Arrays, strings and pointers together

Why it exists

In C the name of an array is silently turned into a pointer in most expressions, and text is stored as an array of characters with no length field. These two facts make C compact, and they make a large share of C exam questions tricky: the same code can be read as "array" or as "pointer", and sizeof, assignment and & treat the two differently.

Prerequisites

Precise definitions

Arrays versus pointers

An array name decays to a pointer to its first element in nearly every expression (see the arrays page). The exceptions are the operand of sizeof, the operand of unary &, and a string literal used to initialise an array. That does not make an array a pointer. The two are different objects:

a[i] and p[i] are both computed as *(a + i) and *(p + i), so the syntax of indexing is the same. The difference is that for a the array's cells start at a, whereas for p the program must first load the pointer, then add.

a versus &a. Both have the same numerical address, but different types. a + 1 moves one element (4 bytes). &a + 1 moves one whole array (20 bytes). This is a classic trick question.

Strings

• A string in C is a sequence of char values ending with a null character '\0' (the byte whose value is 0). There is no length stored anywhere. Functions find the end by looking for the null character.
• The length of a string is the number of characters before the null character. strlen("GATE") is 4. The size of the array that holds it is at least 5. sizeof("GATE") is 5.
• A string literal such as "GATE" is an array of char with static storage duration. Modifying a string literal is undefined behaviour. On the usual platform it is placed in read-only memory (see the memory layout page), so the attempt crashes.
• 'a' is a character constant. In C its type is int (in C++ it is char), so sizeof 'a' is 4 wherever int is 4 bytes, as on gcc/x86-64. "a" is a string literal: two bytes, 'a' and '\0'.

Three ways to hold a string

The animation below shows the first and the third together. The array holds its own private copy of the text. The pointer holds only an address, and the text it points at is somewhere else.

The standard string functions (<string.h>)

== between two strings compares the addresses, not the contents. Use strcmp(a, b) == 0 to test equality.

Arrays of pointers versus two-dimensional arrays

const char *p[] = {"ab", "cde", "f"}; is an array of three pointers, each aiming at a literal of its own length. char q[3][4] = {"ab", "cde", "f"}; is a rectangle of $3 \times 4 = 12$ chars, each row padded with zeros. The first costs $3 \times 8$ bytes for the pointers (plus the literals), the second costs 12.

How it works

Most programs that walk a string use a pointer that advances until it meets the null character. Two idioms appear again and again, and both are exam favourites.

Length by pointer.

int len(const char *s) {
    const char *p = s;
    while (*p) p++;
    return (int)(p - s);
}

*p is false only at the null character (value 0), so the loop stops there. p - s is the number of elements walked, which is the length.

Copy by pointer.

while ((*d++ = *s++))
    ;

Read it as: copy *s into *d, then advance both pointers. The value of the whole assignment expression is the character just copied. The loop therefore ends after the null character has been copied, so the destination is correctly terminated.

Worked examples

#include <stdio.h>
#include <string.h>
 
int main(void) {
    char s[] = "GATE";
    char t[10] = "GATE";
    const char *u = "GATE";
    printf("%zu %zu %zu %zu\n", sizeof s, sizeof t, sizeof u, strlen(t));
    return 0;
}

5 10 8 4

#include <stdio.h>
 
int main(void) {
    int a[5] = {10, 20, 30, 40, 50};
    printf("%td %td\n", (char *)(a + 1) - (char *)a, (char *)(&a + 1) - (char *)a);
    return 0;
}

4 20

#include <stdio.h>
 
int main(void) {
    char s[] = "ABCDEF";
    printf("%s %c %s %c\n", s + 3, *(s + 1), &s[4], "GATE"[2]);
    return 0;
}

DEF B EF T

#include <stdio.h>
 
int main(void) {
    char s[] = "gate";
    for (char *p = s; *p; p++) *p = *p - 'a' + 'A';
    printf("%s\n", s);
    return 0;
}

