Arrays and strings in C

ACompressed recap

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An array is a row of same-type elements; a[i] is *(a + i) and sits at base + i·w; the name decays to &a[0] except in sizeof and &a; array parameters are pointers (sizeof gives 4), so functions can change elements and must be told the length.

2-D arrays are row-major: &a[i][j] = B + (i·C + j)·w; a + 1 jumps a row; parameters need the column count. int *p[N] is an array of pointers, int (*p)[N] a pointer to an array.

Strings are char arrays ending in '\0'; literals are read-only; strlen stops before \0 while sizeof counts the array; use strcmp, not ==.

BMerged rule card

1.5.1 1-D arrays; a[i] == *(a+i) == i[a]; arrays decaying to pointers; passing arrays to functions; the sizeof trap

• int a[n]: indices 0 … n−1; element i at base + i·w.
• Partial initializer zero-fills; int a[] = {…} sizes itself.
• a[i] ≡ *(a + i) ≡ i[a]; p[i] works for any pointer.
• a decays to &a[0] except in sizeof a and &a (whole array).
• Array parameter = pointer: sizeof gives 4 here; pass the length.
• Out-of-range index = undefined; arrays cannot be assigned.

1.5.2 2-D arrays; row-major layout; pointer to array vs array of pointers; reading complex declarations

• T a[R][C]: &a[i][j] = B + (i·C + j)·w (row-major).
• a[i][j] ≡ ((a + i) + j) ≡ *(a[i] + j).
• a decays to T (*)[C]; a + 1 jumps a row; a[i] + 1 jumps an element.
• Parameter: int m[][C] or int (*m)[C] — columns required.
• int *p[N] array of pointers; int (*p)[N] pointer to array.
• Read declarations: name → right ([] , ()) → left (*), parentheses first.

1.5.3 Strings: char arrays, '\0', read-only string literals, strlen vs sizeof, string functions and tracing string code

• String = chars + '\0'; "abc" takes 4 bytes.
• char s[] = "…" writable copy; char *p = "…" points to a read-only literal.
• strlen counts before \0; sizeof counts the whole array (or 4 for a pointer).
• strcpy, strcat, strcmp (0, 0, >0), strncpy in string.h; compare with strcmp, not ==.
• %s prints from a pointer up to \0; p + k prints the tail.
• *p + 1 ≠ *(p + 1).

CInterleaved GATE-style questions

Q1MCQ2 marksOutput

What does this print?

int a[5] = {1, 2, 3, 4, 5};
int *p = a + 1;
*p++ += 10;
*(p + 1) = *p * 2;
p[-2] = 0;
printf("%d %d %d %d %d\n", a[0], a[1], a[2], a[3], a[4]);

• (A) 0 12 13 6 5
• (B) 0 12 3 6 5
• (C) 1 12 3 6 5
• (D) 0 2 13 26 5

Q2NAT2 marksAddress

float f[6][8]; starts at 4000 with 4-byte floats. What is the address of f[5][2]?

Numerical answer: ________

Q3NAT1 markStrings

What is strlen("data\0base")?

Numerical answer: ________

Q4MCQ1 marksizeof

In main, int a[8]; (4-byte ints). What is sizeof(a) / sizeof(a[0])?

• (A) 4
• (B) 32
• (C) 8
• (D) 1

Q5MCQ2 marksOutput

What does this print?

char s[] = "abcdef";
char *p = s;
p += 2;
*p = 'X';
printf("%s %s %c %c\n", s, p + 1, *p + 1, *(p - 1));

• (A) abcdef cdef d b
• (B) abXdef def X c
• (C) abXdef def Y b
• (D) abXdef Xdef Y b

Q6MCQ1 markDeclarations

In int *x[5];, x is:

• (A) a function
• (B) an array of 5 pointers to int
• (C) a pointer to an array of 5 ints
• (D) a pointer to int

Q7MSQ2 marksArrays

For int m[4][5]; which are true?

• (A) m[1] + 1 points to m[1][1]
• (B) m + 1 points to row 1
• (C) m[1][5] is the same element as m[2][0]
• (D) sizeof(m[0]) is 20 with 4-byte ints

Q8MCQ2 marksOutput

What does this print?

int a[] = {4, 8, 15, 16};
int *p = a;
int s = 0;
while (p < a + 4)
    s += *p++;
printf("%d\n", s);

• (A) 4
• (B) 27
• (C) 12
• (D) 43

Q9MCQ1 markFunctions

Inside void f(char s[]), sizeof(s) with 4-byte pointers is:

• (A) the length + 1
• (B) 4
• (C) depends on the caller
• (D) the length of the string

DAnswer Vault