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C language determine length of array depending on amount of digits entered by the user

How do I determine the size of my array in C?Improve INSERT-per-second performance of SQLite?Fastest sort of fixed length 6 int arrayC program: Breaking an int into an int array[]Restrict user input to 8 digit numberC program: Checking digits entered are 1s and 0s only using arraysUnexpected behaviour when scanf used inside while loop to read an arrayArray size different from what is entered - CNeed to write a program that asks the user to fill in a 2D array then checks how many times a digit between 0 and 9 was entered in those two rowsUser defined array elements and array size in C

1

I have the below code in which a user enters their card number, with this each digit is stored as an element in the variable array digits[].

What I am trying to achieve is to have the for loop stop after it has stored all digits, however, I am struggling to set the loop as per the length of the entered card number.

I have tried the below code using sizeof(cardNo) stored in the variable length and setting length as the loop condition.

Let's say the user enters 1234 the result I am receiving with the below code being run is 43210000, when what I am looking for is just 4321.

if I set int digits to digits[4] and the loop condition to < 4 it will give me 4321 but obviously this restricts me to user only entering 4 digits when in fact I want this to be flexible i.e. if they enter 6 digits it will give me 123456 if they enter 8 digits it will give me 12345678 etc.

Any ideas or suggestions, please?

long cardNo = get_long("Enter Card Number: ");
int length = sizeof(cardNo);
int count = 0;
int digits[length];

for (int i = 0; i < length; i++) 
 digits[i] = cardNo % 10;
 cardNo /= 10;
 printf("%in", digits[i]);
 
printf("n");

c

share|improve this question

edited Mar 7 at 16:02

static_cast

1,07211521

asked Mar 7 at 15:29

CoderCoder

9911

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Allocate the maximum. And again, as in every question for this exercise (and there are hundresds around) I suggest to use string representation of the number rather than numerical value, unless there are strict requirements.
  
  – Eugene Sh.
  Mar 7 at 15:31
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  First: Use malloc to this allocate dynamically memoery. Second: length should be size_t type. I think cardNo too unless you want negative number handle.
  
  – Igor Galczak
  Mar 7 at 15:33
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  sizeof(cardNo) is not what you think it is.
  
  – Jabberwocky
  Mar 7 at 15:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @IgorGalczak It doesn't have to be size_t.
  
  – StaceyGirl
  Mar 7 at 15:38
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @StaceyGirl I write should be because of: wiki.sei.cmu.edu/confluence/display/c/…
  
  – Igor Galczak
  Mar 7 at 15:44
  

  
  
  
  

 | 
show 1 more comment

1

I have the below code in which a user enters their card number, with this each digit is stored as an element in the variable array digits[].

What I am trying to achieve is to have the for loop stop after it has stored all digits, however, I am struggling to set the loop as per the length of the entered card number.

I have tried the below code using sizeof(cardNo) stored in the variable length and setting length as the loop condition.

Let's say the user enters 1234 the result I am receiving with the below code being run is 43210000, when what I am looking for is just 4321.

if I set int digits to digits[4] and the loop condition to < 4 it will give me 4321 but obviously this restricts me to user only entering 4 digits when in fact I want this to be flexible i.e. if they enter 6 digits it will give me 123456 if they enter 8 digits it will give me 12345678 etc.

Any ideas or suggestions, please?

long cardNo = get_long("Enter Card Number: ");
int length = sizeof(cardNo);
int count = 0;
int digits[length];

for (int i = 0; i < length; i++) 
 digits[i] = cardNo % 10;
 cardNo /= 10;
 printf("%in", digits[i]);
 
printf("n");

c

share|improve this question

edited Mar 7 at 16:02

static_cast

1,07211521

asked Mar 7 at 15:29

CoderCoder

9911

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Allocate the maximum. And again, as in every question for this exercise (and there are hundresds around) I suggest to use string representation of the number rather than numerical value, unless there are strict requirements.
  
  – Eugene Sh.
  Mar 7 at 15:31
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  First: Use malloc to this allocate dynamically memoery. Second: length should be size_t type. I think cardNo too unless you want negative number handle.
  
  – Igor Galczak
  Mar 7 at 15:33
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  sizeof(cardNo) is not what you think it is.
  
  – Jabberwocky
  Mar 7 at 15:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @IgorGalczak It doesn't have to be size_t.
  
  – StaceyGirl
  Mar 7 at 15:38
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @StaceyGirl I write should be because of: wiki.sei.cmu.edu/confluence/display/c/…
  
  – Igor Galczak
  Mar 7 at 15:44
  

  
  
  
  

 | 
show 1 more comment

I have the below code in which a user enters their card number, with this each digit is stored as an element in the variable array digits[].

