In a block of code, I need to get an input from the user and print it right away. I'm a bit confused with good practice between these two snippets.
Snippet #1:
int i;
while(){
scanf("%d", &i);
printf("%d", i);
}
I'm using the variable i only within the loop. So, should I declare and use it within the loop itself to maintain good practice of using scopes?
If I do the same,
Snippet #2:
while(){
int i;
scanf("%d", &i);
printf("%d", i);
}
the variable i is declared on every iteration of the loop! These re-declarations consume more processing power and re-allocation of memory again and again
Which approach is better and why?
You said:
the variable i is declared on every iteration of the loop! These re-declarations consume more processing power and re-allocation of memory again and again
This is an invalid assumption on your part. The C compiler will calculate the total memory the function needs and allocate for that usage up front. For local variables, that allocation is actually just a shift of a pointer.
To illustrate, if I have a function:
void a_function () {
extern int x, n1, n2;
while (--n1) {
int i;
scanf("%d", &i);
x += i;
}
while (--n2) {
int i, j;
scanf("%d %d", &i, &j);
x += i + j;
}
}
The compiler will allocate space for 2 integers on entry into the function (or none, if it decides it can do everything in registers).
Related
Im trying to make a program which says how many times a specific digit appears on a 100 numbers sequence.
Meanwhile I got this error and I can´t understand what is the solution to this. I´d appreciate if you could get me some tip or the solution.
#include <stdio.h>
int main() {
int i, m, digit, val[99], count=0;
printf("Enter a number:");
scanf("%d", &val[0]);
while (val[0] < 0) {
printf("Enter a number:");
scanf("%d", &val[0]);
}
for (i=1;i<101;i++) {
val[i]=val[0]++;
printf("%d\n", val[i]);
}
printf("Enter a digit:");
scanf("%d", &m);
while (m<0||m>9) {
printf("Enter a digit:");
scanf("%d", &m);
}
do {
digit=val[i]%10;
val[i]=val[i]/10;
if (digit==m) {
count++;
}
}while (val[i]>0);
printf("The digit %d is printed %d times in this sequence.", m, count);
}
In the for loop you step outside of the array val of which the last index is 98. Instead of hard-coding the length of the array in several places it is more convenient to use a length macro, like this:
#define LEN(anArray) (sizeof (anArray) / sizeof (anArray)[0])
...
for (i = 1; i < LEN(val); i++) {
...
Also, in the do-while loop the index i is outside of the array bounds of val. You also need to check the return value of scanf to make sure the input is valid. The last printf statement also needs a trailing newline.
Edit: Note that LEN only handles "real" arrays; arrays passed to functions are received as pointers.
You allocated only int /* ... */ val[99] (only val[0] to val[98] are available) and accessed upto val[100] because the loop condition is i<101.
This will lead to dangerous out-of-range write (undefined behaior).
Allocate enough elements like int /* ... */ val[101] or fix the loop condition not to cause out-of-range access.
Also you didn't set value of i after the for (i=1;i<101;i++) loop, so value of uninitialized element will be used in the do ... while loop. Values of uninitialized elements of non-static local variables are indeterminate and using the value invokes undefned behavior.
Set i to proper value before the loop or change the indice i to proper thing.
Why is this code not running after printing of array if I take value of n>=9?
#include <stdio.h>
#include <math.h>
float mean_function(float array[],int n);
int main() {
int i,n;
float array[n],mean,sum=0,s2,summation,deno,C[i],elements;
printf("Enter No of Elements\n");
scanf("%d",&n);
printf("Enter Elements\n");
for(i=0;i<n;i++){
scanf("%f",&array[i]);
printf("%f",array[i]);
}
printf("sample variance(s2) : (sum((x-mean)*(x-mean)))/(n-1) /n");
printf("population variance(sigma2) : (sum((x-u)*(x-u))/n");
mean_function(array,n);
for(i=0;i<n;i++) {
deno=((array[i]-mean)*(array[i]-mean));
C[i]=deno;
summation=summation+C[i];
}
s2=((summation)/(n-1));
printf("s2=%f \n",s2);
}
float mean_function(float array[],int n) {
int i;
float sum=0,mean;
for(i=0;i<n;i++){ sum=sum+array[i]; }
mean=(sum/n);
return mean;
}
Why is this code not running after printing of array if I take value
of n>=9?
Some thoughts about your code (and about building programs in steps):
Arrays in C don't change in size once defined. VLAs are out for a variety of reasons. malloc() is in.
