Arrays

Array Definition, Explicit Initialization

Nonsensical but illustrative exercise: count digits, whitespace, and others.

countchars-explicit-init.c

#include <stdio.h>

int main(void)
{
    int c, i;
    int nwhite = 0, nother = 0;
    int ndigit[10];

    // explicit array initialization
    for (i=0; i<10; i++)
        ndigit[i] = 0;

    while ((c = getchar()) != EOF)
        if (c >= '0' && c <= '9')
            ++ndigit[c-'0'];
        else if (c == ' ' || c == '\t' || c == '\n')
            ++nwhite;
        else
            ++nother;

    // output stats
    for (i=0; i<10; i++)
        printf("#%c: %d\n", '0'+i, ndigit[i]);

    printf("#white: %d\n#other: %d\n", 
           nwhite, nother);
    return 0;
}

Array Access

You access the n-th element of the array using the index ([n]) operator.

int n = 7;   // for example
int number;
ndigit[n] = 666;  // write value 666 in n-th array position
number = ndigit[n];   // read value from n-th array position (and store into 'number')

Discussion: Initialization

int nwhite = 0, nother = 0;	Counter for whitespace and rest
int ndigit[10]; for (i = 0; i < 10; ++i) ndigit[i] = 0;	10 counters for digits 0..9 Attention: Indexes start at 0

Discussion: if, else

while ((c = getchar()) != EOF) if (c >= '0' && c <= '9') ++ndigit[c-'0']; else if (c == ' ' || c == '\t' || c == '\n') ++nwhite; else ++nother;

There is only if and else No elif (as in Python, for example) ⟶ second if is nested

Array Initializer

countchars-array-initializer.c

#include <stdio.h>

int main(void)
{
    int c, i;
    int nwhite = 0, nother = 0;
    int ndigit[10] = { 0, 0, 0, 0, 0, 0, 0, 0, 0 };

    while ((c = getchar()) != EOF)
        if (c >= '0' && c <= '9')
            ++ndigit[c-'0'];
        else if (c == ' ' || c == '\t' || c == '\n')
            ++nwhite;
        else
            ++nother;

    // output stats
    for (i=0; i<10; i++)
        printf("#%c: %d\n", '0'+i, ndigit[i]);

    printf("#white: %d\n#other: %d\n", 
           nwhite, nother);
    return 0;
}

Discussion: Initializer

int ndigit[10] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };	Array initialized as 10 zeroes
int ndigit[10] = { 1, 2, 3, 4 };	Array initialized as { 1, 2, 3, 4, 0, 0, 0, 0, 0, 0 }
int ndigit[10] = { 0 };	Array initialized as 10 zeroes

Initialization using memset()

countchars-memset-init.c

#include <stdio.h>
#include <string.h>

int main(void)
{
    int c, i;
    int nwhite = 0, nother = 0;
    int ndigit[10];

    memset(ndigit, 0, sizeof(ndigit));

    while ((c = getchar()) != EOF)
        if (c >= '0' && c <= '9')
            ++ndigit[c-'0'];
        else if (c == ' ' || c == '\t' || c == '\n')
            ++nwhite;
        else
            ++nother;

    // output stats
    for (i=0; i<10; i++)
        printf("#%c: %d\n", '0'+i, ndigit[i]);

    printf("#white: %d\n#other: %d\n", 
           nwhite, nother);
    return 0;
}

Discussion: memset()

See memset() manual page.

memset(ndigit, 0, sizeof(ndigit));

For the purpose of parameter passing, ndigit is a pointer to the 0-th element ⟶ the beginning of the array. sizeof() operator: size in bytes ⟶ 10*sizeof(int) (40)