What I am trying to achieve is to have the for loop stop after it has stored all digits, however, I am struggling to set the loop as per the length of the entered card number.

I have tried the below code using sizeof(cardNo) stored in the variable length and setting length as the loop condition.

Let's say the user enters 1234 the result I am receiving with the below code being run is 43210000, when what I am looking for is just 4321.

if I set int digits to digits[4] and the loop condition to < 4 it will give me 4321 but obviously this restricts me to user only entering 4 digits when in fact I want this to be flexible i.e. if they enter 6 digits it will give me 123456 if they enter 8 digits it will give me 12345678 etc.

Any ideas or suggestions, please?

long cardNo = get_long("Enter Card Number: ");
int length = sizeof(cardNo);
int count = 0;
int digits[length];

for (int i = 0; i < length; i++) 
 digits[i] = cardNo % 10;
 cardNo /= 10;
 printf("%in", digits[i]);
 
printf("n");

c

share|improve this question

edited Mar 7 at 16:02

static_cast

1,07211521

asked Mar 7 at 15:29

CoderCoder

9911

long cardNo = get_long("Enter Card Number: ");
int length = sizeof(cardNo);
int count = 0;
int digits[length];

for (int i = 0; i < length; i++) 
 digits[i] = cardNo % 10;
 cardNo /= 10;
 printf("%in", digits[i]);
 
printf("n");

share|improve this question

edited Mar 7 at 16:02

static_cast

1,07211521

asked Mar 7 at 15:29

CoderCoder

9911

share|improve this question

edited Mar 7 at 16:02

static_cast

1,07211521

asked Mar 7 at 15:29

CoderCoder

9911

edited Mar 7 at 16:02

static_cast

1,07211521

edited Mar 7 at 16:02

static_cast

1,07211521

asked Mar 7 at 15:29

CoderCoder

9911

asked Mar 7 at 15:29

CoderCoder

9911

3 Answers
3

active

oldest

votes

3

The length of a number is based on its logarithm base 10:

#include <math.h> //log10
....
 int len = log10(cardNo) + 1;
...

Don't forget to link with the math library (gcc ... -lm)

share|improve this answer

edited Mar 7 at 15:40

answered Mar 7 at 15:35

pmgpmg

84.6k999170

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It's a long not an int. FPU roundoff burns man.
  
  – Joshua
  Mar 7 at 15:39
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Joshua: the argument cardNo is automatically converted to double
  
  – pmg
  Mar 7 at 15:42
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @pmg this seems to do the trick, thanks. Could you be kind enough to give me a quick explanation behind the code? This way I will know what's going on for next time :)
  
  – Coder
  Mar 7 at 15:44
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  This is what logarithms do. log10(x) is the number you have to raise 10 to to obtain x. 10^3 (power, not xor) is 1000, so log10(1000) is 3; I add 1 to get 4. log10(4863) is 3.<something> and it needs 4 digits. The assigment to an integer discards anything after the decimal point.
  
  – pmg
  Mar 7 at 15:49
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Coder: Fails for 999999999999999, 9999999999999999, 99999999999999999, 999999999999999999
  
  – Joshua
  Mar 7 at 16:13
  

  
  
  
  

 | 
show 1 more comment

0

Idiom: measure-allocate-generate.

size_t n =0;
for (; i < cardno; ) 
 cardNo /= 10;
 n++;
 
if(!n)n++;//0
int*digits=malloc(n*sizeof(int));

share|improve this answer

edited Mar 7 at 16:16

answered Mar 7 at 15:38

JoshuaJoshua

24.2k550103

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  sizeof(int) is not a good idea, modifications may yield errors.
  
  – Jose
  Mar 7 at 16:34
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Jose: Calls to malloc without *sizeof() are really bad ideas unless char.
  
  – Joshua
  Mar 7 at 16:45
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  I think @Jose meant that int *digits = malloc(n * sizeof *digits); will make it easier to avoid errors in case digits change type (struct baseddigit *digits = malloc(n * sizeof (int)) /* error here */;). @Jose: excuse me for butting in :)
  
  – pmg
  Mar 7 at 20:53
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, exactly @pmg. Thank you for your explanation, definitely better than mine.
  
  – Jose
  Mar 8 at 8:11
  

  
  
  
  

add a comment | 

0

The code may be simpler, considering a maximum amount of numbers (being developer's choice to store the fixed-size container on the stack, as static, etc):

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define STRINGIFY(x) STRINGIFY2(x)
#define STRINGIFY2(x) #x

#define MAX_NUMBERS 16

int main() 
 char cardNum[MAX_NUMBERS + 1];
 size_t sizeCard;

 printf("Enter card numbersn");
 scanf("%" STRINGIFY(MAX_NUMBERS) "s", cardNum);
 sizeCard = strlen(cardNum);
 for (int i = 0; i < sizeCard; i++) 
 printf("%cn", cardNum[i]);
 
 printf("n");

share|improve this answer

edited Mar 8 at 8:20

answered Mar 7 at 15:53

JoseJose

1,291516

add a comment | 

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3

The length of a number is based on its logarithm base 10:

#include <math.h> //log10
....
 int len = log10(cardNo) + 1;
...

