->count; i++) {
        total += s->values[i];
    }
    return s->count ? (double) total / (double) s->count : 0.0;
}

int series_push(Series *s, int value) {
    if (s->count >= 8) {
        return 0;
    }
    s->values[s->count++] = value;
    return 1;
}

How it works

A reader takes const T * and promises not to write.
A writer takes T * and says so in the type.
The compiler enforces the promise, so it is real documentation.

Keywords and builtins used here

const
double
for
if
int
long
return
series_mean
series_push
size_t
struct
typedef

The run, in numbers

Lines: 20
Characters to type: 354
Tokens: 127
Three-star pace: 100 tpm

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Step 4 of 4 in Signatures, step 4 of 11 in APIs, errors & testing.

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