C Cheat Sheet & Exam Prep

Use this page for last-minute revision. Every row links back to the full lesson.

#Program skeleton

#include <stdio.h>

int main(void) {
    // declarations
    // input
    // processing
    // output
    return 0;
}

#Data types and format specifiers

* typical 64-bit sizes; check with sizeof. See Variables & Data Types.

#Control flow at a glance

if (cond) { } else if (cond2) { } else { }
switch (n) { case 1: ...; break; default: ...; }
for (int i = 0; i < n; i++) { }
while (cond) { }
do { } while (cond);     // runs at least once
break;     // leave loop/switch
continue;  // skip to next iteration

#Pointers in one table

#Must-practise programs

#Armstrong number (asked very often)

A number is an Armstrong number if the sum of its digits, each raised to the number of digits, equals the number: 153 = 1³ + 5³ + 3³.

#include <stdio.h>

int isArmstrong(int n) {
    int digits = 0, sum = 0;
    for (int t = n; t > 0; t /= 10) digits++;
    for (int t = n; t > 0; t /= 10) {
        int d = t % 10, p = 1;
        for (int i = 0; i < digits; i++) p *= d;
        sum += p;
    }
    return sum == n;
}

int main(void) {
    printf("Armstrong numbers below 10000: ");
    for (int i = 1; i < 10000; i++)
        if (isArmstrong(i)) printf("%d ", i);
    printf("\n");
    return 0;
}

Armstrong numbers below 10000: 1 2 3 4 5 6 7 8 9 153 370 371 407 1634 8208 9474

#Mistakes that cost marks

#Theory questions to prepare

1. Features of C and the structure of a C program.
2. Compilation stages: preprocessor, compiler, assembler, linker.
3. Difference between while and do-while; between break and continue.
4. Call by value vs call by reference with an example.
5. Recursion vs iteration.
6. Storage classes with scope, lifetime and default values.
7. Structure vs union.
8. malloc vs calloc; what a memory leak is.
9. File opening modes.
10. Macro vs function.