Cheat Sheet
printf() and scanf() format
printf("%d", n);
FORMAT ARGUMENT TYPE
%d, %i int decimal
%u unsigned int
%x unsigned int hexadecimal
%#x unsigned int hexadecimal with prefix 0x
%f double
%Lf long double
%e, %E double scientific notation
%c int to print a character
%s char * string (character array ended with '\0')
%p void * print memory address
%g, %G double %f or %e depending on the length
scanf("%d", &n);
FORMAT ARGUMENT TYPE
%d int * &n, store the input integer in n
%ld long *
%lld long long *
%u unsigned int *
%f float * read float
%lf double * read double
%Lf long double * read long double
%c char * read 3 characters %3c
%s char * read a string until whitespace
%n int * with %s, to get string length
char a[100]; int len;
scanf("%s%n", a, &len);
len will have the string length
Frequently used functions
#include <string.h>
char str[10];
scanf("%s", str);
to get the string length using strlen(str)
to compare two strings strcmp(str1, str2) ==0 if equal
to compare the first n chars of two strings strncmp(str1, str2, n) ==0 if equal
to copy str2 to str1 strcpy(str1, str2)
to copy the first n chars of str2 to str1 strncpy(str1,str2, n) remember to add '\0' to str1
#include <ctype.h>
isspace(ch), islower(ch), isupper(ch), isdigit(ch)
isalpha(ch), toupper(ch), tolower(ch)
To create a 5-by-5 two-dimensional array, we need to write
int a[5][5];
It will be indexed as follows:
How to read the following data?
1 2 3 4 5 e
#include <stdio.h>
int main(void)
{
int x;
while (scanf("%d", &x) == 1) {
printf("x=%d\n", x);
}
return 0;
}
How to read the following data?
2
L 5 2
D 5 3
#include <stdio.h>
int main(void)
{
char ch;
int i, n, row, col;
scanf("%d", &n);
for (i=0; i<n; i++) {
while(getchar()!='\n');
scanf("%c%d%d", &ch, &row, &col);
}
return 0;
}
Using for loops to print a two-dimensional array
for(i = 0; i < row; i++) {
for (j = 0; j < col; j++) {
printf("%5d", A[i][j]);
}
printf("\n");
}
Using bubble sort to rearrange an array A
for (i = 0; i < n; i++) {
for (j = 1; j < n; j++) {
if (A[j] > A[j-1]) {
/* swap A[j] A[j-1] */
}
}
}
operators:
! && || == != + - * / %
> < >= <=
How to avoid common errors and how to debug for OJ
1. Put the arrays in the 'global' area. Set their size bigger than required. Avoid using variable-length arrays (e.g. int arr[n];). Keep the array size fix (e.g., int arr[1000];).
2. After writing the code for reading input data, you may print out the data to check if your code reads them correctly. Do not proceed to write subsequent code before you confirm that.
3. If your program crashes, usually it is caused by memory related errors. Check the ranges of for-loops to see if your code attempts to read or write the elements out of the arrays’ boundary.
*(a+i) is equivalent to a[i]
(a+i) is equivalent to &a[i]
qsort :
you have to include <stdlib.h>
usage :
void qsort (void *array, size_t count, size_t size, comparison_fn_t compare);
qsort an int array
int compare_int (const void *a, const void *b)
{
const int *va = (const int *) a;
const int *vb = (const int *) b;
return *va-*vb;
}
qsort a double array
int compare_double (const void *a, const void *b)
{
const double *da = (const double *) a;
const double *db = (const double *) b;
return (*da > *db) - (*da < *db);
}