Use double, unless there is a specific reason to use floats.
Define and initialize one variable per line. Uninit vars can only result in an error as mentioned by #Jens.
Function declarations at the top (which you have done)
During development, there is no need to complicate things with a scanf (at least initially). It only adds an unwarranted layer of complexity. If you are testing statistical functions (mean, variance), put numbers in a pre-defined static array and verify functionality first.
C[i] as been declared with uninitialized i.
For this initial phase of building this program, I include a basic program.
I am not a fan of zero space between tokens (but ignore that)
Consider calling your array something other than 'array'.
Calculating the size of the samples array allows you to change the number of elements without changing anything else in code; which adds another layer of complexity to an already difficult phase.
#include <stdio.h>
#include <math.h>
double sample_mean(double* p, int n);
int main()
{
double samples[] = {1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 16.5, 2.3};
double mean = 0.0;
int size_samples = sizeof samples/sizeof(double);
printf("size_samples = %d\n", size_samples);
mean = sample_mean(samples, size_samples);
printf("Mean = %.2lf", mean);
}
// -------------------------------
double sample_mean(double* p, int n)
{
double mean = 0.0;
double total = 0.0;
for(int i = 0; i < n; i++)
total += *p++;
mean = total/n;
return mean;
}
Once this functionality is present (saved), you can start working on other stat functions. This way you can work step by step to get closer to the desired outcome.
Next up you can define sample_variance(double* p, int n) and work on that knowing that additional(new errors) are not coming from your code written so far.
Output:
size_samples = 8
Mean = 5.24
I hope it helps.
The code is likely not running because array[n] is declared with an uninitialized n. At the time you read n with scanf(), the array does not automatically "grow into the right size". You should either declare array big enough, or if you really want it to be user-defined, use malloc to allocate it (read the comp.lang.c FAQ) and all Stackoverflow questions tagged array...)
In addition, the scanf at some point fails. Note that when you enter numbers, you also have the "Enter" as a newline ('\n') in the input stream. You never read the newline so the next scanf fails.
This becomes obvious when you actually check the return value from scanf with code like this:
if (scanf("%f", &array[i]) == 1) {
/* successfully converted 1 item */
}
else {
/* scanf failed */
}
Usually what you want is to skip whitespace in the input. You do this by placing a space in the scanf format. Note that a single space tells scanf to skip any amount of white-space.
if (scanf(" %f", &array[i]) == 1) {
My cousin has a school project and we can't figure out why is the array different the second time it's printed when there is no values changing in between?
Basically you enter a number which states how many rows/columns will the matrix have, and during first loop he assigns a number to every position and prints out the random number. However, the second time we go through the matrix the numbers are different and it seems that they are copied through the matrix from bottom left corner to top right corner for some reason. It seems strange to us because we never assign a different value to a position in the array after defining it for the first time.
int i,j,n,matrica[i][j],suma=0;
srand(time(NULL));
printf("\nunesi prirodan broj N[3,20] = \n");
scanf("%d",&n);
for(i=0;i<n;i++) {
for(j=0;j<n;j++) {
matrica[i][j]=rand()%100;
printf("%d, %d = %4d ",i, j, matrica[i][j]);
if(j==n-1) {
printf("\n");
}
}
}
printf("\n");
for(i=0;i<n;i++) {
for(j=0;j<n;j++) {
printf("%d, %d = %4d ", i, j, matrica[i][j]);
if(j==n-1) {
printf("\n");
}
}
}
And here is the result of this (the code I pasted here has 2 prints, and in the image there is 3 but every time you go through the matrix after the first time it's going to be the same):
We need to use malloc to allocate the dynamic amount of memory.
After
scanf("%d",&n) // PS You should check the return value - read the manual page
Put
matrica = malloc(sizeof(int) * n * n);
And declare it as
int *matrica;
Then replace
matrica[i][j]
with
matrica[i * n + j]
And after you have finished with matrica - use free i.e.
free(matrica);
int i,j,n,matrica[i][j]
At this point I must ask, what value do you think i and j will have? Right there you're invoking undefined behaviour by referring to variables declared with automatic storage duration which you've not initialised. Anything after this point is... undefined behaviour.
Having said that, I noticed a few other parts that look strange. Which book are you reading? The reason I ask is that the people I know to be reading reputable textbooks don't have these problems, thus your textbook (or resource, whatever) mustn't be working for you...
