Programming in C

Data Concepts in C

• Constants are fixed values that do not change during execution (literal or defined via #define).
• Variables are named storage locations declared with a data type before use.
• Declaration example: int count; or float avg = 0.0f;.

Data Types, Size and Values

Standard fundamental types commonly used in exams are listed below.

Storage Classes

• auto — default for local variables (automatic storage duration).
• static — retains value between calls; internal linkage if file-scope.
• extern — declares a global variable defined elsewhere.
• register — hint to keep variable in CPU register (often ignored by modern compilers).

Remember exam questions often ask for scope and lifetime of each storage class.

Operators and Precedence

• Operator categories: arithmetic, relational, logical, bitwise, assignment, and ternary.
• Precedence determines evaluation order. Higher precedence operators evaluate first.

Operator associativity is usually left-to-right; assignment and some unary operators associate right-to-left.

Statements and Expressions

• Declaration statements: introduce variables and optionally initialize them.
• Expression statements: evaluate expressions and may produce side effects (a = b + c;).
• Compound statements (blocks): groups of statements enclosed in { and } defining scope.

Input-Output Statements

• Standard I/O: printf(), scanf(), puts().
• Format specifiers: %d int, %f float, %c char, %s string.
• Always pass address with scanf() for non-array variables: scanf("%d", &x).

int x;
printf("Enter a number: ");
scanf("%d", &x);
printf("You entered %d
", x);

Selection Statements

• if executes a block when condition is true.
• if-else selects between two branches.
• switch selects cases based on integer/enum; use break to prevent fall-through.
• goto exists but is discouraged; used rarely for error handling in legacy code.

if (a > b) {
  printf("a is greater
");
} else {
  printf("b is greater or equal
");
}

switch (ch) {
  case 'a': doA(); break;
  case 'b': doB(); break;
  default: doDefault();
}

Loop Constructs

• for — initialization; condition; update. Use when count known.
• while — condition-checked before each iteration.
• do-while — body executes first, condition checked afterwards.
• Jumps — break, continue (skip), and goto (discouraged).

// for loop
for (int i = 0; i < n; i++) {
  sum += i;
}

// while loop
int i = 0;
while (i < n) {
  sum += i;
  i++;
}

// do-while
int i = 0;
do {
  sum += i;
  i++;
} while (i < n);

Overflow

• Overflow occurs when a calculation produces a value outside a type's range.
• Signed integer overflow is undefined behaviour in C and must be avoided.
• Unsigned overflow wraps modulo 2^n; prefer larger types or checks when needed.

Debugging Exercise

#include <stdio.h>

int main() {
  char s[5];
  printf("Enter string: ");
  scanf("%s", s);   // possible overflow if user types >4 chars
  printf("You entered: %s
", s)
  return 0;
}

Programming Exercise

• Implement a program that reads n integers and finds their sum and average, using loops and appropriate types.
• Implement factorial using iterative loop and check for overflow for large n.