Don't forget to link with the math library (gcc ... -lm)

share|improve this answer

edited Mar 7 at 15:40

answered Mar 7 at 15:35

pmgpmg

84.6k999170

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It's a long not an int. FPU roundoff burns man.
  
  – Joshua
  Mar 7 at 15:39
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Joshua: the argument cardNo is automatically converted to double
  
  – pmg
  Mar 7 at 15:42
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @pmg this seems to do the trick, thanks. Could you be kind enough to give me a quick explanation behind the code? This way I will know what's going on for next time :)
  
  – Coder
  Mar 7 at 15:44
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  This is what logarithms do. log10(x) is the number you have to raise 10 to to obtain x. 10^3 (power, not xor) is 1000, so log10(1000) is 3; I add 1 to get 4. log10(4863) is 3.<something> and it needs 4 digits. The assigment to an integer discards anything after the decimal point.
  
  – pmg
  Mar 7 at 15:49
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Coder: Fails for 999999999999999, 9999999999999999, 99999999999999999, 999999999999999999
  
  – Joshua
  Mar 7 at 16:13
  

  
  
  
  

 | 
show 1 more comment

3

The length of a number is based on its logarithm base 10:

#include <math.h> //log10
....
 int len = log10(cardNo) + 1;
...

Don't forget to link with the math library (gcc ... -lm)

share|improve this answer

edited Mar 7 at 15:40

answered Mar 7 at 15:35

pmgpmg

84.6k999170

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It's a long not an int. FPU roundoff burns man.
  
  – Joshua
  Mar 7 at 15:39
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Joshua: the argument cardNo is automatically converted to double
  
  – pmg
  Mar 7 at 15:42
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @pmg this seems to do the trick, thanks. Could you be kind enough to give me a quick explanation behind the code? This way I will know what's going on for next time :)
  
  – Coder
  Mar 7 at 15:44
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  This is what logarithms do. log10(x) is the number you have to raise 10 to to obtain x. 10^3 (power, not xor) is 1000, so log10(1000) is 3; I add 1 to get 4. log10(4863) is 3.<something> and it needs 4 digits. The assigment to an integer discards anything after the decimal point.
  
  – pmg
  Mar 7 at 15:49
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Coder: Fails for 999999999999999, 9999999999999999, 99999999999999999, 999999999999999999
  
  – Joshua
  Mar 7 at 16:13
  

  
  
  
  

 | 
show 1 more comment

The length of a number is based on its logarithm base 10:

#include <math.h> //log10
....
 int len = log10(cardNo) + 1;
...

Don't forget to link with the math library (gcc ... -lm)

share|improve this answer

edited Mar 7 at 15:40

answered Mar 7 at 15:35

pmgpmg

84.6k999170

#include <math.h> //log10
....
 int len = log10(cardNo) + 1;
...

share|improve this answer

edited Mar 7 at 15:40

answered Mar 7 at 15:35

pmgpmg

84.6k999170

answered Mar 7 at 15:35

pmgpmg

84.6k999170

answered Mar 7 at 15:35

pmgpmg

84.6k999170

0

Idiom: measure-allocate-generate.

size_t n =0;
for (; i < cardno; ) 
 cardNo /= 10;
 n++;
 
if(!n)n++;//0
int*digits=malloc(n*sizeof(int));

share|improve this answer

edited Mar 7 at 16:16

answered Mar 7 at 15:38

JoshuaJoshua

24.2k550103

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  sizeof(int) is not a good idea, modifications may yield errors.
  
  – Jose
  Mar 7 at 16:34
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Jose: Calls to malloc without *sizeof() are really bad ideas unless char.
  
  – Joshua
  Mar 7 at 16:45
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  I think @Jose meant that int *digits = malloc(n * sizeof *digits); will make it easier to avoid errors in case digits change type (struct baseddigit *digits = malloc(n * sizeof (int)) /* error here */;). @Jose: excuse me for butting in :)
  
  – pmg
  Mar 7 at 20:53
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, exactly @pmg. Thank you for your explanation, definitely better than mine.
  
  – Jose
  Mar 8 at 8:11
  

  
  
  
  

add a comment | 

0

Idiom: measure-allocate-generate.

size_t n =0;
for (; i < cardno; ) 
 cardNo /= 10;
 n++;
 
if(!n)n++;//0
int*digits=malloc(n*sizeof(int));

share|improve this answer

edited Mar 7 at 16:16

answered Mar 7 at 15:38

JoshuaJoshua

24.2k550103

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  sizeof(int) is not a good idea, modifications may yield errors.
  