I can't read the commentary inside of the string literals, which is a shame, since that's usually quite valuable contextual information to have in a question. Nonetheless, moving on, if this were me, I'd probably declare a pointer to an array n of int, after asking for n, like so:
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
int main(void) {
size_t n;
printf("Enter n, please: ");
fflush(stdout);
if (scanf("%zu", &n) != 1 || n == 0 || SIZE_MAX / n < n) {
puts("Invalid input or arithmetic overflow...");
return -1;
}
int (*array)[n] = malloc(n * sizeof *array);
if (!array) {
puts("Allocation error...");
return -1;
}
/* now you can use array[0..(n-1)][0..(n-1)] as you might expect */
free(array);
}
This should work for quite high numbers, much higher than int array[n][n]; would in its place... and it gives you that option to tell the user it was an "Allocation error...", rather than just SIGSEGV, SIGILL, SIGBUS or something...
... but nothing would be more optimal than just saving the seed you use to generate the random numbers, and the user input; that's only two integers, no need for dynamic allocation. There's no point storing what rand generates, amd you realise this, right? rand can generate that output purely using register storage, the fastest memory commonly available in our processors. You won't beat it with arrays, not meaningfully, and not... just not.
I have a function that calculates geometric mean (nth root of product of terms, where n is number of terms), and the only thing I'm having trouble with is figuring out how to keep track of how many terms there were. The only thing I could come up with was a global variable, but I know I shouldn't do that. How can I do it without the global variable?
float count;
float geometricMean(float n) {
char again;
float j;
printf("Input a number: ");
scanf("%f", &j);
printf("Another number? (y/n) ");
scanf(" %c", &again);
if (again == 'y') {
j *= geometricMean(n+1);
if (n==1)
return pow(j, (1/count));
else {
return j;
}
}
else {
count = n;
return j;
}
}
Use a static variable. Static variables don't change between function calls.
At the start of your function, write something like:
float geometricMean(float n) {
static int count = 0;
count ++;
// Rest of the code;
}
Each time you call your function, count will increase by 1 (it won't go back to 0).
Go here: What does "static" mean?
if you want to know more about static variables.
EDIT: Please don't go out using static variables just like if it was a global, just with a smaller scope. It will lead to undesired results! If you're planning to change the value of a static variable in each iteration of your function, like I did here, please make you sure you make crystal clear what those alterations are and use them right after defining your static if possible.
I want to add numbers to an array using scanf
What did i do wrong? it says expected an expression on the first bracket { in front of i inside the scanf...
void addScores(int a[],int *counter){
int i=0;
printf("please enter your score..");
scanf_s("%i", a[*c] = {i});
}//end add scores
I suggest:
void addScores(int *a, int count){
int i;
for(i = 0; i < count; i++) {
printf("please enter your score..");
scanf("%d", a+i);
}
}
Usage:
int main() {
int scores[6];
addScores(scores, 6);
}
a+i is not friendly to newcomer.
I suggest
scanf("%d", &a[i]);
Your code suggests that you expect that your array will be dynamically resized; but that's not what happens in C. You have to create an array of the right size upfront. Assuming that you allocated enough memory in your array for all the scores you might want to collect, the following would work:
#include <stdio.h>
int addScores(int *a, int *count) {
return scanf("%d", &a[(*count)++]);
}
int main(void) {
int scores[100];
int sCount = 0;
int sumScore = 0;
printf("enter scores followed by <return>. To finish, type Q\n");
while(addScores(scores, &sCount)>0 && sCount < 100);
printf("total number of scores entered: %d\n", --sCount);
while(sCount >= 0) sumScore += scores[sCount--];
printf("The total score is %d\n", sumScore);
}
A few things to note:
The function addScores doesn't keep track of the total count: that variable is kept in the main program
A simple mechanism for end-of-input: if a letter is entered, scanf will not find a number and return a value of 0
Simple prompts to tell the user what to do are always an essential part of any program - even a simple five-liner.
There are more compact ways of writing certain expressions in the above - but in my experience, clarity ALWAYS trumps cleverness - and the compiler will typically optimize out any apparent redundancy. Thus - don't be afraid of extra parentheses to make sure you will get what you intended.
If you do need to dynamically increase the size of your array, look at realloc. It can be used in conjunction with malloc to create arrays of variable size. But it won't work if your initial array is declared as in the above code snippet.
Testing for a return value (of addScores, and thus effectively of scanf) >0 rather than !=0 catches the case where someone types ctrl-D ("EOF") to terminate input. Thanks #chux for the suggestion!