Control structures, arrays and strings notes — Unit 2

Without control statements a program is a straight line: every statement runs once, in order. Real problems need choices and repetition, and structured programming allows exactly three ways to arrange them.

C has no boolean type in C89. A condition evaluates to an int: 1 for true, 0 for false. Any non-zero value counts as true, which is the source of several classic bugs.

if (marks >= 40)
    printf("Pass\n");

if (n % 2 == 0)
    printf("Even\n");
else
    printf("Odd\n");

• Braces are optional for a single statement but required for a block — always write them.
• No semicolon after if (cond); a stray one makes the body an empty statement.
• else always binds to the nearest unmatched if, whatever your indentation suggests.

if (m >= 90)      printf("O");
else if (m >= 80) printf("A");
else if (m >= 70) printf("B");
else if (m >= 40) printf("C");
else              printf("F");

• Order matters: test the narrowest range first, or every mark above 40 prints C.
• Exactly one branch of a ladder executes — the first true one.
• A nested if is an if inside another if; use it when the second test only makes sense if the first passed.

if (a > b) {
    if (a > c) printf("%d", a);
    else       printf("%d", c);
} else {
    if (b > c) printf("%d", b);
    else       printf("%d", c);
}

big = (a > b) ? a : b;
printf("%s\n", (n % 2 == 0) ? "Even" : "Odd");
/* nested — readable only in small doses */
grade = (m >= 80) ? 'A' : (m >= 40) ? 'C' : 'F';

• ?: is the only ternary (three-operand) operator in C.
• It is an expression, so it can appear inside printf or an assignment.
• goto label; jumps unconditionally; the label sits in the same function.
• Accepted use of goto: breaking out of deeply nested loops on an error.

Every correct loop has three parts, whichever form you choose: an initialisation, a condition that eventually becomes false, and an update that moves towards it. A missing update is the definition of an infinite loop.

int choice;
do {
    printf("1 Add  2 View  3 Exit: ");
    scanf("%d", &choice);
    /* handle the choice here */
} while (choice != 3);

With i = 10 and the condition i < 5, while prints nothing while do-while prints once. That single difference is the whole comparison question.

for (i = 1; i <= 5; i++) printf("%d ", i);   /* 1 2 3 4 5 */
for (i = 10; i >= 1; i -= 2) printf("%d ", i); /* 10 8 6 4 2 */
for (i = 0, j = 9; i < j; i++, j--) { }        /* two counters */
for (;;) { break; }                            /* infinite, exited by break */

1. 1.Initialisation runs once, before the first test.
2. 2.Condition is tested before every iteration.
3. 3.Body executes when the condition is true.
4. 4.Update runs after the body, then back to step 2.

for (i = 1; i <= 4; i++) {
    for (j = 1; j <= i; j++)
        printf("* ");
    printf("\n");
}
/*  *
    * *
    * * *
    * * * *  */

• Outer loop counts rows, inner loop counts columns in that row.
• Total iterations of the inner body = sum of the inner counts, not rows × columns, when the inner limit depends on i.
• The newline belongs to the outer loop; putting it inside the inner loop breaks every pattern question.

for (i = 1; i <= 3; i++) {
    for (j = 1; j <= 3; j++)
        printf("%4d", i * j);
    printf("\n");
}

An array is a collection of elements of the same type stored in contiguous memory, reached by a single name and an index that starts at 0.

int marks[5];                       /* 5 ints, values garbage   */
int a[5] = {10, 20, 30, 40, 50};    /* full initialisation      */
int b[5] = {1, 2};                  /* rest become 0            */
int c[] = {4, 8, 12};               /* size inferred as 3       */
printf("%d", a[0]);                 /* first element, 10       */
printf("%d", a[4]);                 /* last element,  50       */

• Valid indices are 0 to size − 1; a[5] in the example above is out of bounds.
• C does not check bounds — an out-of-range write corrupts other memory silently.
• The array name alone stands for the address of its first element.

int sum = 0;
for (i = 0; i < n; i++) sum += a[i];
float avg = (float) sum / n;

int big = a[0];
for (i = 1; i < n; i++) if (a[i] > big) big = a[i];

int pos = -1;                       /* linear search for key */
for (i = 0; i < n; i++)
    if (a[i] == key) { pos = i; break; }

• Initialise the maximum with a[0], never with 0 — that fails for all-negative data.
• Cast before dividing for the average, or integer division loses the fraction.
• Linear search: worst case n comparisons; binary search needs sorted data and takes about log₂n.

for (i = 0; i < n - 1; i++)
    for (j = 0; j < n - 1 - i; j++)
        if (a[j] > a[j + 1]) {
            t = a[j]; a[j] = a[j + 1]; a[j + 1] = t;
        }

After each pass the largest remaining element is in place, which is why the inner limit shrinks by i. Selection sort instead finds the minimum of the unsorted part and swaps it into position — same O(n²) cost, far fewer swaps.

int m[2][3] = {{1, 2, 3}, {4, 5, 6}};

for (i = 0; i < r; i++)
    for (j = 0; j < c; j++)
        s[i][j] = a[i][j] + b[i][j];

• m[i][j] — first index is the row, second the column; both start at 0.
• Stored in row-major order: m[0][0], m[0][1], m[0][2], m[1][0] …
• Addition needs identical orders; multiplication needs columns of A = rows of B.
• Multiplication uses three nested loops, with sum reset to 0 before the innermost one.

A string in C is a char array terminated by the null character '\'. The terminator is what every library function looks for, so a char array without it is not a string.

char s1[10] = "Hi";                 /* H i \ + 7 unused */
char s2[] = "Hi";                   /* size 3            */
char s3[3] = {'H', 'i', '\'};      /* explicit          */
char *s4 = "Hi";                    /* pointer to literal */

• scanf("%s", name) stops at the first space and needs no & (an array name is already an address).
• For a full line use fgets(name, sizeof name, stdin) — gets() has no length limit and is removed from modern C.
• Print with printf("%s", name) or puts(name).

• strlen("Hello") is 5, but the array needs 6 bytes.
• s1 = s2 does not copy a string; only strcpy does.
• if (s1 == s2) compares addresses; use strcmp(s1, s2) == 0 for content.
• The destination must be large enough — strcat into a full array overflows it.

int len = 0;
while (s[len] != '\') len++;          /* own strlen */

for (i = 0, j = len - 1; i < j; i++, j--) {
    char t = s[i]; s[i] = s[j]; s[j] = t;  /* reverse   */
}

int pal = 1;                            /* palindrome */
for (i = 0, j = len - 1; i < j; i++, j--)
    if (s[i] != s[j]) { pal = 0; break; }

• Counting vowels: compare against 'a','e','i','o','u' in both cases, or use tolower() from ctype.h.
• Counting words: increment when the current character is not a space and the previous one was.
• 'A' is 65 and 'a' is 97, so s[i] − 32 converts lower case to upper case.

• Can you compare while and do-while in four points with an example?
• Can you dry run a nested loop and predict the exact printed pattern?
• Can you write bubble sort and show pass 1 on given data?
• Can you explain why strcmp is needed instead of ==?
• Can you write your own strlen, strrev and palindrome check?