C Programming Fundamentals for GATE

C Programming is a foundational topic in the GATE Computer Science & IT syllabus, contributing 4–8 marks every year. Questions heavily test operator precedence, side-effects, evaluation order, pointer arithmetic, recursion, and memory representation.

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1. Operator Precedence & Associativity

When an expression contains multiple operators, operator precedence determines which operator is evaluated first. When operators have equal precedence, associativity dictates whether the expression is evaluated from left-to-right (L → R) or right-to-left (R → L).

Precedence Rank	Operators	Description	Associativity
1 (Highest)	() [] -> .	Postfix, Subscript, Member Access	Left-to-Right
2	++ -- + - ! ~ * & sizeof	Prefix Increment/Decrement, Unary operations, Address-of, Dereference	Right-to-Left
3	* / %	Multiplicative (Arithmetic)	Left-to-Right
4	+ -	Additive (Arithmetic)	Left-to-Right
5	<< >>	Bitwise Shift Left / Right	Left-to-Right
6	< <= > >=	Relational Comparisons	Left-to-Right
7	== !=	Equality Comparisons	Left-to-Right
8	&	Bitwise AND	Left-to-Right
9	^	Bitwise XOR	Left-to-Right
10	|	Bitwise OR	Left-to-Right
11	&&	Logical AND (Short-circuit)	Left-to-Right
12	||	Logical OR (Short-circuit)	Left-to-Right
13	?:	Conditional (Ternary)	Right-to-Left
14	= += -= *= /= %=	Assignment Operators	Right-to-Left
15 (Lowest)	,	Comma Operator	Left-to-Right

Key Takeaway for GATE: Unary operators, Ternary (?:), and Assignment operators (=) evaluate from Right to Left (R → L). Almost all other operators evaluate from Left to Right (L → R).

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2. Modulus Operator (%) Rules

1. Integer Types Only: The modulus operator % operates exclusively on integral types (int, char, long). Floating-point operands (float, double) trigger compile-time errors.
2. Sign of Result: In C99 and later standards, the result of a % b takes the sign of the dividend (a):
   • 7 % 3 $\rightarrow$ 1
   • -7 % 3 $\rightarrow$ -1
   • 7 % -3 $\rightarrow$ 1
   • -7 % -3 $\rightarrow$ -1

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3. Truth Values & Short-Circuit Evaluation

In C:

• Any non-zero numeric value or non-null address evaluates to True (1).
• 0, '\0' (null character), and NULL evaluate to False (0).

Short-Circuiting Behavior:

• Logical AND (&&): If the left-hand side operand evaluates to 0 (False), the right-hand side operand is never executed.
• Logical OR (||): If the left-hand side operand evaluates to non-zero (True), the right-hand side operand is never executed.

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4. scanf() and printf() Return Values

scanf() Return Value

scanf() returns the number of input items successfully matched and assigned.

int a, b;
int count = scanf("%d %d", &a, &b);
// If user inputs "10 20", count is 2.
// If user inputs "10 abc", count is 1.

printf() Return Value

printf() returns the total number of characters printed to stdout.

int n = printf("GATE2026\n"); // Prints 9 characters (including '\n')
// n will be 9.

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Solved GATE Exam Practice Questions

Question 1 (GATE CS)

What is the output of the following C program snippet?

#include <stdio.h>
int main() {
    int x = 1, y = 0, z = 5;
    int result = x || ++y && ++z;
    printf("%d %d %d\n", x, y, z);
    return 0;
}

Options:

• (A) 1 1 6
• (B) 1 0 5
• (C) 1 1 5
• (D) 1 0 6

Correct Answer: (B) 1 0 5

Explanation:

• In the expression x || ++y && ++z, || has lower precedence than &&, but || is evaluated left-to-right with short-circuiting behavior.
• x is 1 (True). Since the left operand of || is True, short-circuit evaluation kicks in.
• The right operand (++y && ++z) is skipped entirely. Thus, y remains 0 and z remains 5.
• Output: 1 0 5.