GATE

#include <stdio.h>
 
int main(void) {
    char src[] = "AB";
    char dst[3] = {'x', 'y', 'z'};
    char *s = src, *d = dst;
    int steps = 0;
    while ((*d++ = *s++)) steps++;
    printf("%d %zu %c%c%d\n", steps, (size_t)(d - dst), dst[0], dst[1], dst[2]);
    return 0;
}

2 3 AB0

#include <stdio.h>
#include <string.h>
 
int main(void) {
    char a[] = "GATE";
    char b[] = "GATE";
    const char *p = a, *q = b;
    printf("%d %d\n", p == q, strcmp(a, b) == 0);
    return 0;
}

0 1

#include <stdio.h>
#include <string.h>
 
int main(void) {
    const char *p[] = {"ab", "cde", "f"};
    char q[3][4] = {"ab", "cde", "f"};
    printf("%zu %zu %zu %zu\n", sizeof p, sizeof q, strlen(p[1]), strlen(q[1]));
    return 0;
}

24 12 3 3

Cheat box

Traps and GATE patterns

Connections

The arrays page covers indexing and addresses, and pointers the arithmetic used here. Where a string literal usually lives (read-only memory, which is why modifying one normally crashes) is a fact about the memory layout. Passing an array to a function really passes a pointer, shown on the functions page. Strings in the theory of computation are a different object: mathematical sequences with no terminator (alphabets, strings and languages).

PYQs

Consider the following function written the C
programming language.
 void foo (char *a) {
    if (*a && *a != '   ') {
        foo (a+1);
        putchar (*a);
    }
}
The output of the above function on input
"ABCD EFGH" is

Consider the following C program:

#include < stdio.h >
int main(){
int a;
int arr[5] = {30,50,10};
int *ptr;
ptr = &arr[0] + 1;
a = *ptr;
(*ptr)++;
ptr++;
printf("%d", a + (*ptr) + arr[1]);
return 0;
}

The output of the above program is__________  .
(Answer in integer)

Consider the following C program:

#include < stdio.h >
void stringcopy(char *, char *);
int main(){
    char a[30] = "@#Hello World!";
    stringcopy(a, a + 2);
    printf("%s\n", a);
    return 0;
}
void stringcopy(char *s, char *t) {
    while(*t)
        *s++ = *t++;
}

Which ONE of the following will be the output of the
program?

Consider the following C program:
#include <stdio.h>
int main() {
    int a[] = {2, 4, 6, 8, 10};
    int i, sum = 0, *b = a + 4;
    for (i = 0; i < 5; i++)
        sum = sum + (*b - i) - *(b - i);
    printf("%d\n", sum);
    return 0;
}
The output of the above C program is ____ .

Consider the following C program:
#include <stdio.h>
int main (){
   int arr []={1,2,3,4,5,6,7,8,9,0,1,2,5},
   *ip = arr+4;
    printf ("%d\n", ip[1]);
    return 0;
}

The number that will be displayed on execution of the
program is _____.

Consider the following C program.
#include<stdio.h>
#include<string.h>
int main ()  {
      char* c = "GATECSIT2017";
      char* p = c;
      printf("%d", (int) strlen (c + 2[p] - 6[p] - 1));
      return 0;
}
The output of the program is _________ .

Consider the following C program segment.
# include <stdio.h>
int main( )
{
char s1[7] = "1234", *p;
p = s1 + 2;
*p = '0' ;
printf ("%s", s1);
}
What will be printed by the program?

What does the following fragment of C-program print?
char c[] = "GATE2011";
char *p =c;
printf("%s", p + p[3] - p[1]) ;

#include<stdio.h>
int f(int *a, int n)
{
    if(n <= 0) return 0;
    else if(*a % 2 == 0)
    return *a+f(a+1, n-1);
  Else   return *a - f(a+1, n-1);
}
int main()
{
  int a[] = {12, 7, 13, 4, 11, 6};
printf("%d", f(a, 6));
getchar();
  return 0;
}
 What is the value printed by the following C
program

Self-check

#include <stdio.h>
 
int main(void) {
    char s[] = "COMPILER";
    char *p = s + 2;
    printf("%s %c %td\n", p, p[3], p - s);
    return 0;
}

MPILER L 2

Revision card

References