  – Jose
  Mar 7 at 16:34
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Jose: Calls to malloc without *sizeof() are really bad ideas unless char.
  
  – Joshua
  Mar 7 at 16:45
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  I think @Jose meant that int *digits = malloc(n * sizeof *digits); will make it easier to avoid errors in case digits change type (struct baseddigit *digits = malloc(n * sizeof (int)) /* error here */;). @Jose: excuse me for butting in :)
  
  – pmg
  Mar 7 at 20:53
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, exactly @pmg. Thank you for your explanation, definitely better than mine.
  
  – Jose
  Mar 8 at 8:11
  

  
  
  
  

add a comment | 

Idiom: measure-allocate-generate.

size_t n =0;
for (; i < cardno; ) 
 cardNo /= 10;
 n++;
 
if(!n)n++;//0
int*digits=malloc(n*sizeof(int));

share|improve this answer

edited Mar 7 at 16:16

answered Mar 7 at 15:38

JoshuaJoshua

24.2k550103

size_t n =0;
for (; i < cardno; ) 
 cardNo /= 10;
 n++;
 
if(!n)n++;//0
int*digits=malloc(n*sizeof(int));

share|improve this answer

edited Mar 7 at 16:16

answered Mar 7 at 15:38

JoshuaJoshua

24.2k550103

answered Mar 7 at 15:38

JoshuaJoshua

24.2k550103

answered Mar 7 at 15:38

JoshuaJoshua

24.2k550103

0

The code may be simpler, considering a maximum amount of numbers (being developer's choice to store the fixed-size container on the stack, as static, etc):

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define STRINGIFY(x) STRINGIFY2(x)
#define STRINGIFY2(x) #x

#define MAX_NUMBERS 16

int main() 
 char cardNum[MAX_NUMBERS + 1];
 size_t sizeCard;

 printf("Enter card numbersn");
 scanf("%" STRINGIFY(MAX_NUMBERS) "s", cardNum);
 sizeCard = strlen(cardNum);
 for (int i = 0; i < sizeCard; i++) 
 printf("%cn", cardNum[i]);
 
 printf("n");

share|improve this answer

edited Mar 8 at 8:20

answered Mar 7 at 15:53

JoseJose

1,291516

add a comment | 

0

The code may be simpler, considering a maximum amount of numbers (being developer's choice to store the fixed-size container on the stack, as static, etc):

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define STRINGIFY(x) STRINGIFY2(x)
#define STRINGIFY2(x) #x

#define MAX_NUMBERS 16

int main() 
 char cardNum[MAX_NUMBERS + 1];
 size_t sizeCard;

 printf("Enter card numbersn");
 scanf("%" STRINGIFY(MAX_NUMBERS) "s", cardNum);
 sizeCard = strlen(cardNum);
 for (int i = 0; i < sizeCard; i++) 
 printf("%cn", cardNum[i]);
 
 printf("n");

share|improve this answer

edited Mar 8 at 8:20

answered Mar 7 at 15:53

JoseJose

1,291516

add a comment | 

The code may be simpler, considering a maximum amount of numbers (being developer's choice to store the fixed-size container on the stack, as static, etc):

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define STRINGIFY(x) STRINGIFY2(x)
#define STRINGIFY2(x) #x

#define MAX_NUMBERS 16

int main() 
 char cardNum[MAX_NUMBERS + 1];
 size_t sizeCard;

 printf("Enter card numbersn");
 scanf("%" STRINGIFY(MAX_NUMBERS) "s", cardNum);
 sizeCard = strlen(cardNum);
 for (int i = 0; i < sizeCard; i++) 
 printf("%cn", cardNum[i]);
 
 printf("n");

share|improve this answer

edited Mar 8 at 8:20

answered Mar 7 at 15:53

JoseJose

1,291516

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define STRINGIFY(x) STRINGIFY2(x)
#define STRINGIFY2(x) #x

#define MAX_NUMBERS 16

int main() 
 char cardNum[MAX_NUMBERS + 1];
 size_t sizeCard;

 printf("Enter card numbersn");
 scanf("%" STRINGIFY(MAX_NUMBERS) "s", cardNum);
 sizeCard = strlen(cardNum);
 for (int i = 0; i < sizeCard; i++) 
 printf("%cn", cardNum[i]);
 
 printf("n");

share|improve this answer

edited Mar 8 at 8:20

answered Mar 7 at 15:53

JoseJose

1,291516

answered Mar 7 at 15:53

JoseJose

1,291516

answered Mar 7 at 15:53

JoseJose

1,291516